[libcxx-commits] [libcxx] [libc++] Disallow specializing `common_reference` (PR #141465)

[Nikolas Klauser via libcxx-commits]

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@@ -97,6 +97,9 @@ Potentially breaking changes
 - The implementation of ``num_put::do_put`` has been replaced to improve the performance, which can lead to different
   output when printing pointers.
 
+- User-defined specializations of ``std::common_reference`` are disallowed with the ``[[clang::no_specializations]]``
+  attribute. To customize the common reference type, ``std::basic_common_reference`` should be specialized instead.
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philnik777 wrote:

```suggestion
- User-defined specializations of ``std::common_reference`` are diagnosed now. To customize the common reference type, ``std::basic_common_reference`` should be specialized instead.
```
I don't think it's relevant how we do that.

https://github.com/llvm/llvm-project/pull/141465