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

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

Does this issue reproduce with the latest release?

Yes

What did you do?

I'm trying to make some sort of Nullable constraint which would indicate if that type can be set to nil or not.

Right now I'm only trying to detect pointers and slices.

One way to detect a pointer type (which works) is the following:

type Ptr[T any] interface {
	~*T
}

func SetNil[U any, T Ptr[U]](val T) {
	val = nil
}

func main() {
    a := 0
    SetNil(&a)
}

It can also be done with slice type:

type Slice[T any] interface {
	~[]T
}

func SetNil[U any, T Slice[U]](val T) {
	val = nil
}

func main() {
    SetNil([]int{})
}

However if I try to combine those into a Nullable constraint the type inference breaks for U.

https://go.dev/play/p/SbBJnOUeFVP?v=gotip

Providing the types U and T explicitly works tho.

What did you expect to see?

I'm expecting the compiler to properly infer Uand T types. As there are multiple options for T but only one of them is valid, it should be able to try to deduce U just like it does when using the Ptr[T] or Slice[T] constraint.

What did you see instead?

./prog.go:20:8: cannot infer U (prog.go:15:13)

Go build failed.

You should be able to reproduce this with the go.dev/play link provided above.