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Dana Jansens
bad9beddc7
Using a named constraint inside itself is problematic: - If there were not require decls written above, it identifies as an empty set. This makes `Z(Self)` essentially disappear in the identified facet type, which produces "no use of Self" diagnostics while the user can see a use of Self in the code. - It won't include require decls that are written after, and so `require T impls Z` won't actually enforce that `T` impls all of `Z`. Previously this was an error because using the named constraint would require it to be identified, and it's not identified until it is complete. But this will change in proposal #6902. So that proposal also includes changes to preserve diagnostics for incorrect use of a named constraint before it's complete, which is implemented here. Discussed in open discussion [on 2026-03-12](https://docs.google.com/document/d/1mjllGO3ZCL4qGt9uJHUtcxKoHAGEY7Y999ie4EtBWB8/edit?tab=t.0#heading=h.1dvbbrp5a6t3). The new tests exposed a bug where we're not copying named constraints in a facet type on the RHS of `where .Self impls` into the facet type on the left, which is now fixed. The `fail_require_impls_incomplete_self_in_period_self_impls.carbon` test would not diagnose its error without this fix.