enums with ostream<< overloads are printed as their underlying type (as integers)

[nlguillemot]

Description

I had some enums where I defined an ostream<< operator, and I want to use it to print my enums rather than printing their underlying value like doctest seems to do by default.

Steps to reproduce

enum class Foo {};

std::ostream& operator<<(std::ostream& os, Foo) {
  return os << "Foo";
}

int main() {
    assert(doctest::toString(Foo()) == "Foo"); // Fails, toString returns "0"
}

(disclaimer: I have not tested the example code above.)

Extra information

I found a workaround but it only works for GCC (see: #121 (comment))

I suggest to tweak doctest's enum printing code so that it only prints enums by their underlying type if they do not meet the requirements of has_insertion_operator.