GCC versions/flags and FAM

Question

I'm trying to port our build system from make to CMake , and I encountered a problem that surprisingly not "Googleable" Our code is C++ 11/14, compiles fine with GCC6.2, the make applies zillion switches when invoking GCC, mostly pedantic warnings. I built a CMake system that compiles (GCC 6.3) most of the code without a problem but some modules failed to build because of the following

flexible array member 'blahblah' not at end of 'struct'

Aside why it appears in the C++ code. Why did it compile in the make based system? AFAIK, flexible array is not a part of C++ standard. GCC specific extension? What command line switch controls FAM behavior? How do I make it compile as it did in the original make system?

In case someone needs a snippet of compiled code

struct Foo
{
    int _10;
    double _20;
    int a[];
};

struct Bar
{
    Foo foo;
    double _1;
    int _2;
}

To add more context, the cmake file

cmake_minimum_required(VERSION 3.9)

project(foo VERSION ${FOO_VERSION} DESCRIPTION "foo")

set(CMAKE_CXX_STANDARD 14)

set(CMAKE_C_STANDARD 99)

add_executable(foo foo.cpp foo_backup.cpp main.cpp)

set_target_properties(foo PROPERTIES VERSION ${PROJECT_VERSION})

target_include_directories(foo PUBLIC ${CMAKE_SOURCE_DIR}/lib/include ${CMAKE_SOURCE_DIR}/lib/include/bar)

Answer 1

How do I make it compile as it did in the original make system

Revert to GCC 6.2. This -pedantic error was introduced for C++ in GCC 6.3.

See compilation with 6.2 and compilation with 6.3

Disabling pedantic compilation will remove the error but entail other relaxations.