Possible template & constexpr–if incompatibility

I wanted to calculate e value at compile–time (don't worry, not a homework), yet something went wrong.

template<size_t limit = 3, class result = std::ratio<0, 1>, size_t factorial = 1, size_t count = 1>
constexpr double e_impl() {
    if constexpr(limit == 0) {
        return static_cast<double>(result{}.num) / result{}.den;
    }
    return e_impl<limit - 1, std::ratio_add<result, std::ratio<1, factorial>>, factorial * count, count + 1>();
}

While calculated values are correct, compiler throws an error about overflow in template. It seems like limit variable goes out of range (below 0), but it shouldn't happen as the 0–case is being handled by the if constexpr(…) statement.

So the question is, am I wrong and that behavior should be expected, or is it a compiler bug? Compiled with GCC 7.1.0.