[Clang] FunctionEffects: Make a separate diagnostic group for redeclarations/overrides where effects are implicit. (#148690)

The current function effect diagnostics include these behaviors:

When you declare a function `nonblocking` (typically in a header) and
then omit the attribute on the implementation (or any other
redeclaration), Clang warns: attribute 'nonblocking' on function does
not match previous declaration.

But if a `nonblocking` function is a C++ virtual method, then overrides
are implicitly nonblocking; the attribute doesn't need to be explicitly
stated.

These behaviors are arguably inconsistent -- and also, both, more
pedantic than the rest of the function effect diagnostics.

This PR accomplishes two things:

- Separates the diagnostic on a redeclaration into a new group,
`-Wfunction-effect-redeclarations`, so it can be disabled independently.

- Adds a second diagnostic to this new group, for the case of an
override method missing the attribute. (This helps in a situation where
I'm trying to add `nonblocking` via a macro that does other things and I
want to know that the macro is missing on an override declaration.)

---------

Co-authored-by: Doug Wyatt <dwyatt@apple.com>
Co-authored-by: Sirraide <aeternalmail@gmail.com>

clang/docs/ReleaseNotes.rst

@@ -710,6 +710,12 @@ Improvements to Clang's diagnostics
   pointer, provided it can be proven that the pointer only points to
   ``[[noreturn]]`` functions.
 
+- 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/include/clang/Basic/DiagnosticGroups.td

@@ -1293,6 +1293,7 @@ def ThreadSafetyBeta : DiagGroup<"thread-safety-beta">;
 // Warnings and notes related to the function effects system which underlies
 // the nonblocking and nonallocating attributes.
 def FunctionEffects : DiagGroup<"function-effects">;
+def FunctionEffectRedeclarations : DiagGroup<"function-effect-redeclarations">;
 def PerfConstraintImpliesNoexcept : DiagGroup<"perf-constraint-implies-noexcept">;
 
 // Uniqueness Analysis warnings

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -11530,17 +11530,28 @@ def note_in_evaluating_default_argument : Note<
 def warn_invalid_add_func_effects : Warning<
   "attribute '%0' should not be added via type conversion">,
   InGroup<FunctionEffects>, DefaultIgnore;
-def warn_mismatched_func_effect_override : Warning<
-  "attribute '%0' on overriding function does not match base declaration">,
-  InGroup<FunctionEffects>, DefaultIgnore;
-def warn_mismatched_func_effect_redeclaration : Warning<
-  "attribute '%0' on function does not match previous declaration">,
-  InGroup<FunctionEffects>, DefaultIgnore;
+def warn_conflicting_func_effect_override
+    : Warning<"attribute '%0' on overriding function conflicts with base "
+              "declaration">,
+      InGroup<FunctionEffects>,
+      DefaultIgnore;
 def warn_conflicting_func_effects : Warning<
   "effects conflict when merging declarations; kept '%0', discarded '%1'">,
   InGroup<FunctionEffects>, DefaultIgnore;
 def err_func_with_effects_no_prototype : Error<
   "'%0' function must have a prototype">;
+// These are more pedantic: in redeclarations and virtual method overrides,
+// the effect attribute(s) should be restated.
+def warn_mismatched_func_effect_override
+    : Warning<"overriding function is missing '%0' attribute from base "
+              "declaration">,
+      InGroup<FunctionEffectRedeclarations>,
+      DefaultIgnore;
+def warn_mismatched_func_effect_redeclaration
+    : Warning<
+          "redeclaration is missing '%0' attribute from previous declaration">,
+      InGroup<FunctionEffectRedeclarations>,
+      DefaultIgnore;
 
 } // end of sema category
 

clang/lib/Sema/SemaDeclCXX.cpp

@@ -18682,7 +18682,7 @@ bool Sema::CheckOverridingFunctionAttributes(CXXMethodDecl *New,
         case FunctionEffectDiff::OverrideResult::NoAction:
           break;
         case FunctionEffectDiff::OverrideResult::Warn:
-          Diag(New->getLocation(), diag::warn_mismatched_func_effect_override)
+          Diag(New->getLocation(), diag::warn_conflicting_func_effect_override)
               << Diff.effectName();
           Diag(Old->getLocation(), diag::note_overridden_virtual_function)
               << Old->getReturnTypeSourceRange();
@@ -18695,6 +18695,14 @@ bool Sema::CheckOverridingFunctionAttributes(CXXMethodDecl *New,
           QualType ModQT = Context.getFunctionType(NewFT->getReturnType(),
                                                    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.
+            Diag(New->getLocation(), diag::warn_mismatched_func_effect_override)
+                << Diff.effectName();
+            Diag(Old->getLocation(), diag::note_overridden_virtual_function)
+                << Old->getReturnTypeSourceRange();
+          }
           break;
         }
         }

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

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -fblocks -fcxx-exceptions -verify -Wfunction-effects %s
-// RUN: %clang_cc1 -fsyntax-only -fblocks -verify -x c -std=c23 -Wfunction-effects %s
+// RUN: %clang_cc1 -fsyntax-only -fblocks -fcxx-exceptions -verify -Wfunction-effects -Wfunction-effect-redeclarations %s
+// RUN: %clang_cc1 -fsyntax-only -fblocks -verify -x c -std=c23 -Wfunction-effects -Wfunction-effect-redeclarations %s
 
 #if !__has_attribute(nonblocking)
 #error "the 'nonblocking' attribute is not available"
@@ -127,29 +127,35 @@ 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]];
-  virtual void nonallocating() noexcept [[clang::nonallocating]];
+  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 {
   void f1() [[clang::nonblocking]] override;
-  void nonblocking() noexcept override;
-  void nonallocating() noexcept override;
+  void nonblocking() noexcept override; // expected-warning {{overriding function is missing 'nonblocking' attribute from base declaration}}
+  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 ---
 
 void f2();
 void f2() [[clang::nonblocking]]; // expected-note {{previous declaration is here}}
-void f2(); // expected-warning {{attribute 'nonblocking' on function does not match previous declaration}}
+void f2(); // expected-warning {{redeclaration is missing 'nonblocking' attribute from previous declaration}}
 // Note: we verify that the attribute is actually seen during the constraints tests.
 
 void f3() [[clang::blocking]]; // expected-note {{previous declaration is here}}