| Issue |
58192
|
| Summary |
variant's operator<=> is missing a requires-clause
|
| Labels |
libc++,
c++20
|
| Assignees |
|
| Reporter |
CaseyCarter
|
[variant.syn] declares variant's spaceship operator as:
```c++
template<class... Types> requires (three_way_comparable<Types> && ...)
constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
operator<=>(const variant<Types...>&, const variant<Types...>&);
```
libc++ implements this operator, but without the _requires-clause_ `requires (three_way_comparable<Types> && ...)`:
https://github.com/llvm/llvm-project/blob/e2378d343828df73dd640fd1d1f5e02feb4ac0ef/libcxx/include/variant#L1641-L1643
The absence of the _requires-clause_ causes the operator to sometimes be present under conditions in which the standard specifies it should not be, such as a variant with an alternative type that implements `<=>` but not `==` (https://godbolt.org/z/f7Mh3vMxM):
```c++
struct X {
bool operator==(const X&) const = delete;
auto operator<=>(const X&) const = default;
};
template <class T>
concept can_spaceship = requires(const T& x, const T& y) {
x <=> y;
};
static_assert(!can_spaceship<std::variant<X>>);
```
The `static_assert` in this example program fails with libc++ but should pass. If fact, libc++ has a test case for this exact behavior:
https://github.com/llvm/llvm-project/blob/e2378d343828df73dd640fd1d1f5e02feb4ac0ef/libcxx/test/std/utilities/variant/variant.relops/three_way.pass.cpp#L174
which will also need to be fixed.
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