[clang] [C++20][Modules] Implement P1857R3 Modules Dependency Discovery (PR #107168)

[Hubert Tong via cfe-commits]

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@@ -986,6 +989,14 @@ def warn_module_conflict : Warning<
   InGroup<ModuleConflict>;
 
 // C++20 modules
+def err_pp_module_name_is_macro : Error<
+  "%select{module|partition}0 name component %1 cannot be a object-like macro">;
+def err_pp_module_expected_ident : Error<
+  "expected %select{identifier after '.' in |}0module name">;
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hubert-reinterpretcast wrote:

The `.` is technically part of the _pp-tokens_ following the module name in such a case. In other words, this is a special case where we _do_ currently diagnose the `;`/`[` restriction on the _pp-token_ following the module name/partition in the preprocessor.

Contrast with the case where the module name is followed by `(` and the last component is the name of a function-like macro. In that case, the `;`/`[` restriction does not immediately apply (because it applies after macro expansion), but we currently diagnose it in phase 7 using the `;`/`[` restriction.

TL;DR: Instead of having this preprocessor diagnostic, there should be preprocessor diagnostics for the disallowed `(` case and for the `;`/`[` restriction.

https://github.com/llvm/llvm-project/pull/107168