x/tools/gopls: regression in implementationsV2 for concrete methods

[adonovan]

example$ head -99 $(find . -type f)
==> ./go.mod <==
module example.com

go 1.12

==> ./a/a.go <==
package a

type A int

func (A) F() {}

==> ./b/b.go <==
package b

import (
	"example.com/a"
	"example.com/c"
)

type B interface{ F() }

var _ B = a.A(0)
var _ B = c.C(0)

var _ = B.F

==> ./c/c.go <==
package c

type C int

func (C) F() {}

==> ./d/d.go <==
package d

import "example.com/b"

var _ interface{} = b.B.F

example$ ../xtools/gopls-v0.7.5  references   ./a/a.go:#33 
/Users/adonovan/w/example/b/b.go:13:11-12
/Users/adonovan/w/example/d/d.go:5:25-26
example$ ../xtools/gopls-v0.11.0 references   ./a/a.go:#33 
/Users/adonovan/w/example/b/b.go:13:11-12
/Users/adonovan/w/example/d/d.go:5:25-26
example$ ../xtools/gopls.head  references   ./a/a.go:#33 
/Users/adonovan/w/example/d/d.go:5:25-26

This example applies the "implementations" query to method A.F. The correct answer includes both references to B.F, one in package b and one in package d. However, the new ("v2") implementation of "implementations" fails to report the reference from package b.

The reason is that v2 has completely different algorithms for the local and global cases (with disjoint results), and when it discovers that type A satisfies interface B and thus that it must include B.F among the global search targets, it forgets to search package b for local references to B.F.

[gopherbot]

Change https://go.dev/cl/479315 mentions this issue: gopls/internal/lsp/source: fix references expansion bug