cmd/go2go: compiler treats returning type with generics incorrectly.

[googollee]

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

$ go version
devel +ad307489d4 Tue Jun 16 05:49:35 2020 +0000.

Does this issue reproduce with the latest release?

No. It's a dev branch.

What operating system and processor architecture are you using (go env)?

I'm using https://go2goplay.golang.org/.

What did you do?

package main

type Result(type T) struct {
	t T
}

func F(type T)(v Result(T)) (Result(T)) {
	return v
}

What did you expect to see?

The code should be compiled

What did you see instead?

type checking failed for main
prog.go2:8:9: cannot use v (variable of type Result(T)) as T value in return statement

The return type of F should be Result(T), but the compiler thinks it's T, treating the definition as func F(type T)(v Result(T)) (Result T).

[andybons]

@griesemer @ianlancetaylor

[googollee]

It'd be easy to be fixed by func F(type T)(v Result(T)) Result(T). I think it needs to make it more clear what's (Result(T)) should be.

[griesemer]

This is working as expected. This code will work:

package main

type Result(type T) struct {
	t T
}

func F(type T)(v Result(T)) Result(T) {
	return v
}

Alternatively, you need to write (_ Result(T)) or ((Result(T))). The reason is that (Result(T)) is interpreted as (Result (T)), i.e., the result is named Result and is of parenthesized type (T), for backwards-compatibility.

This is a common mistake people are making, so we are considering changing this. This is also documented in the design draft.

[griesemer]

PS: https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#using-generic-types-as-unnamed-function-parameter-types

[googollee]

@griesemer Thanks for clarifying.

[agnivade]

@griesemer - Sorry to comment on a closed issue, but perhaps it might make sense to add a line in https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#using-generic-types-as-unnamed-function-parameter-types mentioning that we need to write Result(T) as (_ Result(T))/(Result(T))?

When I read that section, it seemed to suggest that the new proposal actually breaks backward compatibility, so there's no need to write code with extra parentheses.

[ianlancetaylor]

@agnivade I'm not sure quite what you are saying. It's OK to write func F() Result(T). The case that doesn't work at the moment is func F() (Result(T)). If you really want those parens then at the moment you need to write func F() (_ Result(T)) or func F() ((Result(T))).

[agnivade]

Ah yea sorry about that. I realize now that I went on a slight tangent. I just wanted some clarification about the syntax added to the doc. In my case, what I needed was (_ Result(T))/((Result(T))), as you can see here. But I didn't realize that and wrote Result(T). It took me some googling to figure this out because it wasn't very clear from the error message, which was this:

/tmp/go2go-run630056446/cache.go2:43:3: expected operand, found 'return' (and 1 more errors)

This is what I would like to be added in the doc. (Or is it already there somewhere and I have missed it?)