[clang] [Sema] Mark alias/ifunc targets used and consider mangled names (PR #87130)

[Fangrui Song via cfe-commits]

================
@@ -1980,6 +1981,23 @@ static void handleWeakRefAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(::new (S.Context) WeakRefAttr(S.Context, AL));
 }
 
+// Mark alias/ifunc target as used. For C++, we look up the demangled name
+// ignoring parameters. This should handle the majority of use cases while
+// leaveing false positives for namespace scope names and false negatives in
+// the presence of overloads.
+static void markUsedForAliasOrIfunc(Sema &S, Decl *D, const ParsedAttr &AL,
+                                    StringRef Str) {
+  char *Demangled = llvm::itaniumDemangle(Str, /*ParseParams=*/false);
----------------
MaskRay wrote:

I agree that `ifunc` is ItaniumABI only and `alias` might be used by Windows but I don't know any users.

While we can add `S.getASTContext().getCXXABIKind() != TargetCXXABI::Microsoft` check for downstream users, it doesn't appear to add any value? `ifunc(mangled)` `alias(mangled)` referring to non-existent definitions are rejected by another diagnostic.

https://github.com/llvm/llvm-project/pull/87130