gopls/internal/lsp/source: fix signatureHelp with pointer receivers

I just ran into this today. Alas, it took us too long to fix this.

Fixes golang/go#61189

Change-Id: Iac28a6ba30d099cb2bb6f9d761ee658e190aa07c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/539677
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/lsp/source/types_format.go

@@ -312,7 +312,11 @@ func FormatVarType(ctx context.Context, snapshot Snapshot, srcpkg Package, obj *
 			return types.TypeString(obj.Type(), qf), nil // in generic function
 		}
 		if decl.Recv != nil && len(decl.Recv.List) > 0 {
-			if x, _, _, _ := typeparams.UnpackIndexExpr(decl.Recv.List[0].Type); x != nil {
+			rtype := decl.Recv.List[0].Type
+			if e, ok := rtype.(*ast.StarExpr); ok {
+				rtype = e.X
+			}
+			if x, _, _, _ := typeparams.UnpackIndexExpr(rtype); x != nil {
 				return types.TypeString(obj.Type(), qf), nil // in method of generic type
 			}
 		}

gopls/internal/regtest/marker/testdata/signature/generic.txt

@@ -0,0 +1,21 @@
+This test checks signature help on generic signatures.
+
+-- g.go --
+package g
+
+type M[K comparable, V any] map[K]V
+
+// golang/go#61189: signatureHelp must handle pointer receivers. 
+func (m *M[K, V]) Get(k K) V {
+	return (*m)[k]
+}
+
+func Get[K comparable, V any](m M[K, V], k K) V {
+	return m[k]
+}
+
+func _() {
+	var m M[int, string]
+	_ = m.Get(0)  //@signature("(", "Get(k int) string", 0)
+	_ = Get(m, 0) //@signature("0", "Get(m M[int, string], k int) string", 1)
+}