cmd/compile: fix broken IR for iface -> eface

For implementing interface to empty interface conversion, the compiler
generate code like:

	var res *uint8
	res = itab
	if res != nil {
		res = res.type
	}

However, itab has type *uintptr, so the assignment is broken. The
problem is not shown up, until CL 450215, which call typecheck on this
broken assignment.

To fix this, just cast itab to *uint8 when doing the conversion.

Fixes #56768

Change-Id: Id42792d18e7f382578b40854d46eecd49673792c
Reviewed-on: https://go-review.googlesource.com/c/go/+/451256
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/cmd/compile/internal/walk/convert.go

@@ -80,13 +80,15 @@ func walkConvInterface(n *ir.ConvExpr, init *ir.Nodes) ir.Node {
 
 	var typeWord ir.Node
 	if toType.IsEmptyInterface() {
-		// Implement interface to empty interface conversion.
+		// Implement interface to empty interface conversion:
-		// res = itab
+		//
+		// var res *uint8
+		// res = (*uint8)(unsafe.Pointer(itab))
 		// if res != nil {
 		//    res = res.type
 		// }
 		typeWord = typecheck.Temp(types.NewPtr(types.Types[types.TUINT8]))
-		init.Append(ir.NewAssignStmt(base.Pos, typeWord, itab))
+		init.Append(ir.NewAssignStmt(base.Pos, typeWord, typecheck.Conv(typecheck.Conv(itab, types.Types[types.TUNSAFEPTR]), typeWord.Type())))
 		nif := ir.NewIfStmt(base.Pos, typecheck.Expr(ir.NewBinaryExpr(base.Pos, ir.ONE, typeWord, typecheck.NodNil())), nil, nil)
 		nif.Body = []ir.Node{ir.NewAssignStmt(base.Pos, typeWord, itabType(typeWord))}
 		init.Append(nif)

test/fixedbugs/issue56768.go

@@ -0,0 +1,37 @@
+// compile
+
+// 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 p
+
+type I interface {
+	M()
+}
+
+type slice []any
+
+func f() {
+	ss := struct{ i I }{}
+
+	_ = [...]struct {
+		s slice
+	}{
+		{
+			s: slice{ss.i},
+		},
+		{
+			s: slice{ss.i},
+		},
+		{
+			s: slice{ss.i},
+		},
+		{
+			s: slice{ss.i},
+		},
+		{
+			s: slice{ss.i},
+		},
+	}
+}