Add unnecessary-virtual-specifier to -Wextra

Author: DKLoehr

clang/docs/ReleaseNotes.rst

@@ -434,9 +434,9 @@ Improvements to Clang's diagnostics
 - The ``-Wsign-compare`` warning now treats expressions with bitwise not(~) and minus(-) as signed integers
   except for the case where the operand is an unsigned integer
   and throws warning if they are compared with unsigned integers (##18878).
-- The ``-Wunnecessary-virtual-specifier`` warning has been added to warn about
-  methods which are marked as virtual inside a ``final`` class, and hence can
-  never be overridden.
+- The ``-Wunnecessary-virtual-specifier`` warning (included in ``-Wextra``) has
+  been added to warn about methods which are marked as virtual inside a
+  ``final`` class, and hence can never be overridden.
 
 - Improve the diagnostics for chained comparisons to report actual expressions and operators (#GH129069).
 

clang/include/clang/Basic/DiagnosticGroups.td

@@ -421,13 +421,12 @@ def CXX11WarnSuggestOverride : DiagGroup<"suggest-override">;
 def WarnUnnecessaryVirtualSpecifier : DiagGroup<"unnecessary-virtual-specifier"> {
   code Documentation = [{
 Warns when a ``final`` class contains a virtual method (including virtual
-destructors). Since ``final`` classes cannot be subclassed, their methods
-cannot be overridden, and hence the ``virtual`` specifier is useless.
+destructors) that does not override anything. Since ``final`` classes cannot
+be subclassed, their methods cannot be overridden, so there is no point to
+introducing new ``virtual`` methods.
 
 The warning also detects virtual methods in classes whose destructor is
 ``final``, for the same reason.
-
-The warning does not fire on virtual methods which are also marked ``override``.
   }];
 }
 
@@ -1163,6 +1162,7 @@ def Extra : DiagGroup<"extra", [
     FUseLdPath,
     CastFunctionTypeMismatch,
     InitStringTooLongMissingNonString,
+    WarnUnnecessaryVirtualSpecifier,
   ]>;
 
 def Most : DiagGroup<"most", [

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -882,6 +882,11 @@ if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   # The LLVM libraries have no stable C++ API, so -Wnoexcept-type is not useful.
   append("-Wno-noexcept-type" CMAKE_CXX_FLAGS)
 
+  # LLVM has a policy of including virtual "anchor" functions to control
+  # where the vtable is emitted. In `final` classes, these are exactly what
+  # this warning detects: unnecessary virtual methods.
+  add_flag_if_supported("-Wno-unnecessary-virtual-specifier" CXX_SUPPORTS_UNNECESSARY_VIRTUAL_FLAG)
+
   if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     append("-Wnon-virtual-dtor" CMAKE_CXX_FLAGS)
   endif()