[clang] [clang] Turn -Wenum-constexpr-conversion into a hard error (PR #102364)

[Carlos Galvez via cfe-commits]

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@@ -49,6 +49,9 @@ C++ Specific Potentially Breaking Changes
   few users and can be written as ``__is_same(__remove_cv(T), decltype(nullptr))``,
   which GCC supports as well.
 
+- The warning `-Wenum-constexpr-conversion` has been upgraded into a hard
+  compiler error that cannot be suppressed, as required by the C++ Standard.
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carlosgalvezp wrote:

That's a great formulation so I'll go straight ahead and copy it here, thanks! Added further clarification

https://github.com/llvm/llvm-project/pull/102364