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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package parser implements a parser for Go source files. Input may be
// provided in a variety of forms (see the various Parse* functions); the
// output is an abstract syntax tree (AST) representing the Go source. The
// parser is invoked through one of the Parse* functions.
//
package parser

import (
        "fmt"
        "go/ast"
        "go/scanner"
        "go/token"
)

// The parser structure holds the parser's internal state.
type parser struct {
        file *token.File
        scanner.ErrorVector
        scanner scanner.Scanner

        // Tracing/debugging
        mode   Mode // parsing mode
        trace  bool // == (mode & Trace != 0)
        indent uint // indentation used for tracing output

        // Comments
        comments    []*ast.CommentGroup
        leadComment *ast.CommentGroup // last lead comment
        lineComment *ast.CommentGroup // last line comment

        // Next token
        pos token.Pos   // token position
        tok token.Token // one token look-ahead
        lit string      // token literal

        // Non-syntactic parser control
        exprLev int // < 0: in control clause, >= 0: in expression

        // Ordinary identifier scopes
        pkgScope   *ast.Scope        // pkgScope.Outer == nil
        topScope   *ast.Scope        // top-most scope; may be pkgScope
        unresolved []*ast.Ident      // unresolved identifiers
        imports    []*ast.ImportSpec // list of imports

        // Label scope
        // (maintained by open/close LabelScope)
        labelScope  *ast.Scope     // label scope for current function
        targetStack [][]*ast.Ident // stack of unresolved labels
}

func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
        p.file = fset.AddFile(filename, fset.Base(), len(src))
        var m scanner.Mode
        if mode&ParseComments != 0 {
                m = scanner.ScanComments
        }
        p.scanner.Init(p.file, src, p, m)

        p.mode = mode
        p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)

        p.next()

        // set up the pkgScope here (as opposed to in parseFile) because
        // there are other parser entry points (ParseExpr, etc.)
        p.openScope()
        p.pkgScope = p.topScope

        // for the same reason, set up a label scope
        p.openLabelScope()
}

func (p *parser) errors() error {
        m := scanner.Sorted
        if p.mode&SpuriousErrors == 0 {
                m = scanner.NoMultiples
        }
        return p.GetError(m)
}

// ----------------------------------------------------------------------------
// Scoping support

func (p *parser) openScope() {
        p.topScope = ast.NewScope(p.topScope)
}

func (p *parser) closeScope() {
        p.topScope = p.topScope.Outer
}

func (p *parser) openLabelScope() {
        p.labelScope = ast.NewScope(p.labelScope)
        p.targetStack = append(p.targetStack, nil)
}

func (p *parser) closeLabelScope() {
        // resolve labels
        n := len(p.targetStack) - 1
        scope := p.labelScope
        for _, ident := range p.targetStack[n] {
                ident.Obj = scope.Lookup(ident.Name)
                if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
                        p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
                }
        }
        // pop label scope
        p.targetStack = p.targetStack[0:n]
        p.labelScope = p.labelScope.Outer
}

func (p *parser) declare(decl, data interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
        for _, ident := range idents {
                assert(ident.Obj == nil, "identifier already declared or resolved")
                obj := ast.NewObj(kind, ident.Name)
                // remember the corresponding declaration for redeclaration
                // errors and global variable resolution/typechecking phase
                obj.Decl = decl
                obj.Data = data
                ident.Obj = obj
                if ident.Name != "_" {
                        if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
                                prevDecl := ""
                                if pos := alt.Pos(); pos.IsValid() {
                                        prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
                                }
                                p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
                        }
                }
        }
}

func (p *parser) shortVarDecl(decl *ast.AssignStmt, list []ast.Expr) {
        // Go spec: A short variable declaration may redeclare variables
        // provided they were originally declared in the same block with
        // the same type, and at least one of the non-blank variables is new.
        n := 0 // number of new variables
        for _, x := range list {
                if ident, isIdent := x.(*ast.Ident); isIdent {
                        assert(ident.Obj == nil, "identifier already declared or resolved")
                        obj := ast.NewObj(ast.Var, ident.Name)
                        // remember corresponding assignment for other tools
                        obj.Decl = decl
                        ident.Obj = obj
                        if ident.Name != "_" {
                                if alt := p.topScope.Insert(obj); alt != nil {
                                        ident.Obj = alt // redeclaration
                                } else {
                                        n++ // new declaration
                                }
                        }
                } else {
                        p.errorExpected(x.Pos(), "identifier")
                }
        }
        if n == 0 && p.mode&DeclarationErrors != 0 {
                p.error(list[0].Pos(), "no new variables on left side of :=")
        }
}

// The unresolved object is a sentinel to mark identifiers that have been added
// to the list of unresolved identifiers. The sentinel is only used for verifying
// internal consistency.
var unresolved = new(ast.Object)

func (p *parser) resolve(x ast.Expr) {
        // nothing to do if x is not an identifier or the blank identifier
        ident, _ := x.(*ast.Ident)
        if ident == nil {
                return
        }
        assert(ident.Obj == nil, "identifier already declared or resolved")
        if ident.Name == "_" {
                return
        }
        // try to resolve the identifier
        for s := p.topScope; s != nil; s = s.Outer {
                if obj := s.Lookup(ident.Name); obj != nil {
                        ident.Obj = obj
                        return
                }
        }
        // all local scopes are known, so any unresolved identifier
        // must be found either in the file scope, package scope
        // (perhaps in another file), or universe scope --- collect
        // them so that they can be resolved later
        ident.Obj = unresolved
        p.unresolved = append(p.unresolved, ident)
}

// ----------------------------------------------------------------------------
// Parsing support

func (p *parser) printTrace(a ...interface{}) {
        const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " +
                ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
        const n = uint(len(dots))
        pos := p.file.Position(p.pos)
        fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
        i := 2 * p.indent
        for ; i > n; i -= n {
                fmt.Print(dots)
        }
        fmt.Print(dots[0:i])
        fmt.Println(a...)
}

func trace(p *parser, msg string) *parser {
        p.printTrace(msg, "(")
        p.indent++
        return p
}

// Usage pattern: defer un(trace(p, "..."));
func un(p *parser) {
        p.indent--
        p.printTrace(")")
}

// Advance to the next token.
func (p *parser) next0() {
        // Because of one-token look-ahead, print the previous token
        // when tracing as it provides a more readable output. The
        // very first token (!p.pos.IsValid()) is not initialized
        // (it is token.ILLEGAL), so don't print it .
        if p.trace && p.pos.IsValid() {
                s := p.tok.String()
                switch {
                case p.tok.IsLiteral():
                        p.printTrace(s, p.lit)
                case p.tok.IsOperator(), p.tok.IsKeyword():
                        p.printTrace("\"" + s + "\"")
                default:
                        p.printTrace(s)
                }
        }

        p.pos, p.tok, p.lit = p.scanner.Scan()
}

// Consume a comment and return it and the line on which it ends.
func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
        // /*-style comments may end on a different line than where they start.
        // Scan the comment for '\n' chars and adjust endline accordingly.
        endline = p.file.Line(p.pos)
        if p.lit[1] == '*' {
                // don't use range here - no need to decode Unicode code points
                for i := 0; i < len(p.lit); i++ {
                        if p.lit[i] == '\n' {
                                endline++
                        }
                }
        }

        comment = &ast.Comment{p.pos, p.lit}
        p.next0()

        return
}

// Consume a group of adjacent comments, add it to the parser's
// comments list, and return it together with the line at which
// the last comment in the group ends. An empty line or non-comment
// token terminates a comment group.
//
func (p *parser) consumeCommentGroup() (comments *ast.CommentGroup, endline int) {
        var list []*ast.Comment
        endline = p.file.Line(p.pos)
        for p.tok == token.COMMENT && endline+1 >= p.file.Line(p.pos) {
                var comment *ast.Comment
                comment, endline = p.consumeComment()
                list = append(list, comment)
        }

