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cmd/go2go: adding strings together is not defined #39623

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aykevl opened this issue Jun 16, 2020 · 5 comments
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cmd/go2go: adding strings together is not defined #39623

aykevl opened this issue Jun 16, 2020 · 5 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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Unreleased

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@aykevl
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aykevl commented Jun 16, 2020

I've been experimenting a bit with generics at https://go2goplay.golang.org/ and I think I found a bug.

What did you do?

See: https://go2goplay.golang.org/p/efS6x6s-9NI
I tried concatenating a string:

type IntString interface {
	type int, string
}

func Sum(type T IntString)(s []T) (result T) {
	for _, v := range s {
		result += v // this fails even though it's possible to add strings together
	}
	return
}

What did you expect to see?

Strings can be added (concatenated) together, so I would expect it to also work when using generics.
Specialized variant of the same function, which does compile:

func SumString(s []string) (result string) {
	for _, v := range s {
		result += v
	}
	return
}

What did you see instead?

It reports the following error:

type checking failed for main
prog.go2:13:3: invalid operation: operator + not defined for result (variable of type T)

Both integers and strings support the + operator so I would expect this to work.
@ChrisHines
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There seems to be some interaction with the int type in the constraint here. This works https://go2goplay.golang.org/p/nYbMg_uv31l:

package main

import "fmt"

func main() {
	fmt.Println(Add("Hello", ", world!"))
}

func Add(type T addable)(x, y T) T {
	return x + y
}

type addable interface {
	type string
}
Hello, world!

But this does not https://go2goplay.golang.org/p/Ksywhb4RF6x:

package main

import "fmt"

func main() {
	fmt.Println(Add("Hello", ", world!"))
}

func Add(type T addable)(x, y T) T {
	return x + y
}

type addable interface {
	type string, int
}
type checking failed for main
prog.go2:10:9: invalid operation: operator + not defined for x (variable of type T)

@griesemer griesemer self-assigned this Jun 17, 2020
@griesemer
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Yes, this is a bug in the type checker; thanks for reporting.

One of the more complicated things to get right is computing the subset of operations that are valid for all types of a type list. In this case it should have been easy, but obviously there's a bug somewhere.

@ianlancetaylor ianlancetaylor added the NeedsFix The path to resolution is known, but the work has not been done. label Jun 17, 2020
@ianlancetaylor ianlancetaylor added this to the Unreleased milestone Jun 17, 2020
@ianlancetaylor ianlancetaylor changed the title go2go: adding strings together is not defined cmd/go2go: adding strings together is not defined Jun 17, 2020
@gopherbot
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Change https://golang.org/cl/238359 mentions this issue: [dev.go2go] go/types: use correct predicate when checking binary addition

gopherbot pushed a commit that referenced this issue Jun 17, 2020
@griesemer
[dev.go2go] go/types: use correct predicate when checking binary addi…
0b89d8b 
…tion

See also https://team-review.git.corp.google.com/c/golang/go2-dev/+/759562
for the analogous problem for another predicate.

Fixes #39623.

Change-Id: I39efcb07c337691eb202de166e2cb8d6b5d5ee18
Reviewed-on: https://go-review.googlesource.com/c/go/+/238359
Reviewed-by: Robert Griesemer <gri@golang.org>
@griesemer
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This is now fixed in dev.go2go. Will be fixed in the playground as soon as it's updated.

@mattn mattn mentioned this issue Jun 17, 2020
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@martisch martisch mentioned this issue Jun 17, 2020
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@james-antill
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FWIW I can confirm that it works in playground now.

@golang golang locked and limited conversation to collaborators Jun 17, 2021
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@aykevl @ChrisHines @ianlancetaylor @james-antill @griesemer @gopherbot