[clang] [clang] Generate note on declaration for nodiscard-related attributes (PR #112289)

[Erich Keane via cfe-commits]

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@@ -302,27 +307,43 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
     if (const Decl *FD = CE->getCalleeDecl()) {
       if (ShouldSuppress)
         return;
-      if (FD->hasAttr<PureAttr>()) {
-        Diag(Loc, diag::warn_unused_call) << R1 << R2 << "pure";
-        return;
-      }
-      if (FD->hasAttr<ConstAttr>()) {
-        Diag(Loc, diag::warn_unused_call) << R1 << R2 << "const";
+
+      const InheritableAttr *A = nullptr;
+      if (const auto *PA = FD->getAttr<PureAttr>())
+        A = PA;
+      else if (const auto *CA = FD->getAttr<ConstAttr>())
+        A = CA;
+
+      if (A) {
+        StringRef type = (isa<PureAttr>(A) ? "pure" : "const");
+        Diag(Loc, diag::warn_unused_call) << R1 << R2 << type;
+        if (const auto *ND = dyn_cast<NamedDecl>(OffendingDecl)) {
+          if (!ND->getIdentifier()->getBuiltinID())
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erichkeane wrote:

Hmm... thats actually pretty strange: https://godbolt.org/z/q6EMMs6fa
If you make it NOT an error for the first declaration, it does what I suspect it SHOULD be doing in all cases, creating the overload set without a location: https://godbolt.org/z/4zeq99bne

So there is perhaps a bug in the builtin recovery we need to understand before we can move on.

https://github.com/llvm/llvm-project/pull/112289