        // add comment group to the comments list
        comments = &ast.CommentGroup{list}
        p.comments = append(p.comments, comments)

        return
}

// Advance to the next non-comment token. In the process, collect
// any comment groups encountered, and remember the last lead and
// and line comments.
//
// A lead comment is a comment group that starts and ends in a
// line without any other tokens and that is followed by a non-comment
// token on the line immediately after the comment group.
//
// A line comment is a comment group that follows a non-comment
// token on the same line, and that has no tokens after it on the line
// where it ends.
//
// Lead and line comments may be considered documentation that is
// stored in the AST.
//
func (p *parser) next() {
        p.leadComment = nil
        p.lineComment = nil
        line := p.file.Line(p.pos) // current line
        p.next0()

        if p.tok == token.COMMENT {
                var comment *ast.CommentGroup
                var endline int

                if p.file.Line(p.pos) == line {
                        // The comment is on same line as the previous token; it
                        // cannot be a lead comment but may be a line comment.
                        comment, endline = p.consumeCommentGroup()
                        if p.file.Line(p.pos) != endline {
                                // The next token is on a different line, thus
                                // the last comment group is a line comment.
                                p.lineComment = comment
                        }
                }

                // consume successor comments, if any
                endline = -1
                for p.tok == token.COMMENT {
                        comment, endline = p.consumeCommentGroup()
                }

                if endline+1 == p.file.Line(p.pos) {
                        // The next token is following on the line immediately after the
                        // comment group, thus the last comment group is a lead comment.
                        p.leadComment = comment
                }
        }
}

func (p *parser) error(pos token.Pos, msg string) {
        p.Error(p.file.Position(pos), msg)
}

func (p *parser) errorExpected(pos token.Pos, msg string) {
        msg = "expected " + msg
        if pos == p.pos {
                // the error happened at the current position;
                // make the error message more specific
                if p.tok == token.SEMICOLON && p.lit[0] == '\n' {
                        msg += ", found newline"
                } else {
                        msg += ", found '" + p.tok.String() + "'"
                        if p.tok.IsLiteral() {
                                msg += " " + p.lit
                        }
                }
        }
        p.error(pos, msg)
}

func (p *parser) expect(tok token.Token) token.Pos {
        pos := p.pos
        if p.tok != tok {
                p.errorExpected(pos, "'"+tok.String()+"'")
        }
        p.next() // make progress
        return pos
}

func (p *parser) expectSemi() {
        if p.tok != token.RPAREN && p.tok != token.RBRACE {
                p.expect(token.SEMICOLON)
        }
}

func assert(cond bool, msg string) {
        if !cond {
                panic("go/parser internal error: " + msg)
        }
}

// ----------------------------------------------------------------------------
// Identifiers

func (p *parser) parseIdent() *ast.Ident {
        pos := p.pos
        name := "_"
        if p.tok == token.IDENT {
                name = p.lit
                p.next()
        } else {
                p.expect(token.IDENT) // use expect() error handling
        }
        return &ast.Ident{pos, name, nil}
}

func (p *parser) parseIdentList() (list []*ast.Ident) {
        if p.trace {
                defer un(trace(p, "IdentList"))
        }

        list = append(list, p.parseIdent())
        for p.tok == token.COMMA {
                p.next()
                list = append(list, p.parseIdent())
        }

        return
}

// ----------------------------------------------------------------------------
// Common productions

// If lhs is set, result list elements which are identifiers are not resolved.
func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
        if p.trace {
                defer un(trace(p, "ExpressionList"))
        }

        list = append(list, p.checkExpr(p.parseExpr(lhs)))
        for p.tok == token.COMMA {
                p.next()
                list = append(list, p.checkExpr(p.parseExpr(lhs)))
        }

        return
}

func (p *parser) parseLhsList() []ast.Expr {
        list := p.parseExprList(true)
        switch p.tok {
        case token.DEFINE:
                // lhs of a short variable declaration
                // but doesn't enter scope until later:
                // caller must call p.shortVarDecl(p.makeIdentList(list))
                // at appropriate time.
        case token.COLON:
                // lhs of a label declaration or a communication clause of a select
                // statement (parseLhsList is not called when parsing the case clause
                // of a switch statement):
                // - labels are declared by the caller of parseLhsList
                // - for communication clauses, if there is a stand-alone identifier
                //   followed by a colon, we have a syntax error; there is no need
                //   to resolve the identifier in that case
        default:
                // identifiers must be declared elsewhere
                for _, x := range list {
                        p.resolve(x)
                }
        }
        return list
}

func (p *parser) parseRhsList() []ast.Expr {
        return p.parseExprList(false)
}

// ----------------------------------------------------------------------------
// Types

func (p *parser) parseType() ast.Expr {
        if p.trace {
                defer un(trace(p, "Type"))
        }

        typ := p.tryType()

        if typ == nil {
                pos := p.pos
                p.errorExpected(pos, "type")
                p.next() // make progress
                return &ast.BadExpr{pos, p.pos}
        }

        return typ
}

// If the result is an identifier, it is not resolved.
func (p *parser) parseTypeName() ast.Expr {
        if p.trace {
                defer un(trace(p, "TypeName"))
        }

        ident := p.parseIdent()
        // don't resolve ident yet - it may be a parameter or field name

        if p.tok == token.PERIOD {
                // ident is a package name
                p.next()
                p.resolve(ident)
                sel := p.parseIdent()
                return &ast.SelectorExpr{ident, sel}
        }

        return ident
}

func (p *parser) parseArrayType(ellipsisOk bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "ArrayType"))
        }

        lbrack := p.expect(token.LBRACK)
        var len ast.Expr
        if ellipsisOk && p.tok == token.ELLIPSIS {
                len = &ast.Ellipsis{p.pos, nil}
                p.next()
        } else if p.tok != token.RBRACK {
                len = p.parseRhs()
        }
        p.expect(token.RBRACK)
        elt := p.parseType()

        return &ast.ArrayType{lbrack, len, elt}
}

func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
        idents := make([]*ast.Ident, len(list))
        for i, x := range list {
                ident, isIdent := x.(*ast.Ident)
                if !isIdent {
                        pos := x.Pos()
                        p.errorExpected(pos, "identifier")
                        ident = &ast.Ident{pos, "_", nil}
                }
                idents[i] = ident
        }
        return idents
}

func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
        if p.trace {
                defer un(trace(p, "FieldDecl"))
        }

        doc := p.leadComment

        // fields
        list, typ := p.parseVarList(false)

        // optional tag
        var tag *ast.BasicLit
        if p.tok == token.STRING {
                tag = &ast.BasicLit{p.pos, p.tok, p.lit}
                p.next()
        }

