Generic instance creation inference

[Steve Heyns]

Ran into this and I was wondering if it was a bug or not. Assuming "Foo" is an interface with a function of "Bar", then the following function should have a type reference of T and a pointer reference of P that implements the Foo interface, and is bound to the Type of T (https://gotipplay.golang.org/p/BKsA7-6MlHV). 

type  Foo interface {

Bar() string
}

func Update[T any, P interface {
Foo
*T
}]() {

Inside that function :
var t T; t.Bar() // gives

t.Bar undefined (type T has no field or method Bar)

var t T; i:= &t; i.Bar() // Gives

i.Bar undefined (type *T is pointer to type parameter, not type parameter)

But the following passes compile, and creates a new instance of type T, with "i" being the pointer to it.

var t T; var i P; i= &t; i.Bar() // Pass

IMO all three ways should work, but curious as to why they don't

Thanks

Nz

[Axel Wagner]

That's a well-known implementation restriction, mentioned in the release notes of Go 1.18:

The Go compiler only supports calling a method m on a value x of type parameter type P if m is explicitly declared by P's constraint interface. Similarly, method values x.m and method expressions P.m also are only supported if m is explicitly declared by P, even though m might be in the method set of P by virtue of the fact that all types in P implement m. We hope to remove this restriction in a future release.

So far, we haven't managed to remove this restriction. It's surprisingly complicated.

I tend to spell that call as `P(&t).Bar()` for what it's worth.

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[Steve Heyns]

Thanks for the reference document, and a compressed way of using it !

Cheers

Nz