Use Unimplemented Primary Template

Primary template is unimplemented, aka. incomplete type.

#include <concepts>
#include <utxcpp/core.hpp>

template <typename T>
class print;

template <std::integral T>
class print<T>
{
public:
	print()
	{
		utx::print("std::integral template");
	}
};

template <std::floating_point T>
class print<T>
{
public:
	print()
	{
		utx::print("std::floating_point template");
	}
};

int main()
{
	print<int>{}; // ok
	print<bool>{}; // ok
	print<utx::Pest>{}; // error
}

Compiling ERROR: gcc

error: invalid use of incomplete type ‘class print<utx::Pest>’

Compiling ERROR: clang

error: implicit instantiation of undefined template 'print<utx::Pest>'

Merged-concepts Primary Template

How to merge std::integral and std::floating_point concepts:

template <typename T>
concept real = std::integral<T> || std::floating_point<T>;

Good news is that utxcpp has already merged them:

utx::kspt::real_number

#include <concepts>
#include <utxcpp/core.hpp>

// Merged-concepts
template <utx::kspt::real_number T>
class print {};

template <std::integral T>
class print<T>
{
public:
	print()
	{
		utx::print("std::integral template");
	}
};

template <std::floating_point T>
class print<T>
{
public:
	print()
	{
		utx::print("std::floating_point template");
	}
};

int main()
{
	print<int>{}; // ok
	print<bool>{}; // ok
	print<utx::Pest>{}; // error
}

Compiling ERROR: gcc

error: template constraint failure for ‘template<class T>  requires  real_number<T> class print’

note: no operand of the disjunction is satisfied

cc1plus: note: set ‘-fconcepts-diagnostics-depth=’ to at least 2 for more detail

Compiling ERROR: clang

error: constraints not satisfied for class template 'print' [with T = utx::Pest]

note: because 'utx::Pest' does not satisfy 'real_number'

note: because 'utx::Pest' does not satisfy 'integral'

note: because 'is_integral_v<utx::Pest>' evaluated to false

note: and 'utx::Pest' does not satisfy 'floating_point'

note: because 'is_floating_point_v<utx::Pest>' evaluated to false

Conclusion

To check incorrect using templates, unimplemented primary template is more human-friendly than merged-concepts, and gcc is more human-friendly than clang (in such a case).