        // analyze case
        var idents []*ast.Ident
        if typ != nil {
                // IdentifierList Type
                idents = p.makeIdentList(list)
        } else {
                // ["*"] TypeName (AnonymousField)
                typ = list[0] // we always have at least one element
                p.resolve(typ)
                if n := len(list); n > 1 || !isTypeName(deref(typ)) {
                        pos := typ.Pos()
                        p.errorExpected(pos, "anonymous field")
                        typ = &ast.BadExpr{pos, list[n-1].End()}
                }
        }

        p.expectSemi() // call before accessing p.linecomment

        field := &ast.Field{doc, idents, typ, tag, p.lineComment}
        p.declare(field, nil, scope, ast.Var, idents...)

        return field
}

func (p *parser) parseStructType() *ast.StructType {
        if p.trace {
                defer un(trace(p, "StructType"))
        }

        pos := p.expect(token.STRUCT)
        lbrace := p.expect(token.LBRACE)
        scope := ast.NewScope(nil) // struct scope
        var list []*ast.Field
        for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
                // a field declaration cannot start with a '(' but we accept
                // it here for more robust parsing and better error messages
                // (parseFieldDecl will check and complain if necessary)
                list = append(list, p.parseFieldDecl(scope))
        }
        rbrace := p.expect(token.RBRACE)

        return &ast.StructType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
}

func (p *parser) parsePointerType() *ast.StarExpr {
        if p.trace {
                defer un(trace(p, "PointerType"))
        }

        star := p.expect(token.MUL)
        base := p.parseType()

        return &ast.StarExpr{star, base}
}

func (p *parser) tryVarType(isParam bool) ast.Expr {
        if isParam && p.tok == token.ELLIPSIS {
                pos := p.pos
                p.next()
                typ := p.tryIdentOrType(isParam) // don't use parseType so we can provide better error message
                if typ == nil {
                        p.error(pos, "'...' parameter is missing type")
                        typ = &ast.BadExpr{pos, p.pos}
                }
                return &ast.Ellipsis{pos, typ}
        }
        return p.tryIdentOrType(false)
}

func (p *parser) parseVarType(isParam bool) ast.Expr {
        typ := p.tryVarType(isParam)
        if typ == nil {
                pos := p.pos
                p.errorExpected(pos, "type")
                p.next() // make progress
                typ = &ast.BadExpr{pos, p.pos}
        }
        return typ
}

func (p *parser) parseVarList(isParam bool) (list []ast.Expr, typ ast.Expr) {
        if p.trace {
                defer un(trace(p, "VarList"))
        }

        // a list of identifiers looks like a list of type names
        //
        // parse/tryVarType accepts any type (including parenthesized
        // ones) even though the syntax does not permit them here: we
        // accept them all for more robust parsing and complain later
        for typ := p.parseVarType(isParam); typ != nil; {
                list = append(list, typ)
                if p.tok != token.COMMA {
                        break
                }
                p.next()
                typ = p.tryVarType(isParam) // maybe nil as in: func f(int,) {}
        }

        // if we had a list of identifiers, it must be followed by a type
        if typ = p.tryVarType(isParam); typ != nil {
                p.resolve(typ)
        }

        return
}

func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
        if p.trace {
                defer un(trace(p, "ParameterList"))
        }

        list, typ := p.parseVarList(ellipsisOk)
        if typ != nil {
                // IdentifierList Type
                idents := p.makeIdentList(list)
                field := &ast.Field{nil, idents, typ, nil, nil}
                params = append(params, field)
                // Go spec: The scope of an identifier denoting a function
                // parameter or result variable is the function body.
                p.declare(field, nil, scope, ast.Var, idents...)
                if p.tok == token.COMMA {
                        p.next()
                }

                for p.tok != token.RPAREN && p.tok != token.EOF {
                        idents := p.parseIdentList()
                        typ := p.parseVarType(ellipsisOk)
                        field := &ast.Field{nil, idents, typ, nil, nil}
                        params = append(params, field)
                        // Go spec: The scope of an identifier denoting a function
                        // parameter or result variable is the function body.
                        p.declare(field, nil, scope, ast.Var, idents...)
                        if p.tok != token.COMMA {
                                break
                        }
                        p.next()
                }

        } else {
                // Type { "," Type } (anonymous parameters)
                params = make([]*ast.Field, len(list))
                for i, x := range list {
                        p.resolve(x)
                        params[i] = &ast.Field{Type: x}
                }
        }

        return
}

func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
        if p.trace {
                defer un(trace(p, "Parameters"))
        }

        var params []*ast.Field
        lparen := p.expect(token.LPAREN)
        if p.tok != token.RPAREN {
                params = p.parseParameterList(scope, ellipsisOk)
        }
        rparen := p.expect(token.RPAREN)

        return &ast.FieldList{lparen, params, rparen}
}

func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
        if p.trace {
                defer un(trace(p, "Result"))
        }

        if p.tok == token.LPAREN {
                return p.parseParameters(scope, false)
        }

        typ := p.tryType()
        if typ != nil {
                list := make([]*ast.Field, 1)
                list[0] = &ast.Field{Type: typ}
                return &ast.FieldList{List: list}
        }

        return nil
}

func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
        if p.trace {
                defer un(trace(p, "Signature"))
        }

        params = p.parseParameters(scope, true)
        results = p.parseResult(scope)

        return
}

func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
        if p.trace {
                defer un(trace(p, "FuncType"))
        }

        pos := p.expect(token.FUNC)
        scope := ast.NewScope(p.topScope) // function scope
        params, results := p.parseSignature(scope)

        return &ast.FuncType{pos, params, results}, scope
}

func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
        if p.trace {
                defer un(trace(p, "MethodSpec"))
        }

        doc := p.leadComment
        var idents []*ast.Ident
        var typ ast.Expr
        x := p.parseTypeName()
        if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
                // method
                idents = []*ast.Ident{ident}
                scope := ast.NewScope(nil) // method scope
                params, results := p.parseSignature(scope)
                typ = &ast.FuncType{token.NoPos, params, results}
        } else {
                // embedded interface
                typ = x
                p.resolve(typ)
        }
        p.expectSemi() // call before accessing p.linecomment

        spec := &ast.Field{doc, idents, typ, nil, p.lineComment}
        p.declare(spec, nil, scope, ast.Fun, idents...)

