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

[Hubert Tong via cfe-commits]

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@@ -466,6 +466,9 @@ def err_pp_embed_device_file : Error<
 
 def ext_pp_extra_tokens_at_eol : ExtWarn<
   "extra tokens at end of #%0 directive">, InGroup<ExtraTokens>;
+def ext_pp_extra_tokens_at_module_directive_eol
+    : ExtWarn<"extra tokens at end of '%0' directive">,
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hubert-reinterpretcast wrote:

Based on the context of CWG2947, this is not an extension. This should be considered a `Warning` IIUC: https://releases.llvm.org/20.1.0/tools/clang/docs/InternalsManual.html#the-diagnostic-kinds-td-files.

There should be a test that programs that trigger this warning are accepted (with a warning) under `-pedantic-errors`.

Additionally, regardless of CWG2947, there is an issue with the wording of the message because the directive does not formally end with the first semicolon.

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