[CMake] Don't add -Wnon-virtual-dtor if affected by GCC PR102168

[MaskRay]

Summary

See the discussion on
https://reviews.llvm.org/rG4852c770fe8703145dd2a35395985646ce57a454
The GCC behavior (https://gcc.gnu.org/PR102168)
seems less useful.

Unconditional -Wnon-virtual-dtor led to other unnecessary workarounds like
6df09d6ccbc0cb72d3278cafb592e9bc0e6b84a1

This patches uses a variant of 4bb5f44c701402462cb93ef00d46d52382f39f11 check to detect GCC PR102168

[MaskRay]

Checked that clang++ build has -Wnon-virtual-dtor and g++ build don't have the option.

[dblaikie]

Looks about right to me, thanks!

[dblaikie]

I don't think the derived class is necessary for the test case, though - could remove that.

[MaskRay]

remove derived

llvm/cmake/modules/HandleLLVMOptions.cmake

if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     check_cxx_compiler_flag("-Wpessimizing-move" CXX_SUPPORTS_PESSIMIZING_MOVE_FLAG)
     append_if(CXX_SUPPORTS_PESSIMIZING_MOVE_FLAG "-Wno-pessimizing-move" CMAKE_CXX_FLAGS)
   endif()
 
   # The LLVM libraries have no stable C++ API, so -Wnoexcept-type is not useful.
   check_cxx_compiler_flag("-Wnoexcept-type" CXX_SUPPORTS_NOEXCEPT_TYPE_FLAG)
   append_if(CXX_SUPPORTS_NOEXCEPT_TYPE_FLAG "-Wno-noexcept-type" CMAKE_CXX_FLAGS)
 
-  append("-Wnon-virtual-dtor" CMAKE_CXX_FLAGS)
+  # Check if -Wnon-virtual-dtor warns for a class marked final, when it has a
+  # friend declaration. If it does, don't add -Wnon-virtual-dtor. The case is
+  # considered unhelpful (https://gcc.gnu.org/PR102168).
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=non-virtual-dtor")
+  CHECK_CXX_SOURCE_COMPILES("class f {};
+                             class base {friend f; public: virtual void anchor();protected: ~base();};
+                             int main() { return 0; }"
+                            CXX_WONT_WARN_ON_FINAL_NONVIRTUALDTOR)
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+  append_if(CXX_WONT_WARN_ON_FINAL_NONVIRTUALDTOR "-Wnon-virtual-dtor" CMAKE_CXX_FLAGS)
+
   append("-Wdelete-non-virtual-dtor" CMAKE_CXX_FLAGS)
 
   # Enable -Wsuggest-override if it's available, and only if it doesn't
   # suggest adding 'override' to functions that are already marked 'final'
   # (which means it is disabled for GCC < 9.2).
   check_cxx_compiler_flag("-Wsuggest-override" CXX_SUPPORTS_SUGGEST_OVERRIDE_FLAG)
   if (CXX_SUPPORTS_SUGGEST_OVERRIDE_FLAG)
     set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})