x/tools/go/ssa: instantiate sel.Recv() on MethodVal.

When a MethodVal selection receiver is a TypeParam while monomorphizing, use the method value from the substituted type.

Fixes golang/go#52835

Change-Id: Iaf174a02b62b0c3ed4aafefdfd2aca195781e9f9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/405594
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>
Run-TryBot: Tim King <taking@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

go/ssa/builder.go

@@ -814,6 +814,13 @@ func (b *builder) expr0(fn *Function, e ast.Expr, tv types.TypeAndValue) Value {
 		case types.MethodVal:
 			// e.f where e is an expression and f is a method.
 			// The result is a "bound".
+
+			if base := fn.typ(sel.Recv()); base != sel.Recv() {
+				// instantiate sel as sel.Recv is not equal after substitution.
+				pkg := fn.declaredPackage().Pkg
+				// mv is the instantiated method value.
+				sel = types.NewMethodSet(base).Lookup(pkg, sel.Obj().Name())
+			}
 			obj := sel.Obj().(*types.Func)
 			rt := fn.typ(recvType(obj))
 			wantAddr := isPointer(rt)

go/ssa/interp/interp_test.go

@@ -129,6 +129,12 @@ var testdataTests = []string{
 	"fixedbugs/issue52342.go",
 }
 
+func init() {
+	if typeparams.Enabled {
+		testdataTests = append(testdataTests, "fixedbugs/issue52835.go")
+	}
+}
+
 // Specific GOARCH to use for a test case in go.tools/go/ssa/interp/testdata/.
 // Defaults to amd64 otherwise.
 var testdataArchs = map[string]string{
@@ -233,7 +239,6 @@ func TestTestdataFiles(t *testing.T) {
 	if err != nil {
 		log.Fatal(err)
 	}
-
 	var failures []string
 	for _, input := range testdataTests {
 		if !run(t, filepath.Join(cwd, "testdata", input)) {

go/ssa/interp/testdata/fixedbugs/issue52835.go

@@ -0,0 +1,27 @@
+package main
+
+var called bool
+
+type I interface {
+	Foo()
+}
+
+type A struct{}
+
+func (a A) Foo() {
+	called = true
+}
+
+func lambda[X I]() func() func() {
+	return func() func() {
+		var x X
+		return x.Foo
+	}
+}
+
+func main() {
+	lambda[A]()()()
+	if !called {
+		panic(called)
+	}
+}