When a static-const primitive is passed as a const-ref argument to function with a variadic template arglist, GCC generates code that references the static-const primitive, and optimizes away the static-const primitive, resulting in a "undefined reference" linker error.

/* Start of Example */
class Foo {
public:
    template <typename... T>
    static void foo(const T&... blah) { }
};

class Test {
public:
    static const int tmp = 1;
};

int main(int argc, char** argv) {
    Foo::foo(Test::tmp); // THIS COMPILES BUT LINKER FAILS
    //Foo::foo(static_cast<int>(Test::tmp)); // THIS COMPILES AND LINKS FINE
}
/* END OF EXAMPLE */

LINKER ERROR:
/tmp/ccfV1opu.o: In function `main':
tmp.cc:(.text+0x10): undefined reference to `Test::tmp'
collect2: ld returned 1 exit status

Since static cast of the static-const int primitive to an int fixes the problem, it's clear that GCC is somehow mishandling how it optimizes away Test::tmp even though it's being used as a const int& in Foo::foo(...)