cmd/compile/internal/gc: better error message for parenthesized go/defer exprs

Change-Id: Ie24d56422ae2196198a6c306716fa867c1442d6e
Reviewed-on: https://go-review.googlesource.com/17043Reviewed-by: Chris Manghane <cmang@golang.org>

src/cmd/compile/internal/gc/parser.go

@@ -1391,13 +1391,18 @@ func (p *parser) pseudocall() *Node {
 		defer p.trace("pseudocall")()
 	}
 
-	// The expression in go/defer must not be parenthesized;
-	// don't drop ()'s so we can report an error.
-	x := p.pexpr(true /* keep_parens */)
-	if x.Op != OCALL {
-		Yyerror("argument to go/defer must be function call")
-	}
+	x := p.pexpr(true) // keep_parens so we can report error below
+	switch x.Op {
+	case OCALL:
 		return x
+	case OPAREN:
+		Yyerror("expression in go/defer must not be parenthesized")
+		// already progressed, no need to advance
+	default:
+		Yyerror("expression in go/defer must be function call")
+		// already progressed, no need to advance
+	}
+	return nil
 }
 
 // go.y:pexpr (partial)

test/fixedbugs/issue4468.go

@@ -19,8 +19,12 @@ type S struct {
 }
 
 func F() {
-	go (F())	// ERROR "must be function call"
-	defer (F())	// ERROR "must be function call"
+	go F            // ERROR "must be function call"
+	defer F         // ERROR "must be function call"
+	go (F)		// ERROR "must be function call|must not be parenthesized"
+	defer (F)	// ERROR "must be function call|must not be parenthesized"
+	go (F())	// ERROR "must be function call|must not be parenthesized"
+	defer (F())	// ERROR "must be function call|must not be parenthesized"
 	var s S
 	(&s.t).F()
 	go (&s.t).F()