Why compiling with C++11 is successful despite using C++14 feature?

GCC v6.1 successfully compiles the code below with flags -std=c++11 -Wall -Wextra -Wpedantic, but produces this warning:

variable templates only available with -std=c++14 or -std=gnu++14

The code:

#include <iostream>

template <typename T>
struct ParamMetadata {
        T min;
        T max;
};

template <class T1, class T2>
class FooMap {};

template <typename T>
// WARNING PRODUCED ON THIS LINE
extern FooMap<int, ParamMetadata<T> > metadataHashmap;

int main() {
        return 0;
}

If this is a feature only available in C++14, why does it compile with C++11 flag just fine and I can run the executable? Shouldn't it be a fatal error?