go/ssa: use origin method in source lookup

Use the origin method when looking for an enclosing source function.
The *types.Func coming from MethodSets.MethodSet may not be in
ssa.Package.objects while the origin will be.

Fixes golang/go#59428

Change-Id: I0f16e60db69b9b79d1cd1f405134c7f0deda1eeb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/491155
Run-TryBot: Tim King <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
gopls-CI: kokoro <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: timothy-king

go/ssa/source.go

@@ -121,7 +121,9 @@ func findNamedFunc(pkg *Package, pos token.Pos) *Function {
 				// Don't call Program.Method: avoid creating wrappers.
 				obj := mset.At(i).Obj().(*types.Func)
 				if obj.Pos() == pos {
-					return pkg.objects[obj].(*Function)
+					// obj from MethodSet may not be the origin type.
+					m := typeparams.OriginMethod(obj)
+					return pkg.objects[m].(*Function)
 				}
 			}
 		}

go/ssa/source_test.go

@@ -24,6 +24,7 @@ import (
 	"golang.org/x/tools/go/loader"
 	"golang.org/x/tools/go/ssa"
 	"golang.org/x/tools/go/ssa/ssautil"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 func TestObjValueLookup(t *testing.T) {
@@ -384,6 +385,19 @@ func TestEnclosingFunction(t *testing.T) {
 		  func init() { println(func(){print(900)}) }`,
 			"900", "main.init#1$1"},
 	}
+	if typeparams.Enabled {
+		tests = append(tests, struct {
+			input  string
+			substr string
+			fn     string
+		}{
+			`package main
+			type S[T any] struct{}
+			func (*S[T]) Foo() { println(1000) }
+			type P[T any] struct{ *S[T] }`,
+			"1000", "(*main.S[T]).Foo",
+		})
+	}
 	for _, test := range tests {
 		conf := loader.Config{Fset: token.NewFileSet()}
 		f, start, end := findInterval(t, conf.Fset, test.input, test.substr)