cmd/compile: struct initialization in "if" doesn't compile

[menghanl]

What version of Go are you using (go version)?

$ go version
go version go1.14.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

https://play.golang.org/p/TACak6vfX1a

package main

import (
	"fmt"
)

type foo struct{}

func main() {
	u := foo{}

	if (u != foo{}) { // With () it works
		fmt.Println("OK")
	}
	
	if u != foo{} { // This doesn't compile
		fmt.Println("hello")
	}
}

What did you expect to see?

No compile error

What did you see instead?

if u != foo{} { does not compile:

./main.go:16:16: syntax error: unexpected { at end of statement
./main.go:19:1: syntax error: non-declaration statement outside function body

Adding parentheses works:

• if (u != foo{}) {
• if u != (foo{}) {

[ianlancetaylor]

This is expected and documented. Search for "ambiguity" in https://golang.org/ref/spec#Composite_literals.