        return spec
}

func (p *parser) parseInterfaceType() *ast.InterfaceType {
        if p.trace {
                defer un(trace(p, "InterfaceType"))
        }

        pos := p.expect(token.INTERFACE)
        lbrace := p.expect(token.LBRACE)
        scope := ast.NewScope(nil) // interface scope
        var list []*ast.Field
        for p.tok == token.IDENT {
                list = append(list, p.parseMethodSpec(scope))
        }
        rbrace := p.expect(token.RBRACE)

        return &ast.InterfaceType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
}

func (p *parser) parseMapType() *ast.MapType {
        if p.trace {
                defer un(trace(p, "MapType"))
        }

        pos := p.expect(token.MAP)
        p.expect(token.LBRACK)
        key := p.parseType()
        p.expect(token.RBRACK)
        value := p.parseType()

        return &ast.MapType{pos, key, value}
}

func (p *parser) parseChanType() *ast.ChanType {
        if p.trace {
                defer un(trace(p, "ChanType"))
        }

        pos := p.pos
        dir := ast.SEND | ast.RECV
        if p.tok == token.CHAN {
                p.next()
                if p.tok == token.ARROW {
                        p.next()
                        dir = ast.SEND
                }
        } else {
                p.expect(token.ARROW)
                p.expect(token.CHAN)
                dir = ast.RECV
        }
        value := p.parseType()

        return &ast.ChanType{pos, dir, value}
}

// If the result is an identifier, it is not resolved.
func (p *parser) tryIdentOrType(ellipsisOk bool) ast.Expr {
        switch p.tok {
        case token.IDENT:
                return p.parseTypeName()
        case token.LBRACK:
                return p.parseArrayType(ellipsisOk)
        case token.STRUCT:
                return p.parseStructType()
        case token.MUL:
                return p.parsePointerType()
        case token.FUNC:
                typ, _ := p.parseFuncType()
                return typ
        case token.INTERFACE:
                return p.parseInterfaceType()
        case token.MAP:
                return p.parseMapType()
        case token.CHAN, token.ARROW:
                return p.parseChanType()
        case token.LPAREN:
                lparen := p.pos
                p.next()
                typ := p.parseType()
                rparen := p.expect(token.RPAREN)
                return &ast.ParenExpr{lparen, typ, rparen}
        }

        // no type found
        return nil
}

func (p *parser) tryType() ast.Expr {
        typ := p.tryIdentOrType(false)
        if typ != nil {
                p.resolve(typ)
        }
        return typ
}

// ----------------------------------------------------------------------------
// Blocks

func (p *parser) parseStmtList() (list []ast.Stmt) {
        if p.trace {
                defer un(trace(p, "StatementList"))
        }

        for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
                list = append(list, p.parseStmt())
        }

        return
}

func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
        if p.trace {
                defer un(trace(p, "Body"))
        }

        lbrace := p.expect(token.LBRACE)
        p.topScope = scope // open function scope
        p.openLabelScope()
        list := p.parseStmtList()
        p.closeLabelScope()
        p.closeScope()
        rbrace := p.expect(token.RBRACE)

        return &ast.BlockStmt{lbrace, list, rbrace}
}

func (p *parser) parseBlockStmt() *ast.BlockStmt {
        if p.trace {
                defer un(trace(p, "BlockStmt"))
        }

        lbrace := p.expect(token.LBRACE)
        p.openScope()
        list := p.parseStmtList()
        p.closeScope()
        rbrace := p.expect(token.RBRACE)

        return &ast.BlockStmt{lbrace, list, rbrace}
}

// ----------------------------------------------------------------------------
// Expressions

func (p *parser) parseFuncTypeOrLit() ast.Expr {
        if p.trace {
                defer un(trace(p, "FuncTypeOrLit"))
        }

        typ, scope := p.parseFuncType()
        if p.tok != token.LBRACE {
                // function type only
                return typ
        }

        p.exprLev++
        body := p.parseBody(scope)
        p.exprLev--

        return &ast.FuncLit{typ, body}
}

// parseOperand may return an expression or a raw type (incl. array
// types of the form [...]T. Callers must verify the result.
// If lhs is set and the result is an identifier, it is not resolved.
//
func (p *parser) parseOperand(lhs bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "Operand"))
        }

        switch p.tok {
        case token.IDENT:
                x := p.parseIdent()
                if !lhs {
                        p.resolve(x)
                }
                return x

        case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
                x := &ast.BasicLit{p.pos, p.tok, p.lit}
                p.next()
                return x

        case token.LPAREN:
                lparen := p.pos
                p.next()
                p.exprLev++
                x := p.parseRhsOrType() // types may be parenthesized: (some type)
                p.exprLev--
                rparen := p.expect(token.RPAREN)
                return &ast.ParenExpr{lparen, x, rparen}

        case token.FUNC:
                return p.parseFuncTypeOrLit()

        default:
                if typ := p.tryIdentOrType(true); typ != nil {
                        // could be type for composite literal or conversion
                        _, isIdent := typ.(*ast.Ident)
                        assert(!isIdent, "type cannot be identifier")
                        return typ
                }
        }

        pos := p.pos
        p.errorExpected(pos, "operand")
        p.next() // make progress
        return &ast.BadExpr{pos, p.pos}
}

func (p *parser) parseSelector(x ast.Expr) ast.Expr {
        if p.trace {
                defer un(trace(p, "Selector"))
        }

        sel := p.parseIdent()

        return &ast.SelectorExpr{x, sel}
}

func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
        if p.trace {
                defer un(trace(p, "TypeAssertion"))
        }

        p.expect(token.LPAREN)
        var typ ast.Expr
        if p.tok == token.TYPE {
                // type switch: typ == nil
                p.next()
        } else {
                typ = p.parseType()
        }
        p.expect(token.RPAREN)

        return &ast.TypeAssertExpr{x, typ}
}

func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
        if p.trace {
                defer un(trace(p, "IndexOrSlice"))
        }

        lbrack := p.expect(token.LBRACK)
        p.exprLev++
        var low, high ast.Expr
        isSlice := false
        if p.tok != token.COLON {
                low = p.parseRhs()
        }
        if p.tok == token.COLON {
                isSlice = true
                p.next()
                if p.tok != token.RBRACK {
                        high = p.parseRhs()
                }
        }
        p.exprLev--
        rbrack := p.expect(token.RBRACK)

        if isSlice {
                return &ast.SliceExpr{x, lbrack, low, high, rbrack}
        }
        return &ast.IndexExpr{x, lbrack, low, rbrack}
}

func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
        if p.trace {
                defer un(trace(p, "CallOrConversion"))
        }

        lparen := p.expect(token.LPAREN)
        p.exprLev++
        var list []ast.Expr
        var ellipsis token.Pos
        for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
                list = append(list, p.parseRhsOrType()) // builtins may expect a type: make(some type, ...)
                if p.tok == token.ELLIPSIS {
                        ellipsis = p.pos
                        p.next()
                }
                if p.tok != token.COMMA {
                        break
                }
                p.next()
        }
        p.exprLev--
        rparen := p.expect(token.RPAREN)

        return &ast.CallExpr{fun, lparen, list, ellipsis, rparen}
}

func (p *parser) parseElement(keyOk bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "Element"))
        }

        if p.tok == token.LBRACE {
                return p.parseLiteralValue(nil)
        }

        x := p.checkExpr(p.parseExpr(keyOk)) // don't resolve if map key
        if keyOk {
                if p.tok == token.COLON {
                        colon := p.pos
                        p.next()
                        return &ast.KeyValueExpr{x, colon, p.parseElement(false)}
                }
                p.resolve(x) // not a map key
        }

        return x
}

func (p *parser) parseElementList() (list []ast.Expr) {
        if p.trace {
                defer un(trace(p, "ElementList"))
        }

        for p.tok != token.RBRACE && p.tok != token.EOF {
                list = append(list, p.parseElement(true))
                if p.tok != token.COMMA {
                        break
                }
                p.next()
        }

        return
}

func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
        if p.trace {
                defer un(trace(p, "LiteralValue"))
        }

        lbrace := p.expect(token.LBRACE)
        var elts []ast.Expr
        p.exprLev++
        if p.tok != token.RBRACE {
                elts = p.parseElementList()
        }
        p.exprLev--
        rbrace := p.expect(token.RBRACE)
        return &ast.CompositeLit{typ, lbrace, elts, rbrace}
}

