go/types, types2: inference fails where it should succeed when considering interface methods

[griesemer]

This code produces an error but we should be able to infer T in this case.

package p

type I[T any] interface {
	m(T)
}

func f5[T any](I[T]) {}

func _() {
	var x interface {
		m(int)
		n()
	}
	f5(x /* ERROR "type interface{m(int); n()} of x does not match inferred type I[int] for I[T]" */)
	f5[int](x) // ok
}

Not a release blocker as it doesn't work either in Go 1.20. But should fix if not too hard.

cc: @findleyr

[gopherbot]

Change https://go.dev/cl/500195 mentions this issue: go/types, types2: handle all type parameters before big unify switch