[clang] Improve the -Wundefined-func-template diagnostic note for invisible template functions (PR #129031)

[Haojian Wu via cfe-commits]

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@@ -5654,6 +5654,8 @@ def warn_func_template_missing : Warning<"instantiation of function %q0 "
   InGroup<UndefinedFuncTemplate>, DefaultIgnore;
 def note_forward_template_decl : Note<
   "forward declaration of template entity is here">;
+def note_unreachable_template_decl
+    : Note<"unreachable declaration of template entity is here">;
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hokein wrote:

> should we expect Clang to sometimes complain about reachability in C++17 and below with -fmodules enabled?

I think yes. Clang already does this — in` Sema::DiagnoseUninstantiableTemplate`, when a reachable function definition is missing, Clang may emit a "missing-import" diagnostic.

However, for this particular testcase, the issue is only triggered with the C++20 standard module feature

> But it looks like the method to judge unreachable decl in this patch is the same for C++20 modules.

Yes, this aligns with my reading of the code. The same code is shared between header modules and C++20 modules.



https://github.com/llvm/llvm-project/pull/129031