cmd/compile: constant-fold loads from constant dictionaries and types

Retrying the original CL with a small modification. The original CL
did not handle the case of reading an itab out of a dictionary
correctly.  When we read an itab out of a dictionary, we must treat
the type inside that itab as maybe being put in an interface.

Original CL: 486895
Revert CL: 490156

Change-Id: Id2dc1699d184cd8c63dac83986a70b60b4e6cbd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/491495
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 50 insertions, 0 deletions

diff --git a/test/typeparam/devirtualize1.go b/test/typeparam/devirtualize1.go
new file mode 100644
index 0000000000..fb26225e6e
--- /dev/null
+++ b/test/typeparam/devirtualize1.go
@@ -0,0 +1,22 @@
+// run
+
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type S struct {
+	x int
+}
+
+func (t *S) M1() {
+}
+
+func F[T any](x T) any {
+	return x
+}
+
+func main() {
+	F(&S{}).(interface{ M1() }).M1()
+}
diff --git a/test/typeparam/devirtualize2.go b/test/typeparam/devirtualize2.go
new file mode 100644
index 0000000000..ca6dc8fa86
--- /dev/null
+++ b/test/typeparam/devirtualize2.go
@@ -0,0 +1,28 @@
+// run
+
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type S struct {
+	x int
+}
+
+func (t *S) M1() {
+}
+func (t *S) M2() {
+}
+
+type I interface {
+	M1()
+}
+
+func F[T I](x T) I {
+	return x
+}
+
+func main() {
+	F(&S{}).(interface{ M2() }).M2()
+}