cmd/compile: add missing shape check in (*Tsubster).tinter

Add a missing shape check in (*Tsubster).tinter when substituting on a
generic type which is an empty interface, analogous to same check in
(*Tsubster).tstruct. Empty structs/interfaces that have rparams (i.e.
are a generic type or a shape type) need to get a new type of their
rparams - they will be different even though they don't have any
fields/methods. Without this shape check, we were not correctly
completing the Token[int] type during substitution in the example in the
issue. This issue only happens for a generic type which is an empty
interface (i.e. doesn't actually use the type param, hence quite unusual).

Added the test case already created by Keith.

Fixes #50841

Change-Id: Ia985b9f52c0e87ed0647b46373e44c51cb748ba4
Reviewed-on: https://go-review.googlesource.com/c/go/+/381175
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

3 files changed, 40 insertions, 0 deletions

diff --git a/test/typeparam/issue50841.dir/a.go b/test/typeparam/issue50841.dir/a.go
new file mode 100644
index 0000000000..37e0233701
--- /dev/null
+++ b/test/typeparam/issue50841.dir/a.go
@@ -0,0 +1,22 @@
+// Copyright 2022 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 a
+
+func Marshal[foobar any]() {
+	_ = NewEncoder[foobar]()
+}
+
+func NewEncoder[foobar any]() *Encoder[foobar] {
+	return nil
+}
+
+type Encoder[foobar any] struct {
+}
+
+func (e *Encoder[foobar]) EncodeToken(t Token[foobar]) {
+
+}
+
+type Token[foobar any] any
diff --git a/test/typeparam/issue50841.dir/b.go b/test/typeparam/issue50841.dir/b.go
new file mode 100644
index 0000000000..f2f70225ff
--- /dev/null
+++ b/test/typeparam/issue50841.dir/b.go
@@ -0,0 +1,11 @@
+// Copyright 2022 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 b
+
+import "a"
+
+func F() {
+	a.Marshal[int]()
+}
diff --git a/test/typeparam/issue50841.go b/test/typeparam/issue50841.go
new file mode 100644
index 0000000000..060a1214cc
--- /dev/null
+++ b/test/typeparam/issue50841.go
@@ -0,0 +1,7 @@
+// compiledir -G=3
+
+// Copyright 2022 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 ignored