Integrated: Downgrade subtyping/cast failures from errors to warnings

[Maurizio Cimadamore]

On Thu, 8 Jan 2026 19:11:03 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This PR reverts all changes to subtyping and cast conversion, meaning that assigning `null` to a null-restricted type is no longer an error.
> I've resurrected the lint `null` category, to issue a warning in such cases.
> 
> There's some stuff left to do, as I realized that the compiler swallows null markers when emitting diagnostics. This is due to a conundrum in the diagnostic formatter calling `stripMetadata`, and that resulting in _all_ metadata (not just type annos) to be lost.

This pull request has now been integrated.

Changeset: 09204505
Author:    Maurizio Cimadamore <mcimadamore at openjdk.org>
URL:       https://git.openjdk.org/valhalla/commit/092045052619c33bcbdee3f25d900ab8b516c4f9
Stats:     208 lines in 11 files changed: 138 ins; 49 del; 21 mod

Downgrade subtyping/cast failures from errors to warnings

Reviewed-by: vromero

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PR: https://git.openjdk.org/valhalla/pull/1872