C++
gcc 8.2.0
$ g++ prog.cc -Wall -Wextra -std=c++17
prog.cc:10:31: error: static assertion failed class X<true> { static_assert(dependent_false<X>::value); }; ^~~~~~~~~~~~~~~~~~ prog.cc: In function 'int main()': prog.cc:13:14: warning: unused variable 'x' [-Wunused-variable] X<false> x; ^