cmd/compile: fix devirtualization bug with unified IR

As a consistency check in devirtualization, when we determine `i` (of
interface type `I`) always has dynamic type `T`, we insert a type
assertion `i.(T)`. This emits an itab check for `go:itab.T,I`, but
it's always true (and so SSA optimizes it away).

However, if `I` is instead the generic interface type `I[T]`, then
`go:itab.T,I[int]` and `go:itab.T,I[go.shape.int]` are equivalent but
distinct itabs. And notably, we'll have originally created the
interface value using the former; but the (non-dynamic) TypeAssertExpr
created by devirtualization would ultimately emit a comparison against
the latter. This comparison would then evaluate false, leading to a
spurious type assertion panic at runtime.

The comparison is just meant as an extra safety check, so it should be
safe to just disable. But for now, it's simpler/safer to just punt on
devirtualization in this case. (The non-unified frontend doesn't
devirtualize this either.)

Change-Id: I6a8809bcfebc9571f32e289fa4bc6a8b0d21ca46
Reviewed-on: https://go-review.googlesource.com/c/go/+/424774
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/cmd/compile/internal/devirtualize/devirtualize.go

@@ -74,6 +74,25 @@ func Call(call *ir.CallExpr) {
 			}
 			return
 		}
+
+		// Further, if sel.X's type has a shape type, then it's a shaped
+		// interface type. In this case, the (non-dynamic) TypeAssertExpr
+		// we construct below would attempt to create an itab
+		// corresponding to this shaped interface type; but the actual
+		// itab pointer in the interface value will correspond to the
+		// original (non-shaped) interface type instead. These are
+		// functionally equivalent, but they have distinct pointer
+		// identities, which leads to the type assertion failing.
+		//
+		// TODO(mdempsky): We know the type assertion here is safe, so we
+		// could instead set a flag so that walk skips the itab check. For
+		// now, punting is easy and safe.
+		if sel.X.Type().HasShape() {
+			if base.Flag.LowerM != 0 {
+				base.WarnfAt(call.Pos(), "cannot devirtualize %v: shaped interface %v", call, sel.X.Type())
+			}
+			return
+		}
 	}
 
 	dt := ir.NewTypeAssertExpr(sel.Pos(), sel.X, nil)

test/typeparam/mdempsky/21.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.
+
+// Test that devirtualization doesn't introduce spurious type
+// assertion failures due to shaped and non-shaped interfaces having
+// distinct itabs.
+
+package main
+
+func main() {
+	F[int]()
+}
+
+func F[T any]() {
+	var i I[T] = X(0)
+	i.M()
+}
+
+type I[T any] interface{ M() }
+
+type X int
+
+func (X) M() {}