[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[via cfe-commits]

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@@ -2302,6 +2302,10 @@ def err_init_list_bad_dest_type : Error<
 def warn_cxx20_compat_aggregate_init_with_ctors : Warning<
   "aggregate initialization of type %0 with user-declared constructors "
   "is incompatible with C++20">, DefaultIgnore, InGroup<CXX20Compat>;
+def warn_cxx20_compat_requires_explicit_init_non_aggregate : Warning<
+  "explicit initialization of field %0 may not be enforced in C++20 as type %1 "
+  "will become a non-aggregate due to the presence of user-declared "
+  "constructors">, DefaultIgnore, InGroup<CXX20Compat>;
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higher-performance wrote:

It seems strictly like a compatibility warning to me -- because it isn't relevant for someone who _doesn't_ want to support later versions of the standard. I'd prefer to keep it that way if possible.

https://github.com/llvm/llvm-project/pull/102040