`<sstream>`, `<syncstream>`: Buffer types assume that allocated pointers are not modified by users

[frederick-vs-ja]

Describe the bug

Currently, MSVC STL's basic_stringbuf relies on that the eback() pointer points to the beginning of the allocated buffer.

STL/stl/inc/sstream

Lines 495 to 497 in e077eb6

	_Al.deallocate(_Ptr_traits::pointer_to(*_Mysb::eback()),
	static_cast<typename allocator_traits<allocator_type>::size_type>(
	(_Mysb::pptr() ? _Mysb::epptr() : _Mysb::egptr()) - _Mysb::eback()));

Likewise, basic_syncbuf requires pbase() to behave the same way.

STL/stl/inc/syncstream

Line 247 in e077eb6

_Getal().deallocate(_Refancy<_Pointer>(streambuf_type::pbase()), _Buf_size);

However, the pointers returned by these functions can be modified by users via setg/setp functions, which means these conditions can't be considered as invariants maintained by the implementation.

Command-line test case

STL/tests/libcxx/expected_results.txt

Lines 1103 to 1107 in e077eb6

	# Not analyzed. Assertion failed: _CrtIsValidHeapPointer(block)
	std/input.output/string.streams/stringbuf/stringbuf.members/str.pass.cpp FAIL
	std/input.output/string.streams/stringbuf/stringbuf.members/view.pass.cpp FAIL
	std/input.output/syncstream/syncbuf/syncstream.syncbuf.cons/dtor.pass.cpp FAIL
	std/input.output/syncstream/syncbuf/syncstream.syncbuf.members/emit.pass.cpp FAIL

Expected behavior

These tests pass.

STL version

e077eb6

(Probably in all versions where basic_stringbuf or basic_syncbuf is available.)

Additional context

Not sure whether this can be fixed without breaking ABI. #4239 may be related, but doesn't fix any part of this issue.