// checkExpr checks that x is an expression (and not a type).
func (p *parser) checkExpr(x ast.Expr) ast.Expr {
        switch unparen(x).(type) {
        case *ast.BadExpr:
        case *ast.Ident:
        case *ast.BasicLit:
        case *ast.FuncLit:
        case *ast.CompositeLit:
        case *ast.ParenExpr:
                panic("unreachable")
        case *ast.SelectorExpr:
        case *ast.IndexExpr:
        case *ast.SliceExpr:
        case *ast.TypeAssertExpr:
                // If t.Type == nil we have a type assertion of the form
                // y.(type), which is only allowed in type switch expressions.
                // It's hard to exclude those but for the case where we are in
                // a type switch. Instead be lenient and test this in the type
                // checker.
        case *ast.CallExpr:
        case *ast.StarExpr:
        case *ast.UnaryExpr:
        case *ast.BinaryExpr:
        default:
                // all other nodes are not proper expressions
                p.errorExpected(x.Pos(), "expression")
                x = &ast.BadExpr{x.Pos(), x.End()}
        }
        return x
}

// isTypeName returns true iff x is a (qualified) TypeName.
func isTypeName(x ast.Expr) bool {
        switch t := x.(type) {
        case *ast.BadExpr:
        case *ast.Ident:
        case *ast.SelectorExpr:
                _, isIdent := t.X.(*ast.Ident)
                return isIdent
        default:
                return false // all other nodes are not type names
        }
        return true
}

// isLiteralType returns true iff x is a legal composite literal type.
func isLiteralType(x ast.Expr) bool {
        switch t := x.(type) {
        case *ast.BadExpr:
        case *ast.Ident:
        case *ast.SelectorExpr:
                _, isIdent := t.X.(*ast.Ident)
                return isIdent
        case *ast.ArrayType:
        case *ast.StructType:
        case *ast.MapType:
        default:
                return false // all other nodes are not legal composite literal types
        }
        return true
}

// If x is of the form *T, deref returns T, otherwise it returns x.
func deref(x ast.Expr) ast.Expr {
        if p, isPtr := x.(*ast.StarExpr); isPtr {
                x = p.X
        }
        return x
}

// If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
func unparen(x ast.Expr) ast.Expr {
        if p, isParen := x.(*ast.ParenExpr); isParen {
                x = unparen(p.X)
        }
        return x
}

// checkExprOrType checks that x is an expression or a type
// (and not a raw type such as [...]T).
//
func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
        switch t := unparen(x).(type) {
        case *ast.ParenExpr:
                panic("unreachable")
        case *ast.UnaryExpr:
        case *ast.ArrayType:
                if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
                        p.error(len.Pos(), "expected array length, found '...'")
                        x = &ast.BadExpr{x.Pos(), x.End()}
                }
        }

        // all other nodes are expressions or types
        return x
}

// If lhs is set and the result is an identifier, it is not resolved.
func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "PrimaryExpr"))
        }

        x := p.parseOperand(lhs)
L:
        for {
                switch p.tok {
                case token.PERIOD:
                        p.next()
                        if lhs {
                                p.resolve(x)
                        }
                        switch p.tok {
                        case token.IDENT:
                                x = p.parseSelector(p.checkExpr(x))
                        case token.LPAREN:
                                x = p.parseTypeAssertion(p.checkExpr(x))
                        default:
                                pos := p.pos
                                p.next() // make progress
                                p.errorExpected(pos, "selector or type assertion")
                                x = &ast.BadExpr{pos, p.pos}
                        }
                case token.LBRACK:
                        if lhs {
                                p.resolve(x)
                        }
                        x = p.parseIndexOrSlice(p.checkExpr(x))
                case token.LPAREN:
                        if lhs {
                                p.resolve(x)
                        }
                        x = p.parseCallOrConversion(p.checkExprOrType(x))
                case token.LBRACE:
                        if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
                                if lhs {
                                        p.resolve(x)
                                }
                                x = p.parseLiteralValue(x)
                        } else {
                                break L
                        }
                default:
                        break L
                }
                lhs = false // no need to try to resolve again
        }

        return x
}

// If lhs is set and the result is an identifier, it is not resolved.
func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "UnaryExpr"))
        }

        switch p.tok {
        case token.ADD, token.SUB, token.NOT, token.XOR, token.AND:
                pos, op := p.pos, p.tok
                p.next()
                x := p.parseUnaryExpr(false)
                return &ast.UnaryExpr{pos, op, p.checkExpr(x)}

        case token.ARROW:
                // channel type or receive expression
                pos := p.pos
                p.next()
                if p.tok == token.CHAN {
                        p.next()
                        value := p.parseType()
                        return &ast.ChanType{pos, ast.RECV, value}
                }

                x := p.parseUnaryExpr(false)
                return &ast.UnaryExpr{pos, token.ARROW, p.checkExpr(x)}

        case token.MUL:
                // pointer type or unary "*" expression
                pos := p.pos
                p.next()
                x := p.parseUnaryExpr(false)
                return &ast.StarExpr{pos, p.checkExprOrType(x)}
        }

        return p.parsePrimaryExpr(lhs)
}

// If lhs is set and the result is an identifier, it is not resolved.
func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
        if p.trace {
                defer un(trace(p, "BinaryExpr"))
        }

        x := p.parseUnaryExpr(lhs)
        for prec := p.tok.Precedence(); prec >= prec1; prec-- {
                for p.tok.Precedence() == prec {
                        pos, op := p.pos, p.tok
                        p.next()
                        if lhs {
                                p.resolve(x)
                                lhs = false
                        }
                        y := p.parseBinaryExpr(false, prec+1)
                        x = &ast.BinaryExpr{p.checkExpr(x), pos, op, p.checkExpr(y)}
                }
        }

        return x
}

// If lhs is set and the result is an identifier, it is not resolved.
// The result may be a type or even a raw type ([...]int). Callers must
// check the result (using checkExpr or checkExprOrType), depending on
// context.
func (p *parser) parseExpr(lhs bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "Expression"))
        }

        return p.parseBinaryExpr(lhs, token.LowestPrec+1)
}

func (p *parser) parseRhs() ast.Expr {
        return p.checkExpr(p.parseExpr(false))
}

func (p *parser) parseRhsOrType() ast.Expr {
        return p.checkExprOrType(p.parseExpr(false))
}

// ----------------------------------------------------------------------------
// Statements

// Parsing modes for parseSimpleStmt.
const (
        basic = iota
        labelOk
        rangeOk
)

