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

[Erich Keane via cfe-commits]

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@@ -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);
+  if (Demangled)
+    Str = Demangled;
+  const DeclarationNameInfo target(&S.Context.Idents.get(Str), AL.getLoc());
+  LookupResult LR(S, target, Sema::LookupOrdinaryName);
+  if (S.LookupQualifiedName(LR, S.getCurLexicalContext()))
+    for (NamedDecl *ND : LR)
+      ND->markUsed(S.Context);
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erichkeane wrote:

This loop doesn't seem quite right either.  First, we need to ensure we're only marking 'functions' as used, not all possible named decls I think.  

Second, we need to ensure we're doing a better job with testing, this doesn't seem to be adding enough test infrastructure to make sure we're picking up the right function.

Finally, what does this mean in the case of template declarations/instantiations?  We need to make sure we're marking those correctly (or not picking them up at all perhaps?).

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