[clang] [clang] Add the candiscard attribute to suppress nodiscard (PR #154943)

[Erich Keane via cfe-commits]

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@@ -1632,21 +1632,34 @@ QualType CallExpr::getCallReturnType(const ASTContext &Ctx) const {
 
 std::pair<const NamedDecl *, const WarnUnusedResultAttr *>
 Expr::getUnusedResultAttrImpl(const Decl *Callee, QualType ReturnType) {
-  // If the callee is marked nodiscard, return that attribute
-  if (Callee != nullptr)
+  // If the callee is marked nodiscard, return that attribute for the
+  // diagnostic. If the callee is marked candiscard, do not diagnose.
+  // If seen on the same level, candiscard beats nodiscard.
+  if (Callee != nullptr) {
+    if (const auto *A = Callee->getAttr<CanDiscardAttr>())
+      return {nullptr, nullptr};
     if (const auto *A = Callee->getAttr<WarnUnusedResultAttr>())
       return {nullptr, A};
+  }
 
-  // If the return type is a struct, union, or enum that is marked nodiscard,
-  // then return the return type attribute.
-  if (const TagDecl *TD = ReturnType->getAsTagDecl())
-    if (const auto *A = TD->getAttr<WarnUnusedResultAttr>())
-      return {TD, A};
-
+  // Walk the return type's (chain of) type aliases. The first alias
+  // that is marked either nodiscard or candiscard ends the walk.
   for (const auto *TD = ReturnType->getAs<TypedefType>(); TD;
-       TD = TD->desugar()->getAs<TypedefType>())
+       TD = TD->desugar()->getAs<TypedefType>()) {
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erichkeane wrote:

Can we make sure we have tests where this is done on a template alias?  We want to make sure semantics for this are sensible there too.

https://github.com/llvm/llvm-project/pull/154943