[clang] [clang][Modules] Complete the implementation of P2615: Meaningful exports (PR #194201)

[Victor Chernyakin via cfe-commits]

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@@ -12854,6 +12854,15 @@ def err_invalid_module_name : Error<"%0 is an invalid name for a module">;
 def err_extern_def_in_header_unit : Error<
   "non-inline external definitions are not permitted in C++ header units">;
 
+def err_meaningless_export : Error<
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localspook wrote:

Can others share an opinion? I'm fine with making it an error only with `-pedantic-errors`, but I want to make sure no one is against, since it's technically nonconforming.

https://github.com/llvm/llvm-project/pull/194201