cmd/compile: type inference fails on a case which looks simple

[zigo101]

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

$ go version
go version go1.21.3 linux/amd64

Does this issue reproduce with the latest release?

What did you do?

package main

func foo[C ~*T | ~[]T, T any](v C) {} // compiles

func main() {
	foo(new(int))       // cannot infer T
	foo(make([]int, 3)) // cannot infer T
}

What did you expect to see?

Compiles.

What did you see instead?

Fails to compile.

[mauri870]

/cc @golang/compiler

[griesemer]

Unfortunately we can't do any inference in this case because the constraint C doesn't have a core type.
Closing because this is a known limitation of the language, not an actual bug.
Eventually we need a concrete proposal to move forward.

[dr2chase]

This case would look simpler if it included a comment explaining what was thought to be happening.

[griesemer]

@dr2chase In both cases, T should be inferred to be int.
@go101 What we need to see to move forward with cases like these (type inference) is really good real-life examples where this matters. Functions operating on type parameters which have no core types can't do a whole lot with values of such type parameters.