cmd/compile: inconsistent generics behaviour

[mmrath]

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

$ go version
go version devel go1.18-f93a63addb Mon Sep 13 20:17:35 2021 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Latest master

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

go env Output

$ go env

What did you do?

The below programs fails to build with error whereas a similar program with string replaced with int compiles fine. Not sure if this is a bug or expected.

./main4.go:9:16: cannot convert in (variable of type parameter T constrained by Constraint) to string

package main4

type Constraint interface {
    ~string
}

func C[T Constraint](in T) {
    _ = string(in)
}

What did you expect to see?

What did you see instead?

[randall77]

@griesemer @findleyr

[griesemer]

Looks like a bug to me. There's several issues around conversions that we need to address.

[toothrot]

Checking in on this issue as it's labeled a release blocker for Go 1.18. Is there any update?

[griesemer]

This will be worked on soon.

[OneOfOne]

Also another case:

func toStrings[T ~string|~[]byte](in []T) []string {
	out := make([]string, 0, len(in))
	for _, v := range in {
		out = append(out, string(v))
	}
	return out
}

[rogpeppe]

Another issue which may be closely related. This code should work but doesn't.
I get the same error as in the description for this issue:

./tst.go:28:17: cannot convert x (variable of type parameter T constrained by I1) to Struct1

https://go2goplay.golang.org/p/JyUKVBNOFfX

type Struct1 struct {
	A int
}

type Struct2 struct {
	A int `json:"a"`
}

type I1 interface {
	Struct1 | Struct2
}

func F[T I1](x T) Struct1 {
	return Struct1(x)
}