Additional layer of indirection, C++ (vc++)

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//Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x86

#include <type_traits>
#include <iostream>

int fn1()    { return 1; }
int fn2(int) { return 2; }

template <typename FN_T, FN_T FN, typename DERIVED>
class test_base
{
	template <typename T = DERIVED, typename = std::enable_if_t<!T::fn_specified>>
	int fn()
	{
		return FN();
	}

public:
	int fn_indirect()
	{
		return static_cast<DERIVED*>(this)->fn();
	}
};

template <class FN_T>
class test_derived : public test_base<typename FN_T::value_type, FN_T::value, test_derived<FN_T>>
{
public:
	static constexpr bool fn_specified = false;
};

template <typename FN_T>
class test_derived<std::integral_constant<FN_T, &fn2>>
	: public test_base<FN_T, &fn2, test_derived<std::integral_constant<FN_T, &fn2>>>
{
	using base = test_base<FN_T, &fn2, test_derived<std::integral_constant<FN_T, &fn2>>>;
	friend base;

public:
	static constexpr bool fn_specified = true;
	int fn()
	{
		return fn2(1);
	}
};

int main(void)
{
	test_derived<std::integral_constant<decltype(&fn1), &fn1>> x1;
	test_derived<std::integral_constant<decltype(&fn2), &fn2>> x2;
    
    std::cout << x1.fn_indirect() << " ";
    
    // comment next line out and it will work
    std::cout << x2.fn_indirect() << std::endl;
    return 0;
}

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Compilation time: 1,92 sec, absolute running time: 0,33 sec, absolute service time: 2,27 sec