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>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -1328,21 +1328,21 @@ func writeITab(lsym *obj.LSym, typ, iface *types.Type, allowNonImplement bool) {
 	// type itab struct {
 	//   inter  *interfacetype
 	//   _type  *_type
-	//   hash   uint32
+	//   hash   uint32 // copy of _type.hash. Used for type switches.
 	//   _      [4]byte
-	//   fun    [1]uintptr // variable sized
+	//   fun    [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter.
 	// }
 	o := objw.SymPtr(lsym, 0, writeType(iface), 0)
 	o = objw.SymPtr(lsym, o, writeType(typ), 0)
 	o = objw.Uint32(lsym, o, types.TypeHash(typ)) // copy of type hash
 	o += 4                                        // skip unused field
+	if !completeItab {
+		// If typ doesn't implement iface, make method entries be zero.
+		o = objw.Uintptr(lsym, o, 0)
+		entries = entries[:0]
+	}
 	for _, fn := range entries {
-		if !completeItab {
-			// If typ doesn't implement iface, make method entries be zero.
-			o = objw.Uintptr(lsym, o, 0)
-		} else {
-			o = objw.SymPtrWeak(lsym, o, fn, 0) // method pointer for each method
-		}
+		o = objw.SymPtrWeak(lsym, o, fn, 0) // method pointer for each method
 	}
 	// Nothing writes static itabs, so they are read only.
 	objw.Global(lsym, int32(o), int16(obj.DUPOK|obj.RODATA))

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{})
+}

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]()
+}