// parseSimpleStmt returns true as 2nd result if it parsed the assignment
// of a range clause (with mode == rangeOk). The returned statement is an
// assignment with a right-hand side that is a single unary expression of
// the form "range x". No guarantees are given for the left-hand side.
func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
        if p.trace {
                defer un(trace(p, "SimpleStmt"))
        }

        x := p.parseLhsList()

        switch p.tok {
        case
                token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
                token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
                token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
                token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
                // assignment statement, possibly part of a range clause
                pos, tok := p.pos, p.tok
                p.next()
                var y []ast.Expr
                isRange := false
                if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
                        pos := p.pos
                        p.next()
                        y = []ast.Expr{&ast.UnaryExpr{pos, token.RANGE, p.parseRhs()}}
                        isRange = true
                } else {
                        y = p.parseRhsList()
                }
                as := &ast.AssignStmt{x, pos, tok, y}
                if tok == token.DEFINE {
                        p.shortVarDecl(as, x)
                }
                return as, isRange
        }

        if len(x) > 1 {
                p.errorExpected(x[0].Pos(), "1 expression")
                // continue with first expression
        }

        switch p.tok {
        case token.COLON:
                // labeled statement
                colon := p.pos
                p.next()
                if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
                        // Go spec: The scope of a label is the body of the function
                        // in which it is declared and excludes the body of any nested
                        // function.
                        stmt := &ast.LabeledStmt{label, colon, p.parseStmt()}
                        p.declare(stmt, nil, p.labelScope, ast.Lbl, label)
                        return stmt, false
                }
                // The label declaration typically starts at x[0].Pos(), but the label
                // declaration may be erroneous due to a token after that position (and
                // before the ':'). If SpuriousErrors is not set, the (only) error re-
                // ported for the line is the illegal label error instead of the token
                // before the ':' that caused the problem. Thus, use the (latest) colon
                // position for error reporting.
                p.error(colon, "illegal label declaration")
                return &ast.BadStmt{x[0].Pos(), colon + 1}, false

        case token.ARROW:
                // send statement
                arrow := p.pos
                p.next()
                y := p.parseRhs()
                return &ast.SendStmt{x[0], arrow, y}, false

        case token.INC, token.DEC:
                // increment or decrement
                s := &ast.IncDecStmt{x[0], p.pos, p.tok}
                p.next()
                return s, false
        }

        // expression
        return &ast.ExprStmt{x[0]}, false
}

func (p *parser) parseCallExpr() *ast.CallExpr {
        x := p.parseRhsOrType() // could be a conversion: (some type)(x)
        if call, isCall := x.(*ast.CallExpr); isCall {
                return call
        }
        p.errorExpected(x.Pos(), "function/method call")
        return nil
}

func (p *parser) parseGoStmt() ast.Stmt {
        if p.trace {
                defer un(trace(p, "GoStmt"))
        }

        pos := p.expect(token.GO)
        call := p.parseCallExpr()
        p.expectSemi()
        if call == nil {
                return &ast.BadStmt{pos, pos + 2} // len("go")
        }

        return &ast.GoStmt{pos, call}
}

func (p *parser) parseDeferStmt() ast.Stmt {
        if p.trace {
                defer un(trace(p, "DeferStmt"))
        }

        pos := p.expect(token.DEFER)
        call := p.parseCallExpr()
        p.expectSemi()
        if call == nil {
                return &ast.BadStmt{pos, pos + 5} // len("defer")
        }

        return &ast.DeferStmt{pos, call}
}

func (p *parser) parseReturnStmt() *ast.ReturnStmt {
        if p.trace {
                defer un(trace(p, "ReturnStmt"))
        }

        pos := p.pos
        p.expect(token.RETURN)
        var x []ast.Expr
        if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
                x = p.parseRhsList()
        }
        p.expectSemi()

        return &ast.ReturnStmt{pos, x}
}

func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
        if p.trace {
                defer un(trace(p, "BranchStmt"))
        }

        pos := p.expect(tok)
        var label *ast.Ident
        if tok != token.FALLTHROUGH && p.tok == token.IDENT {
                label = p.parseIdent()
                // add to list of unresolved targets
                n := len(p.targetStack) - 1
                p.targetStack[n] = append(p.targetStack[n], label)
        }
        p.expectSemi()

        return &ast.BranchStmt{pos, tok, label}
}

func (p *parser) makeExpr(s ast.Stmt) ast.Expr {
        if s == nil {
                return nil
        }
        if es, isExpr := s.(*ast.ExprStmt); isExpr {
                return p.checkExpr(es.X)
        }
        p.error(s.Pos(), "expected condition, found simple statement")
        return &ast.BadExpr{s.Pos(), s.End()}
}

func (p *parser) parseIfStmt() *ast.IfStmt {
        if p.trace {
                defer un(trace(p, "IfStmt"))
        }

        pos := p.expect(token.IF)
        p.openScope()
        defer p.closeScope()

        var s ast.Stmt
        var x ast.Expr
        {
                prevLev := p.exprLev
                p.exprLev = -1
                if p.tok == token.SEMICOLON {
                        p.next()
                        x = p.parseRhs()
                } else {
                        s, _ = p.parseSimpleStmt(basic)
                        if p.tok == token.SEMICOLON {
                                p.next()
                                x = p.parseRhs()
                        } else {
                                x = p.makeExpr(s)
                                s = nil
                        }
                }
                p.exprLev = prevLev
        }

        body := p.parseBlockStmt()
        var else_ ast.Stmt
        if p.tok == token.ELSE {
                p.next()
                else_ = p.parseStmt()
        } else {
                p.expectSemi()
        }

        return &ast.IfStmt{pos, s, x, body, else_}
}

func (p *parser) parseTypeList() (list []ast.Expr) {
        if p.trace {
                defer un(trace(p, "TypeList"))
        }

        list = append(list, p.parseType())
        for p.tok == token.COMMA {
                p.next()
                list = append(list, p.parseType())
        }

        return
}

func (p *parser) parseCaseClause(typeSwitch bool) *ast.CaseClause {
        if p.trace {
                defer un(trace(p, "CaseClause"))
        }

        pos := p.pos
        var list []ast.Expr
        if p.tok == token.CASE {
                p.next()
                if typeSwitch {
                        list = p.parseTypeList()
                } else {
                        list = p.parseRhsList()
                }
        } else {
                p.expect(token.DEFAULT)
        }

        colon := p.expect(token.COLON)
        p.openScope()
        body := p.parseStmtList()
        p.closeScope()

        return &ast.CaseClause{pos, list, colon, body}
}

func isTypeSwitchAssert(x ast.Expr) bool {
        a, ok := x.(*ast.TypeAssertExpr)
        return ok && a.Type == nil
}

func isTypeSwitchGuard(s ast.Stmt) bool {
        switch t := s.(type) {
        case *ast.ExprStmt:
                // x.(nil)
                return isTypeSwitchAssert(t.X)
        case *ast.AssignStmt:
                // v := x.(nil)
                return len(t.Lhs) == 1 && t.Tok == token.DEFINE && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0])
        }
        return false
}

func (p *parser) parseSwitchStmt() ast.Stmt {
        if p.trace {
                defer un(trace(p, "SwitchStmt"))
        }

        pos := p.expect(token.SWITCH)
        p.openScope()
        defer p.closeScope()

        var s1, s2 ast.Stmt
        if p.tok != token.LBRACE {
                prevLev := p.exprLev
                p.exprLev = -1
                if p.tok != token.SEMICOLON {
                        s2, _ = p.parseSimpleStmt(basic)
                }
                if p.tok == token.SEMICOLON {
                        p.next()
                        s1 = s2
                        s2 = nil
                        if p.tok != token.LBRACE {
                                // A TypeSwitchGuard may declare a variable in addition
                                // to the variable declared in the initial SimpleStmt.
                                // Introduce extra scope to avoid redeclaration errors:
                                //
                                //      switch t := 0; t := x.(T) { ... }
                                //
                                // (this code is not valid Go because the first t will
                                // cannot be accessed and thus is never used, the extra
                                // scope is needed for the correct error message).
                                //
                                // If we don't have a type switch, s2 must be an expression.
                                // Having the extra nested but empty scope won't affect it.
                                p.openScope()
                                defer p.closeScope()
                                s2, _ = p.parseSimpleStmt(basic)
                        }
                }
                p.exprLev = prevLev
        }

