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Source code

#pragma once

#include <stdexcept>

template<int nLine>
struct InstantiationLocation final {
	friend auto InstantiatedFlag(InstantiationLocation) noexcept;           // (1)
};

template<typename Location>
struct SetInstantiation final {                                             // (2)
	friend auto InstantiatedFlag(Location) noexcept {}
};

template<typename Location, auto UniqueTag>
[[nodiscard]] consteval auto Instantiate() noexcept {                       // (3)
	if constexpr(requires { InstantiatedFlag(Location()); }) {
		return false;
	} else {
		SetInstantiation<Location>();
		return true;
	}
}

template<typename Location>
void AssertSingleInstantiation() {
	static constinit bool bIsInstantiated = false;
	if(bIsInstantiated) {
        throw std::runtime_error("Should only be instantiated once!");
    }
	bIsInstantiated = true;
}

template<typename Location, auto UniqueTag>
inline const auto c_AssertSingleInstantiationBeforeMain = []() noexcept {
	AssertSingleInstantiation<Location>();
	return 0;
}();

#define ASSERT_SINGLE_INSTANTIATION { \
	using Location = InstantiationLocation<__LINE__>; \
	(void) c_AssertSingleInstantiationBeforeMain<Location, []{}>; \
	static_assert(Instantiate<Location, []{}>(), "Should only be instantiated once!"); \
}

template<int>
void MySpecialFunction() {
    ASSERT_SINGLE_INSTANTIATION
}