conditional constexpr functions

I have here the case that a function could potentially be constexpr. Normally one adds the constexpr and use the constant evaluation only if the context allows it. However the following code complaints despite not using it in a constexpr context:

template <typename T>
struct Wrapper
{
    friend constexpr bool operator==(const Wrapper& crLhs, const Wrapper& crRhs) noexcept
    {
        return crLhs.m_t == crRhs.m_t;
    }

    T m_t = {};
};

Using Visual Studio 2017 15.9.20 this gives 'error C3615: constexpr function 'operator ==' cannot result in a constant expression' when e.g. instantiated for std::string. The information is correct but I am not instantiating it in a constexpr context.

void f()
{
   bool b;

   Wrapper<int>  a;

   b = a == a; //ok

   Wrapper<std::string> c;

   b = c == c;  //C3615, but not using constexpr context
}

I can apply a workaround it by using a member template or drop the constexpr but is there fancy trick here to have the best of both worlds (i.e. constexpr when applicable)?