[libc++][test] Non-rebindable `test_alloc` in string.capacity/deallocate_size.pass.cpp

[StephanTLavavej]

This test_alloc has a rebind struct, but not a rebinding constructor:

llvm-project/libcxx/test/std/strings/basic.string/string.capacity/deallocate_size.pass.cpp

Lines 22 to 46 in 564fd62

	template <class T, class Sz>
	struct test_alloc {
	typedef Sz size_type;
	typedef typename std::make_signed<Sz>::type difference_type;
	typedef T value_type;
	typedef value_type* pointer;
	typedef const value_type* const_pointer;
	typedef typename std::add_lvalue_reference<value_type>::type reference;
	typedef typename std::add_lvalue_reference<const value_type>::type const_reference;
	template <class U>
	struct rebind {
	typedef test_alloc<U, Sz> other;
	};
	TEST_CONSTEXPR_CXX14 pointer allocate(size_type n, const void* = nullptr) {
	allocated_ += n;
	return std::allocator<value_type>().allocate(n);
	}
	TEST_CONSTEXPR_CXX14 void deallocate(pointer p, size_type s) {
	allocated_ -= s;
	std::allocator<value_type>().deallocate(p, s);
	}
	};

This fails to meet the allocator requirements. MSVC's STL rejects this because we attempt to rebind the allocator (to an internal "container proxy" type).

I observe other requirement deficiencies here (no comparison operators) but the rebinding one is the specific problem for us.

[frederick-vs-ja]

When attempting to fix this, I realized that MSVC STL's "container proxy" objects can make allocated_ have different values when they are present... I wish assert(allocated_ == 0 || allocated_ >= i) passes as the "different value" should be larger than normal.