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cmd/compile: if expression having parenthesis produces compilation error #49996

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pkositsyn opened this issue Dec 6, 2021 · 1 comment
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@pkositsyn
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@pkositsyn pkositsyn commented Dec 6, 2021

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

$ go version go1.16.4 linux/amd64
and
$ go version go1.17.4 linux/amd64

What did you do?

https://go.dev/play/p/eIJEXrkKD8m

package main

import "log"

func main() {
	if T{}.Name() == "T" {
		log.Println("T")
	}
}

type T struct {}

func (t T) Name() string { return "T" }

I'd like to mention, that compiling with new(T) or extra parenthesis around T{}.Name() was successful. Seems to me that if T{ was parsed as beginning of the if scope.

At a first glance seems like two situations can be distinguished by having/not having a method call after parenthesis is closed

What did you expect to see?

Code compiled successfully

What did you see instead?

./prog.go:6:8: syntax error: unexpected . at end of statement
./prog.go:9:1: syntax error: non-declaration statement outside function body
@ALTree
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@ALTree ALTree commented Dec 6, 2021

A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the keyword and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening brace of the literal is erroneously parsed as the one introducing the block of statements. To resolve the ambiguity, the composite literal must appear within parentheses.

if x == (T{a,b,c}[i]) { … }
if (x == T{a,b,c}[i]) { … }

https://go.dev/ref/spec#Composite_literals

@ALTree ALTree closed this as completed Dec 6, 2021
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