prog.cpp: In function 'void f()':
prog.cpp:9:7: error: passing 'const A' as 'this' argument discards qualifiers [-fpermissive]
a1.f(); // Only works in C++11. For C++14 the method has to be declared f() const.
^
prog.cpp:3:16: note: in call to 'constexpr int A::f()'
constexpr int f() { return 0; }
^