Adding new warning: not eliding copy on return

Author: grigorypas

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12445,6 +12445,10 @@ def warn_zero_as_null_pointer_constant : Warning<
   "zero as null pointer constant">,
   InGroup<DiagGroup<"zero-as-null-pointer-constant">>, DefaultIgnore;
 
+def warn_not_eliding_copy_on_return : Warning<
+  "not eliding copy on return">, 
+  InGroup<DiagGroup<"nrvo">>, DefaultIgnore;
+
 def err_nullability_cs_multilevel : Error<
   "nullability keyword %0 cannot be applied to multi-level pointer type %1">;
 def note_nullability_type_specifier : Note<

clang/lib/Sema/SemaDecl.cpp

@@ -16093,8 +16093,11 @@ void Sema::computeNRVO(Stmt *Body, FunctionScopeInfo *Scope) {
 
   for (unsigned I = 0, E = Scope->Returns.size(); I != E; ++I) {
     if (const VarDecl *NRVOCandidate = Returns[I]->getNRVOCandidate()) {
-      if (!NRVOCandidate->isNRVOVariable())
+      if (!NRVOCandidate->isNRVOVariable()) {
+        Diag(Returns[I]->getRetValue()->getExprLoc(),
+             diag::warn_not_eliding_copy_on_return);
         Returns[I]->setNRVOCandidate(nullptr);
+      }
     }
   }
 }

clang/test/SemaCXX/warn-nrvo.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fsyntax-only -Wnrvo -verify %s
+struct MyClass {
+    int value;
+    int c;
+    MyClass(int v) : value(v), c(0) {}
+    MyClass(const MyClass& other) : value(other.value) { c++; }
+};
+
+MyClass create_object(bool condition) {
+    MyClass obj1(1);
+    MyClass obj2(2);
+    if (condition) {
+        return obj1; // expected-warning{{not eliding copy on return}}
+    }
+    return obj2; // expected-warning{{not eliding copy on return}}
+}