Avoid g++ warning spam

[lebedev.ri]

+1, it's super noisy.

[rnk]

Adding -Wno-cast-function-type seems reasonable, but we should do it more carefully.

cmake/modules/HandleLLVMOptions.cmake

583–586

You should put this here and follow this pattern instead to avoid setting the -Wno- flag on old versions of GCC that don't support it. GCC normally does not warn about unrecognized -Wno- flags unless it emits a warning, in which case it warns about all the unrecognized -Wno- flags. That's less than ideal.

cmake/modules/HandleLLVMOptions.cmake

if (LLVM_ENABLE_PEDANTIC AND LLVM_COMPILER_IS_GCC_COMPATIBLE)
     append("-pedantic" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     append("-Wno-long-long" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif()
 
   add_flag_if_supported("-Wcovered-switch-default" COVERED_SWITCH_DEFAULT_FLAG)
   append_if(USE_NO_UNINITIALIZED "-Wno-uninitialized" CMAKE_CXX_FLAGS)
   append_if(USE_NO_MAYBE_UNINITIALIZED "-Wno-maybe-uninitialized" CMAKE_CXX_FLAGS)
 
   # Disable -Wclass-memaccess, a C++-only warning from GCC 8 that fires on
   # LLVM's ADT classes.
   check_cxx_compiler_flag("-Wclass-memaccess" CXX_SUPPORTS_CLASS_MEMACCESS_FLAG)
   append_if(CXX_SUPPORTS_CLASS_MEMACCESS_FLAG "-Wno-class-memaccess" CMAKE_CXX_FLAGS)
 
   # Check if -Wnon-virtual-dtor warns even though the class is marked final.
   # If it does, don't add it. So it won't be added on clang 3.4 and older.
   # This also catches cases when -Wnon-virtual-dtor isn't supported by
   # the compiler at all.  This flag is not activated for gcc since it will
   # incorrectly identify a protected non-virtual base when there is a friend
   # declaration. Don't activate this in general on Windows as this warning has
   # too many false positives on COM-style classes, which are destroyed with
if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror -Wcomment")
   CHECK_C_SOURCE_COMPILES("// \\\\\\n//\\nint main() {return 0;}"
                           C_WCOMMENT_ALLOWS_LINE_WRAP)
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
   if (NOT C_WCOMMENT_ALLOWS_LINE_WRAP)
     append("-Wno-comment" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   endif()
 
+  # g++ likes to generate a 12 line warning every time the
+  # SchedulerRegistry is referenced, which makes actual warnings and
+  # errors hard to find. Disabling the warning is not ideal, but is
+  # probably better than having other warnings and errors overlooked.
+  if (CMAKE_COMPILER_IS_GNUCXX)
+    add_flag_if_supported("-Wno-cast-function-type" NO_CAST_FUNCTION_TYPE_FLAG)
+  endif()
+
   # Enable -Wstring-conversion to catch misuse of string literals.
   add_flag_if_supported("-Wstring-conversion" STRING_CONVERSION_FLAG)
 endif (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
 
 if (LLVM_COMPILER_IS_GCC_COMPATIBLE AND NOT LLVM_ENABLE_WARNINGS)
   append("-w" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 endif()