Implement P1771

As passed in the Cologne meeting and treated by Core as a DR,
[[nodiscard]] was applied to constructors so that they can be diagnosed
in cases where the user forgets a variable name for a type.

The intent is to enable the library to start using this on the
constructors of scope_guard/lock_guard.

Differential Revision: https://reviews.llvm.org/D64914

llvm-svn: 367027

1 files changed, 19 insertions, 1 deletions

diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 6ef77b8aee6..d2730d3e26d 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -2563,13 +2563,31 @@ bool Expr::isUnusedResultAWarning(const Expr *&WarnE, SourceLocation &Loc,
   case CXXTemporaryObjectExprClass:
   case CXXConstructExprClass: {
     if (const CXXRecordDecl *Type = getType()->getAsCXXRecordDecl()) {
-      if (Type->hasAttr<WarnUnusedAttr>()) {
+      const auto *WarnURAttr = Type->getAttr<WarnUnusedResultAttr>();
+      if (Type->hasAttr<WarnUnusedAttr>() ||
+          (WarnURAttr && WarnURAttr->IsCXX11NoDiscard())) {
         WarnE = this;
         Loc = getBeginLoc();
         R1 = getSourceRange();
         return true;
       }
     }
+
+    const auto *CE = cast<CXXConstructExpr>(this);
+    if (const CXXConstructorDecl *Ctor = CE->getConstructor()) {
+      const auto *WarnURAttr = Ctor->getAttr<WarnUnusedResultAttr>();
+      if (WarnURAttr && WarnURAttr->IsCXX11NoDiscard()) {
+        WarnE = this;
+        Loc = getBeginLoc();
+        R1 = getSourceRange();
+
+        if (unsigned NumArgs = CE->getNumArgs())
+          R2 = SourceRange(CE->getArg(0)->getBeginLoc(),
+                           CE->getArg(NumArgs - 1)->getEndLoc());
+        return true;
+      }
+    }
+
     return false;
   }