[[maybe_unused]] attribute not working

I am trying to ignore the unused parameter warning using the new c++17 attribute [[maybe_unused]], as below.

int main([[maybe_unused]] int argc, char** argv){    //...}

But I still get warning: unused parameter ‘argc’ [-Wunused-parameter] with the following additional warning.

warning: ‘maybe_unused’ attribute directive ignored [-Wattributes]

I'm using g++ (GCC) 7.2.0 with cmake-3.11.3. My compiler flags are as follows.

-std=c++17 -Wall -pedantic -Wextra -Weffc++

I remember using this attribute successfully before, but I have no idea why this is not working now. Could someone show what I am doing wrong here?