cmd/compile: improve error message when if statement brace is on next line

[zigo101]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.7.4 linux/amd64

What did you do?

package main

func main() {
	a := 1
	
	for a = 1; a < 2; a++ // syntax error: missing { after for clause
	{
	}
	
	if a == 1 // missing condition in if statement
	{
	}
}

What did you expect to see?

the two error messages should be alike.

What did you see instead?

the the second error message is not very accurate.

[mvdan]

Happens all the same on go version devel +4cce27a3fa Sat Jan 21 03:20:55 2017 +0000 linux/amd64 .

@golang101 perhaps retitle to something like cmd/compile: wrong error when the opening brace of an if is on a new line

[josharian]

@golang101 thanks for filing issues. Would you mind helping the folks doing triage by identifying the relevant component in the issue title, when you know it? Thanks!

cc @odeke-em

[mvdan]

@josharian did you mean that triage sentence towards me? I haven't requested github access, maybe I should.

[josharian]

It was meant for @golang101. But you should definitely request GitHub access.

[zigo101]

@josharian I would like to.

[gopherbot]

CL https://golang.org/cl/36317 mentions this issue.

[mdempsky]

Syntactically,

if foo
{
}

is the same as

if foo; {}

And the latter produces a "missing condition in if statement" error, which is why the former does too.

It's not obvious that the former deserves a different error message; or that if it does, what exactly should be the criteria for producing something different. I'd like to establish that before we worry about CLs.

[bradfitz]

@mdempsky, I bet only about 2% of Go users know that semicolons are even in the language at all, much less know the insertion rules.

I personally think it deserves a better error message. A fair number of newcomers to the language are going to write code in that style before they discover gofmt.

[odeke-em]

@mdempsky, yeah, I agree with @bradfitz's explanation,
I too think it deserves a better error message.

As we can see in the issue report, the user who reported it found the same problem.

if condition
{
}

wasn't intuitive since visually they already provided the condition a == 1

Getting the same message for

if expression; {
}

that tells me to think about the missing condition in the style

if expression; condition {

even though expression might be a condition.