x/tools/gopls: 'implementations' doesn't work with generics #59224
Description
Even some of the most basic scenarios of 'implementations' are not working at all with generics, for example when the concrete and interface types are both generalized over [T any] and are defined within the same file:
xtools$ cat d/d.go
package d
type Collection[T any] interface {
Push(T) error
Pop() (*T, bool)
}
type C[T any] struct{}
func (C[T]) Push(t T) error { return nil }
func (C[T]) Pop() (*T, bool) {
var t T
return &t, true
}
var _ Collection[int] = C[int]{}
xtools$ go run ./gopls implementation ./d/d.go:#120 # within "Push" concrete method identifier
xtools$
This is not a recent regression:
xtools$ ./gopls-v0.11.0 implementation ./d/d.go:#120
xtools$
A similar lack of results is obtained by querying the Push abstract method, or the Collection interface, or the C concrete type.
(This test case was a further reduction of the one described in https://youtrack.jetbrains.com/issue/GO-12702/Go-to-Declaration-Implementation-not-available-for-generics-interfaces, referenced by golang/vscode-go#2711.)