cmd/compile: incorrect "cannot use .this" error when using _ for multiple type parameters in method declaration

[samherrmann]

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

$ go version
go version go1.18beta1 linux/amd64

Does this issue reproduce with the latest release?

N/A. Type parameters are not available in any other version that are currently available.

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

go env Output

$ go env
GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOVERSION="go1.18beta1"

What did you do?

package main

import (
    "fmt"
)

func main() {
    foo := &Foo[string, int]{
        valueA: "i am a string",
        valueB: 123,
    }
    fmt.Println(foo)
}

type Foo[T1 any, T2 any] struct {
    valueA T1
    valueB T2
}

func (f *Foo[_,_]) String() string {
    return fmt.Sprintf("%v %v", f.valueA, f.valueB)
}

• Playground
• See this StackOverflow post for more details.

What did you expect to see?

I expected the code snippet above to build and run successfully.

What did you see instead?

Build error:

<autogenerated>:1: cannot use .this (type *Foo[string,int]) as type *Foo[go.shape.string_0,go.shape.string_0] in argument to (*Foo[go.shape.string_0,go.shape.int_1]).String

[danscales]

Thanks for the issue report!

This is a problem in the main compiler (not the types2 typechecker). It happens when generating method wrappers. It looks like the use of [_, _] in func (f *Foo[_, _]) String() string { is causing the bug - we are somehow interpreting the blanks as the same type. Works fine if you just replace [_, _] with [T1, T2].

[gopherbot]

Change https://golang.org/cl/375474 mentions this issue: cmd/compile: make sure multiple blanks typeparams are still unique

[gopherbot]

Change https://golang.org/cl/375641 mentions this issue: go/internal/gcimporter: don't add blank type parameters to tparamIndex

[gopherbot]

Change https://golang.org/cl/379194 mentions this issue: cmd/compile: make sure multiple blanks typeparams are still unique