cmd/compile: always write fun[0] in incomplete itab

runtime.getitab need filled fun[0] to identify whether
implemented the interface.

Fixes #51700
Fixes #52228

Change-Id: I0173b98f4e1b45e3a0183a5b60229d289140d1e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/399058
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>

2 files changed, 56 insertions, 0 deletions

diff --git a/test/typeparam/issue51700.go b/test/typeparam/issue51700.go
new file mode 100644
index 0000000000..bf8a1f6289
--- /dev/null
+++ b/test/typeparam/issue51700.go
@@ -0,0 +1,26 @@
+// run
+
+// 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 main
+
+func f[B any](b B) {
+	if b1, ok := any(b).(interface{ m1() }); ok {
+		panic(1)
+		_ = b1.(B)
+	}
+	if b2, ok := any(b).(interface{ m2() }); ok {
+		panic(2)
+		_ = b2.(B)
+	}
+}
+
+type S struct{}
+
+func (S) m3() {}
+
+func main() {
+	f(S{})
+}
diff --git a/test/typeparam/issue52228.go b/test/typeparam/issue52228.go
new file mode 100644
index 0000000000..3fbbde59ab
--- /dev/null
+++ b/test/typeparam/issue52228.go
@@ -0,0 +1,30 @@
+// run
+
+// 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 main
+
+type SomeInterface interface {
+	Whatever()
+}
+
+func X[T any]() T {
+	var m T
+
+	// for this example, this block should never run
+	if _, ok := any(m).(SomeInterface); ok {
+		var dst SomeInterface
+		_, _ = dst.(T)
+		return dst.(T)
+	}
+
+	return m
+}
+
+type holder struct{}
+
+func main() {
+	X[holder]()
+}