__builtin_is_replaceable issues

[jakubjelinek]

struct E trivially_relocatable_if_eligible replaceable_if_eligible {
  E (E &&);
  E &operator= (E &&) = default;
};

static_assert (__builtin_is_replaceable (E), "");

E &&foo ();

void
bar (E &y)
{
  E x = foo ();
  y = foo ();
}

This and also

struct F trivially_relocatable_if_eligible replaceable_if_eligible {
  F (F &&) = default;
  F &operator= (F &&);
};

struct G replaceable_if_eligible { G (G const &) = default; };

struct I replaceable_if_eligible { I &operator= (const I &) = default; };

static_assert (__builtin_is_replaceable (F), "");
static_assert (__builtin_is_replaceable (G), "");
static_assert (__builtin_is_replaceable (I), "");

is where my current GCC implementation disagrees with clang trunk.
Sure, the E and F classes are not default-movable, but all are replaceable_if_eligible, so that is irrelevant.
I believe all of them are eligible for replacement though. None of these have any non-static data members nor bases, so the first two bullets of https://eel.is/c++draft/class.prop#6 don't apply, none of these has deleted dtor.
For the first class I've tried to show in bar function that it can be successfully direct initialized from an xvalue of type E and it can be also assigned from an xvalue of type E.

[jakubjelinek]

@cor3ntin

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Jakub Jelínek (jakubjelinek)

```c struct E trivially_relocatable_if_eligible replaceable_if_eligible { E (E &&); E &operator= (E &&) = default; };

static_assert (__builtin_is_replaceable (E), "");

E &&foo ();

void
bar (E &y)
{
E x = foo ();
y = foo ();
}

This and also
```c
struct F trivially_relocatable_if_eligible replaceable_if_eligible {
  F (F &&) = default;
  F &operator= (F &&);
};

struct G replaceable_if_eligible { G (G const &) = default; };

struct I replaceable_if_eligible { I &operator= (const I &) = default; };

static_assert (__builtin_is_replaceable (F), "");
static_assert (__builtin_is_replaceable (G), "");
static_assert (__builtin_is_replaceable (I), "");

is where my current GCC implementation disagrees with clang trunk.
Sure, the E and F classes are not default-movable, but all are replaceable_if_eligible, so that is irrelevant.
I believe all of them are eligible for replacement though. None of these have any non-static data members nor bases, so the first two bullets of https://eel.is/c++draft/class.prop#6 don't apply, none of these has deleted dtor.
For the first class I've tried to show in bar function that it can be successfully direct initialized from an xvalue of type E and it can be also assigned from an xvalue of type E.

[jakubjelinek]

BTW, how do you plan at constant evaluation time check for whether std::relocate should iterate forward or backward?

#include <functional>

char a, b;
constexpr bool c = std::less{}(&a, &a + 1);
constexpr bool d = std::less{}(&a, &b);

fails with all of g++, clang++ -stdlib=libstdc++ and clang++ -stdlib=libc++, so the

if (less{}(result,first) || less{}(last,result)) {

check described in the paper will not work at constant evaluation time.

[frederick-vs-ja]

BTW, how do you plan at constant evaluation time check for whether std::relocate should iterate forward or backward?

I think we can use existing __is_pointer_in_range. Such technique seems also available for GCC (but not MSVC).

llvm-project/libcxx/include/__utility/is_pointer_in_range.h

Lines 35 to 48 in 80b79ce

	template <class _Tp, class _Up, __enable_if_t<__is_less_than_comparable<const _Tp*, const _Up*>::value, int> = 0>
	_LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_SANITIZE("address") bool
	__is_pointer_in_range(const _Tp* __begin, const _Tp* __end, const _Up* __ptr) {
	_LIBCPP_ASSERT_VALID_INPUT_RANGE(std::__is_valid_range(__begin, __end), "[__begin, __end) is not a valid range");
	if (__libcpp_is_constant_evaluated()) {
	// If this is not a constant during constant evaluation we know that __ptr is not part of the allocation where
	// [__begin, __end) is.
	if (!__builtin_constant_p(__begin <= __ptr && __ptr < __end))
	return false;
	}
	return !__less<>()(__ptr, __begin) && __less<>()(__ptr, __end);
	}