go/parser: more tolerant parsing of const and var decls

Instead, rely on the type checker.

R=adonovan
CC=golang-dev
https://golang.org/cl/10826044

src/pkg/go/parser/parser.go

@@ -2180,8 +2180,9 @@ func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota
 	idents := p.parseIdentList()
 	typ := p.tryType()
 	var values []ast.Expr
-	if p.tok == token.ASSIGN || keyword == token.CONST && (typ != nil || iota == 0) || keyword == token.VAR && typ == nil {
-		p.expect(token.ASSIGN)
+	// always permit optional initialization for more tolerant parsing
+	if p.tok == token.ASSIGN {
+		p.next()
 		values = p.parseRhsList()
 	}
 	p.expectSemi() // call before accessing p.linecomment

src/pkg/go/parser/short_test.go

@@ -47,7 +47,6 @@ var invalids = []string{
 	`package p; func f() { if { /* ERROR "expected operand" */ } };`,
 	`package p; func f() { if ; { /* ERROR "expected operand" */ } };`,
 	`package p; func f() { if f(); { /* ERROR "expected operand" */ } };`,
-	`package p; const c; /* ERROR "expected '='" */`,
 	`package p; func f() { if _ /* ERROR "expected condition" */ = range x; true {} };`,
 	`package p; func f() { switch _ /* ERROR "expected condition" */ = range x; true {} };`,
 	`package p; func f() { for _ = range x ; /* ERROR "expected '{'" */ ; {} };`,