[clang-tidy] Using an aliased name of a class for its copy/move/assignment constructors/operators produces a false positive cppcoreguidelines-special-member-functions warning.

[crud89]

The following class produces a false positive Wcppcoreguidelines-special-member-functions with LLVM 19.1.3:

template <typename T, unsigned ROWS, unsigned COLS> requires (ROWS >= 2 && COLS >= 2)
class Mat {
public:
	static constexpr size_t mat_rows = ROWS;
	static constexpr size_t mat_cols = COLS;

	using scalar_type = T;
	using mat_type = Mat<scalar_type, mat_rows, mat_cols>;

protected:
	using array_type = std::array<scalar_type, mat_rows* mat_cols>;

private:
	array_type m_elements = { };

public:
	constexpr Mat() noexcept = default;

	constexpr Mat(const mat_type& _other) noexcept {
		std::ranges::copy(_other.m_elements, std::begin(m_elements));
	}

	constexpr Mat(mat_type&& _other) noexcept :
		m_elements{ std::move(_other.m_elements) } {
	}

	virtual ~Mat() noexcept = default;

	constexpr auto& operator=(const mat_type& _other) noexcept {
		std::ranges::copy(_other.m_elements, std::begin(m_elements));
		return *this;
	}

	constexpr auto& operator=(mat_type&& _other) noexcept {
		m_elements = std::move(_other.m_elements);
		return *this;
	}
};

This happens due to the alias using mat_type = Mat<scalar_type, mat_rows, mat_cols>;. If I replace the alias mat_type in the constructors and assignment operators with the full class name, the warning goes away.

[chrchr-github]

Reduced:

template<int i>
struct S {
    static constexpr int n = i;
    using U = S<n>;
    S(const U&) = default;
    S(U&&) = default;
    U& operator=(const U& u) = default;
    U& operator=(U&& u) = default;
    virtual ~S() = default;
};

https://godbolt.org/z/3EP3Td9jT
There is an additional false positive hicpp-explicit-conversions.

[nicovank]

Note that this code does not compile on GCC.

<source>:5:19: error: 'S<i>::S(const U&)' cannot be defaulted [-Wtemplate-body]
    5 |     S(const U&) = default;
      |                   ^~~~~~~
<source>:6:14: error: 'S<i>::S(U&&)' cannot be defaulted [-Wtemplate-body]
    6 |     S(U&&) = default;
      |              ^~~~~~~
<source>:7:32: error: 'S<i>::U& S<i>::operator=(const U&)' cannot be defaulted [-Wtemplate-body]
    7 |     U& operator=(const U& u) = default;
      |                                ^~~~~~~
<source>:8:27: error: 'S<i>::U& S<i>::operator=(U&&)' cannot be defaulted [-Wtemplate-body]
    8 |     U& operator=(U&& u) = default;
      |                           ^~~~~~~

[chrchr-github]

MSVC rejects also. But the false positive still occurs when implementations are provided:
https://godbolt.org/z/Thq7bar6d

[nicovank]

Correct, but I think something fishy is going here where the check might technically be right in that no "move constructor" as defined in standard is provided. For example, in the example with implementations provided, you can add another overload S(const S&) {}, which will compile just fine until you use it. In practice everything should work fine because the correct overload is selected anyway.

I'm curious why Clang lets you = default it.