sync/atomic: atomic.Pointer[T] can be misused with type conversions. [1.19 backport]

[gopherbot]

@ianlancetaylor requested issue #56603 to be considered for backport to the next 1.19 minor release.

@gopherbot Please backport to 1.19.

This will make it less likely for people to write invalid type conversions with generic atomic types.

[gopherbot]

Change https://go.dev/cl/448518 mentions this issue: [release-branch.go1.19] sync/atomic: disallow type conversions of atomic.Pointer[T]

[mknyszek]

Consensus is that this is unlikely to break users but if it does, it's better to break them sooner rather than later to avoid these type conversions from propagating. Approving.

[gopherbot]

Closed by merging 0dfdf32 to release-branch.go1.19.

[gopherbot]

Change https://go.dev/cl/452438 mentions this issue: [release-branch.go1.19] sync/atomic: allow linked list of atomic pointers again