go/parser: don't require parens around composite literals inside a composite literal

	   within an if, for, or switch control clause

R=rsc
CC=golang-dev
https://golang.org/cl/943046

src/pkg/go/parser/parser.go

@@ -1105,9 +1105,11 @@ func (p *parser) parseCompositeLit(typ ast.Expr) ast.Expr {
 
 	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}
 }

src/pkg/go/parser/parser_test.go

@@ -38,6 +38,9 @@ var validPrograms = []interface{}{
 	`package main; func f(func() func() func())` + "\n",
 	`package main; func f(...)` + "\n",
 	`package main; func f(float, ...int)` + "\n",
+	`package main; type T []int; var a []bool; func f() { if a[T{42}[0]] {} }` + "\n",
+	`package main; type T []int; func g(int) bool { return true }; func f() { if g(T{42}[0]) {} }` + "\n",
+	`package main; type T []int; func f() { for _ = range []int{T{42}[0]} {} }` + "\n",
 }