[Clang] FunctionEffects: Make a separate diagnostic group for redeclarations/overrides where effects are implicit.

clang/docs/ReleaseNotes.rst

@@ -707,6 +707,12 @@ Improvements to Clang's diagnostics
 - Improve the diagnostics for placement new expression when const-qualified
   object was passed as the storage argument. (#GH143708)
 
+- Added a separate diagnostic group `-Wfunction-effect-redeclarations``, for the more pedantic
+  diagnostics for function effects (``[[clang::nonblocking]]`` and ``[[clang::nonallocating]]``).
+  Moved the warning for a missing (though implied) attribute on a redeclaration into this group.
+  Added a new warning in this group for the case where the attribute is missing/implicit on
+  an override of a virtual method.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaDeclCXX.cpp

@@ -18700,8 +18700,8 @@ bool Sema::CheckOverridingFunctionAttributes(CXXMethodDecl *New,
                                                    NewFT->getParamTypes(), EPI);
           New->setType(ModQT);
           if (Errs.empty()) {
-            // A warning here is somewhat pedantic. Skip this if there was already 
-            // a merge conflict, which is more serious.
+            // A warning here is somewhat pedantic. Skip this if there was
+            // already a merge conflict, which is more serious.
             Diag(New->getLocation(), diag::warn_mismatched_func_effect_override)
                 << Diff.effectName();
             Diag(Old->getLocation(), diag::note_overridden_virtual_function)

clang/test/Sema/attr-nonblocking-sema.cpp

@@ -127,14 +127,15 @@ void type_conversions_2()
 #endif
 
 // --- VIRTUAL METHODS ---
-// Attributes propagate to overridden methods, so no diagnostics except for conflicts.
+// Attributes propagate to overridden methods.
 // Check this in the syntax tests too.
 #ifdef __cplusplus
 struct Base {
   virtual void f1();
   virtual void nonblocking() noexcept [[clang::nonblocking]]; // expected-note {{overridden virtual function is here}}
   virtual void nonallocating() noexcept [[clang::nonallocating]]; // expected-note {{overridden virtual function is here}}
   virtual void f2() [[clang::nonallocating]]; // expected-note {{previous declaration is here}}
+  virtual void f3() [[clang::nonblocking]]; // expected-note {{overridden virtual function is here}}
 };
 
 struct Derived : public Base {
@@ -143,6 +144,11 @@ struct Derived : public Base {
   void nonallocating() noexcept override; // expected-warning {{overriding function is missing 'nonallocating' attribute from base declaration}}
   void f2() [[clang::allocating]] override; // expected-warning {{effects conflict when merging declarations; kept 'allocating', discarded 'nonallocating'}}
 };
+
+template <bool B>
+struct TDerived : public Base {
+  void f3() [[clang::nonblocking(B)]] override; // expected-warning {{attribute 'nonblocking' on overriding function conflicts with base declaration}}
+};
 #endif // __cplusplus
 
 // --- REDECLARATIONS ---