Turn off redundant move warnings

Summary:
The redundant move warning added in GCC 9 can be misleading. Some
moves that it warns for may not be redundant on older compilers,
particularly when type conversions are involved.

Reviewed By: tmikov

Differential Revision: D23893822

fbshipit-source-id: 637211b22942e0c280202f0f3ea1c7f57989c56c

CMakeLists.txt

@@ -408,10 +408,17 @@ if (NOT (GENERATOR_IS_MULTI_CONFIG OR CMAKE_BUILD_TYPE STREQUAL Debug OR CMAKE_C
     option(HERMES_ENABLE_LTO "Build Hermes with LTO" ON)
 endif()
 
-if (GCC_COMPATIBLE)
+if (CMAKE_COMPILER_IS_GNUCXX)
   # Suppress uninteresting warnings about initializing ArrayRef from initializer lists.
   check_cxx_compiler_flag("-Winit-list-lifetime" INIT_LIST_LIFETIME_FLAG)
   append_if(INIT_LIST_LIFETIME_FLAG "-Wno-init-list-lifetime" CMAKE_CXX_FLAGS)
+  # Suppress the redundant move warnings in GCC 9, because it leads to suboptimal
+  # recommendations for older compilers that do not implement C++ Core Issue 1579.
+  check_cxx_compiler_flag("-Wredundant-move" REDUNDANT_MOVE_FLAG)
+  append_if(REDUNDANT_MOVE_FLAG "-Wno-redundant-move" CMAKE_CXX_FLAGS)
+endif()
+
+if (GCC_COMPATIBLE)
   # Don't export symbols unless we explicitly say so
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility=hidden")
 elseif ("${CMAKE_CXX_COMPILER_ID}" MATCHES "MSVC")