go/parser: fix (pathological) corner case

Inside a control clause (if ... {}), composite
literals starting with a type name must be parenthesized.
A composite literal used in the array length expression
of an array composite literal is already parenthesized.
Not a valid program, but syntactically is should
be accepted.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/142760043

src/go/parser/parser.go

@@ -641,6 +641,7 @@ func (p *parser) parseArrayType() ast.Expr {
 	}
 
 	lbrack := p.expect(token.LBRACK)
+	p.exprLev++
 	var len ast.Expr
 	// always permit ellipsis for more fault-tolerant parsing
 	if p.tok == token.ELLIPSIS {
@@ -649,6 +650,7 @@ func (p *parser) parseArrayType() ast.Expr {
 	} else if p.tok != token.RBRACK {
 		len = p.parseRhs()
 	}
+	p.exprLev--
 	p.expect(token.RBRACK)
 	elt := p.parseType()
 

src/go/parser/short_test.go

@@ -39,6 +39,7 @@ var valids = []string{
 	`package p; func ((*T),) m() {}`,
 	`package p; func (*(T),) m() {}`,
 	`package p; func _(x []int) { for range x {} }`,
+	`package p; func _() { if [T{}.n]int{} {} }`,
 }
 
 func TestValid(t *testing.T) {