cmd/compile: generics definition with primitive type leads to weird behavior

[vyskocilm]

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

$ go version
go version devel go1.18-7983830 Tue Oct 5 14:12:44 2021 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes, on latest master.

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

go env Output

$ go env

What did you do?

package main

type vector[T any] []T

func (iv *vector[int]) PushInt(value int) {
    panic("*(vector[int]).PushInt")
}

func main() {
    vs := vector[string]{}
    vs.PushInt("42")
}

What did you expect to see?

I have expected that a primitive type like int won't be allowed as a type parameter. If it's not a compiler error, then PushInt method shall not available for any other instance than vector[int]. I did not find it described in the design document https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md

What did you see instead?

It sounds that [int] is treated as [T any], so PushInt is called for vector[string] too.

panic: *(vector[int]).PushInt

goroutine 1 [running]:
main.(*vector[...]).PushInt(...)
        /tmp/main.go:7
main.main()
        /tmp/main.go:12 +0x27
exit status 2

[tdakkota]

See #47419 (comment).

[vyskocilm]

@tdakkota thanks! This is not obvious - hope that there will be some linter for it.

To summarize it

func foo[T]() { }  // illegal without constraint
func (s someStruct[T]) foo() {} // legal, because constraint is defined on a struct level