[release-branch.go1.19] sync/atomic: disallow type conversions of atomic.Pointer[T]

For #56603.
Fixes #56638.

Change-Id: I6af9d80201025ae4028bfaa4a62e5de9ac0c501d
GitHub-Last-Rev: e6ed5e1
GitHub-Pull-Request: #56604
Reviewed-on: https://go-review.googlesource.com/c/go/+/448275
Reviewed-by: Michael Knyszek <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
(cherry picked from commit 6bead8f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/448518
Run-TryBot: Michael Knyszek <[email protected]>

Author: mateusz834

src/sync/atomic/type.go

@@ -37,6 +37,10 @@ func b32(b bool) uint32 {
 
 // A Pointer is an atomic pointer of type *T. The zero value is a nil *T.
 type Pointer[T any] struct {
+	// Mention T in a field to disallow conversion between Pointer types.
+	// See go.dev/issue/56603 for more details.
+	_ [0]T
+
 	_ noCopy
 	v unsafe.Pointer
 }