        typeSwitch := isTypeSwitchGuard(s2)
        lbrace := p.expect(token.LBRACE)
        var list []ast.Stmt
        for p.tok == token.CASE || p.tok == token.DEFAULT {
                list = append(list, p.parseCaseClause(typeSwitch))
        }
        rbrace := p.expect(token.RBRACE)
        p.expectSemi()
        body := &ast.BlockStmt{lbrace, list, rbrace}

        if typeSwitch {
                return &ast.TypeSwitchStmt{pos, s1, s2, body}
        }

        return &ast.SwitchStmt{pos, s1, p.makeExpr(s2), body}
}

func (p *parser) parseCommClause() *ast.CommClause {
        if p.trace {
                defer un(trace(p, "CommClause"))
        }

        p.openScope()
        pos := p.pos
        var comm ast.Stmt
        if p.tok == token.CASE {
                p.next()
                lhs := p.parseLhsList()
                if p.tok == token.ARROW {
                        // SendStmt
                        if len(lhs) > 1 {
                                p.errorExpected(lhs[0].Pos(), "1 expression")
                                // continue with first expression
                        }
                        arrow := p.pos
                        p.next()
                        rhs := p.parseRhs()
                        comm = &ast.SendStmt{lhs[0], arrow, rhs}
                } else {
                        // RecvStmt
                        if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
                                // RecvStmt with assignment
                                if len(lhs) > 2 {
                                        p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
                                        // continue with first two expressions
                                        lhs = lhs[0:2]
                                }
                                pos := p.pos
                                p.next()
                                rhs := p.parseRhs()
                                as := &ast.AssignStmt{lhs, pos, tok, []ast.Expr{rhs}}
                                if tok == token.DEFINE {
                                        p.shortVarDecl(as, lhs)
                                }
                                comm = as
                        } else {
                                // lhs must be single receive operation
                                if len(lhs) > 1 {
                                        p.errorExpected(lhs[0].Pos(), "1 expression")
                                        // continue with first expression
                                }
                                comm = &ast.ExprStmt{lhs[0]}
                        }
                }
        } else {
                p.expect(token.DEFAULT)
        }

        colon := p.expect(token.COLON)
        body := p.parseStmtList()
        p.closeScope()

        return &ast.CommClause{pos, comm, colon, body}
}

func (p *parser) parseSelectStmt() *ast.SelectStmt {
        if p.trace {
                defer un(trace(p, "SelectStmt"))
        }

        pos := p.expect(token.SELECT)
        lbrace := p.expect(token.LBRACE)
        var list []ast.Stmt
        for p.tok == token.CASE || p.tok == token.DEFAULT {
                list = append(list, p.parseCommClause())
        }
        rbrace := p.expect(token.RBRACE)
        p.expectSemi()
        body := &ast.BlockStmt{lbrace, list, rbrace}

        return &ast.SelectStmt{pos, body}
}

func (p *parser) parseForStmt() ast.Stmt {
        if p.trace {
                defer un(trace(p, "ForStmt"))
        }

        pos := p.expect(token.FOR)
        p.openScope()
        defer p.closeScope()

        var s1, s2, s3 ast.Stmt
        var isRange bool
        if p.tok != token.LBRACE {
                prevLev := p.exprLev
                p.exprLev = -1
                if p.tok != token.SEMICOLON {
                        s2, isRange = p.parseSimpleStmt(rangeOk)
                }
                if !isRange && p.tok == token.SEMICOLON {
                        p.next()
                        s1 = s2
                        s2 = nil
                        if p.tok != token.SEMICOLON {
                                s2, _ = p.parseSimpleStmt(basic)
                        }
                        p.expectSemi()
                        if p.tok != token.LBRACE {
                                s3, _ = p.parseSimpleStmt(basic)
                        }
                }
                p.exprLev = prevLev
        }

        body := p.parseBlockStmt()
        p.expectSemi()

        if isRange {
                as := s2.(*ast.AssignStmt)
                // check lhs
                var key, value ast.Expr
                switch len(as.Lhs) {
                case 2:
                        key, value = as.Lhs[0], as.Lhs[1]
                case 1:
                        key = as.Lhs[0]
                default:
                        p.errorExpected(as.Lhs[0].Pos(), "1 or 2 expressions")
                        return &ast.BadStmt{pos, body.End()}
                }
                // parseSimpleStmt returned a right-hand side that
                // is a single unary expression of the form "range x"
                x := as.Rhs[0].(*ast.UnaryExpr).X
                return &ast.RangeStmt{pos, key, value, as.TokPos, as.Tok, x, body}
        }

        // regular for statement
        return &ast.ForStmt{pos, s1, p.makeExpr(s2), s3, body}
}

func (p *parser) parseStmt() (s ast.Stmt) {
        if p.trace {
                defer un(trace(p, "Statement"))
        }

        switch p.tok {
        case token.CONST, token.TYPE, token.VAR:
                s = &ast.DeclStmt{p.parseDecl()}
        case
                // tokens that may start a top-level expression
                token.IDENT, token.INT, token.FLOAT, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operand
                token.LBRACK, token.STRUCT, // composite type
                token.MUL, token.AND, token.ARROW, token.ADD, token.SUB, token.XOR: // unary operators
                s, _ = p.parseSimpleStmt(labelOk)
                // because of the required look-ahead, labeled statements are
                // parsed by parseSimpleStmt - don't expect a semicolon after
                // them
                if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
                        p.expectSemi()
                }
        case token.GO:
                s = p.parseGoStmt()
        case token.DEFER:
                s = p.parseDeferStmt()
        case token.RETURN:
                s = p.parseReturnStmt()
        case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
                s = p.parseBranchStmt(p.tok)
        case token.LBRACE:
                s = p.parseBlockStmt()
                p.expectSemi()
        case token.IF:
                s = p.parseIfStmt()
        case token.SWITCH:
                s = p.parseSwitchStmt()
        case token.SELECT:
                s = p.parseSelectStmt()
        case token.FOR:
                s = p.parseForStmt()
        case token.SEMICOLON:
                s = &ast.EmptyStmt{p.pos}
                p.next()
        case token.RBRACE:
                // a semicolon may be omitted before a closing "}"
                s = &ast.EmptyStmt{p.pos}
        default:
                // no statement found
                pos := p.pos
                p.errorExpected(pos, "statement")
                p.next() // make progress
                s = &ast.BadStmt{pos, p.pos}
        }

        return
}

// ----------------------------------------------------------------------------
// Declarations

type parseSpecFunction func(p *parser, doc *ast.CommentGroup, iota int) ast.Spec

func parseImportSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
        if p.trace {
                defer un(trace(p, "ImportSpec"))
        }

        var ident *ast.Ident
        switch p.tok {
        case token.PERIOD:
                ident = &ast.Ident{p.pos, ".", nil}
                p.next()
        case token.IDENT:
                ident = p.parseIdent()
        }

        var path *ast.BasicLit
        if p.tok == token.STRING {
                path = &ast.BasicLit{p.pos, p.tok, p.lit}
                p.next()
        } else {
                p.expect(token.STRING) // use expect() error handling
        }
        p.expectSemi() // call before accessing p.linecomment

