[libc++][test] `nasty_char_traits::move` is incompatible with `constexpr`

[StephanTLavavej]

Found while running libc++'s test suite with MSVC's STL.

nasty_char_traits::move is marked constexpr but compares unrelated pointers s1 < s2. This is forbidden, and nasty_char_traits::copy acknowledges this immediately below:

llvm-project/libcxx/test/support/nasty_string.h

Lines 120 to 141 in 38f75d6

	constexpr nasty_char* nasty_char_traits::move(nasty_char* s1, const nasty_char* s2, std::size_t n) {
	nasty_char* r = s1;
	if (s1 < s2) {
	for (; n; --n, ++s1, ++s2)
	assign(*s1, *s2);
	} else if (s2 < s1) {
	s1 += n;
	s2 += n;
	for (; n; --n)
	assign(*--s1, *--s2);
	}
	return r;
	}
	constexpr nasty_char* nasty_char_traits::copy(nasty_char* s1, const nasty_char* s2, std::size_t n) {
	if (!std::is_constant_evaluated()) // fails in constexpr because we might be comparing unrelated pointers
	assert(s2 < s1 || s2 >= s1 + n);
	nasty_char* r = s1;
	for (; n; --n, ++s1, ++s2)
	assign(*s1, *s2);
	return r;
	}

Click to expand compiler error:

With libc++'s test suite, MSVC's STL, and Clang/LLVM, std/strings/basic.string/string.modifiers/string_append/initializer_list.pass.cpp emits this error:

D:\GitHub\STL\llvm-project\libcxx\test\std\strings\basic.string\string.modifiers\string_append\initializer_list.pass.cpp(53,17): error: static assertion expression is not an integral constant expression
  static_assert(test());
                ^~~~~~
D:\GitHub\STL\llvm-project\libcxx\test\support\nasty_string.h(122,10): note: comparison between '&s._Mypair._Myval2._Bx._Buf[3]' and '&{CharT(('a')), CharT(('b')), CharT(('c'))}[0]' has unspecified value
  if (s1 < s2) {
         ^
D:\GitHub\STL\out\x64\out\inc\xstring(3311,13): note: in call to 'move(&s._Mypair._Myval2._Bx._Buf[3], &{CharT(('a')), CharT(('b')), CharT(('c'))}[0], 3)'
            _Traits::move(_Old_ptr + _Old_size, _Ptr, _Count);
            ^
D:\GitHub\STL\out\x64\out\inc\xstring(3152,16): note: in call to '&s->append(&{CharT(('a')), CharT(('b')), CharT(('c'))}[0], 3)'
        return append(_Ilist.begin(), _Convert_size<size_type>(_Ilist.size()));
               ^
D:\GitHub\STL\llvm-project\libcxx\test\std\strings\basic.string\string.modifiers\string_append\initializer_list.pass.cpp(27,5): note: in call to '&s->append({&{CharT(('a')), CharT(('b')), CharT(('c'))}[0], &{CharT(('a')), CharT(('b')), CharT(('c'))}[3]})'
  s.append({CharT('a'), CharT('b'), CharT('c')});
    ^
D:\GitHub\STL\llvm-project\libcxx\test\std\strings\basic.string\string.modifiers\string_append\initializer_list.pass.cpp(44,3): note: in call to 'test()'
  test<nasty_string>();
  ^
D:\GitHub\STL\llvm-project\libcxx\test\std\strings\basic.string\string.modifiers\string_append\initializer_list.pass.cpp(53,17): note: in call to 'test()'
  static_assert(test());
                ^

In microsoft/STL's product code, I have a truly marvelous way to avoid this problem - a linear scan to detect whether the first iterator of the destination is within the source range, in which case a backward loop is necessary. See https://github.com/microsoft/STL/blob/0403d19f5461fd15983737c3f01ec34800ea9275/stl/inc/xstring#L85-L93 .