[clang] [Clang] prevent errors for deduction guides using deduced type aliases (PR #117450)

[Younan Zhang via cfe-commits]

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@@ -583,6 +583,8 @@ Improvements to Clang's diagnostics
 - For an rvalue reference bound to a temporary struct with an integer member, Clang will detect constant integer overflow
   in the initializer for the integer member (#GH46755).
 
+- Clang now prevents errors for deduction guides with deduced type aliases (#GH54909).
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zyn0217 wrote:

This isn't clear enough to me: Someone might ask why we should 'prevent' errors in such cases, and what the rationale is behind it. It might help to reference some standardese to make it unimpeachable.

https://github.com/llvm/llvm-project/pull/117450