        // collect imports
        spec := &ast.ImportSpec{doc, ident, path, p.lineComment, token.NoPos}
        p.imports = append(p.imports, spec)

        return spec
}

func parseConstSpec(p *parser, doc *ast.CommentGroup, iota int) ast.Spec {
        if p.trace {
                defer un(trace(p, "ConstSpec"))
        }

        idents := p.parseIdentList()
        typ := p.tryType()
        var values []ast.Expr
        if typ != nil || p.tok == token.ASSIGN || iota == 0 {
                p.expect(token.ASSIGN)
                values = p.parseRhsList()
        }
        p.expectSemi() // call before accessing p.linecomment

        // Go spec: The scope of a constant or variable identifier declared inside
        // a function begins at the end of the ConstSpec or VarSpec and ends at
        // the end of the innermost containing block.
        // (Global identifiers are resolved in a separate phase after parsing.)
        spec := &ast.ValueSpec{doc, idents, typ, values, p.lineComment}
        p.declare(spec, iota, p.topScope, ast.Con, idents...)

        return spec
}

func parseTypeSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
        if p.trace {
                defer un(trace(p, "TypeSpec"))
        }

        ident := p.parseIdent()

        // Go spec: The scope of a type identifier declared inside a function begins
        // at the identifier in the TypeSpec and ends at the end of the innermost
        // containing block.
        // (Global identifiers are resolved in a separate phase after parsing.)
        spec := &ast.TypeSpec{doc, ident, nil, nil}
        p.declare(spec, nil, p.topScope, ast.Typ, ident)

        spec.Type = p.parseType()
        p.expectSemi() // call before accessing p.linecomment
        spec.Comment = p.lineComment

        return spec
}

func parseVarSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
        if p.trace {
                defer un(trace(p, "VarSpec"))
        }

        idents := p.parseIdentList()
        typ := p.tryType()
        var values []ast.Expr
        if typ == nil || p.tok == token.ASSIGN {
                p.expect(token.ASSIGN)
                values = p.parseRhsList()
        }
        p.expectSemi() // call before accessing p.linecomment

        // Go spec: The scope of a constant or variable identifier declared inside
        // a function begins at the end of the ConstSpec or VarSpec and ends at
        // the end of the innermost containing block.
        // (Global identifiers are resolved in a separate phase after parsing.)
        spec := &ast.ValueSpec{doc, idents, typ, values, p.lineComment}
        p.declare(spec, nil, p.topScope, ast.Var, idents...)

        return spec
}

func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
        if p.trace {
                defer un(trace(p, "GenDecl("+keyword.String()+")"))
        }

        doc := p.leadComment
        pos := p.expect(keyword)
        var lparen, rparen token.Pos
        var list []ast.Spec
        if p.tok == token.LPAREN {
                lparen = p.pos
                p.next()
                for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
                        list = append(list, f(p, p.leadComment, iota))
                }
                rparen = p.expect(token.RPAREN)
                p.expectSemi()
        } else {
                list = append(list, f(p, nil, 0))
        }

        return &ast.GenDecl{doc, pos, keyword, lparen, list, rparen}
}

func (p *parser) parseReceiver(scope *ast.Scope) *ast.FieldList {
        if p.trace {
                defer un(trace(p, "Receiver"))
        }

        par := p.parseParameters(scope, false)

        // must have exactly one receiver
        if par.NumFields() != 1 {
                p.errorExpected(par.Opening, "exactly one receiver")
                par.List = []*ast.Field{{Type: &ast.BadExpr{par.Opening, par.Closing + 1}}}
                return par
        }

        // recv type must be of the form ["*"] identifier
        recv := par.List[0]
        base := deref(recv.Type)
        if _, isIdent := base.(*ast.Ident); !isIdent {
                p.errorExpected(base.Pos(), "(unqualified) identifier")
                par.List = []*ast.Field{{Type: &ast.BadExpr{recv.Pos(), recv.End()}}}
        }

        return par
}

func (p *parser) parseFuncDecl() *ast.FuncDecl {
        if p.trace {
                defer un(trace(p, "FunctionDecl"))
        }

        doc := p.leadComment
        pos := p.expect(token.FUNC)
        scope := ast.NewScope(p.topScope) // function scope

        var recv *ast.FieldList
        if p.tok == token.LPAREN {
                recv = p.parseReceiver(scope)
        }

        ident := p.parseIdent()

        params, results := p.parseSignature(scope)

        var body *ast.BlockStmt
        if p.tok == token.LBRACE {
                body = p.parseBody(scope)
        }
        p.expectSemi()

        decl := &ast.FuncDecl{doc, recv, ident, &ast.FuncType{pos, params, results}, body}
        if recv == nil {
                // Go spec: The scope of an identifier denoting a constant, type,
                // variable, or function (but not method) declared at top level
                // (outside any function) is the package block.
                //
                // init() functions cannot be referred to and there may
                // be more than one - don't put them in the pkgScope
                if ident.Name != "init" {
                        p.declare(decl, nil, p.pkgScope, ast.Fun, ident)
                }
        }

        return decl
}

func (p *parser) parseDecl() ast.Decl {
        if p.trace {
                defer un(trace(p, "Declaration"))
        }

        var f parseSpecFunction
        switch p.tok {
        case token.CONST:
                f = parseConstSpec

        case token.TYPE:
                f = parseTypeSpec

        case token.VAR:
                f = parseVarSpec

        case token.FUNC:
                return p.parseFuncDecl()

        default:
                pos := p.pos
                p.errorExpected(pos, "declaration")
                p.next() // make progress
                decl := &ast.BadDecl{pos, p.pos}
                return decl
        }

        return p.parseGenDecl(p.tok, f)
}

// ----------------------------------------------------------------------------
// Source files

func (p *parser) parseFile() *ast.File {
        if p.trace {
                defer un(trace(p, "File"))
        }

        // package clause
        doc := p.leadComment
        pos := p.expect(token.PACKAGE)
        // Go spec: The package clause is not a declaration;
        // the package name does not appear in any scope.
        ident := p.parseIdent()
        if ident.Name == "_" {
                p.error(p.pos, "invalid package name _")
        }
        p.expectSemi()

        var decls []ast.Decl

        // Don't bother parsing the rest if we had errors already.
        // Likely not a Go source file at all.

        if p.ErrorCount() == 0 && p.mode&PackageClauseOnly == 0 {
                // import decls
                for p.tok == token.IMPORT {
                        decls = append(decls, p.parseGenDecl(token.IMPORT, parseImportSpec))
                }

                if p.mode&ImportsOnly == 0 {
                        // rest of package body
                        for p.tok != token.EOF {
                                decls = append(decls, p.parseDecl())
                        }
                }
        }

        assert(p.topScope == p.pkgScope, "imbalanced scopes")

        // resolve global identifiers within the same file
        i := 0
        for _, ident := range p.unresolved {
                // i <= index for current ident
                assert(ident.Obj == unresolved, "object already resolved")
                ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
                if ident.Obj == nil {
                        p.unresolved[i] = ident
                        i++
                }
        }

        return &ast.File{doc, pos, ident, decls, p.pkgScope, p.imports, p.unresolved[0:i], p.comments}
}