[libc++][test] Update some wstring_convert tests for MSVC quirks


[libc++][test] Update some wstring_convert tests for MSVC quirks

Due to MSVC's decision to encode wchar_t as UTF-16, it rejects wide
character/string literals that expect a character value greater than
\xffff. UTF-16 wchar_t is clearly non-conforming, given that the
standard requires wchar_t to be capable of representing all characters
in the supported wide character execution sets, but rejecting e.g.
\x40003 is a reasonably sane compromise given that encoding choice:
there's an expectation that \xFOO produces a single character in the
resulting literal. Consequently L'\x40003'/L"\x40003" are ill-formed
literals on MSVC. L'\U00040003' is a high surrogate (and produces a
warning about ignoring the "second character" in a multi-character
literal), and L"\U00040003" is a perfectly-valid const wchar_t[3].

This change updates these tests to use universal-character-names instead
of raw values for the intended character values, which technically makes
them portable even to implementations that don't use a unicode
transformation format encoding for their wide character execution
character set. The two-character literal L"\u1005e" is awkward - the
e looks like part of the UCN's hex encoding - but necessary to compile
in '03 mode since '03 didn't allow UCNs to be used for members of the
basic execution character set even in character/string literals.

I've also eliminated the extraneous \x00 "bonus null-terminator" in
some of the string literals which doesn't affect the tested behavior.

I'm sorry about using *L"\U00040003" in conversions.string/to_bytes.pass.cpp,
but it's correct for platforms with 32-bit wchar_t, *and* doesn't
trigger narrowing warnings as did the prior CharT(0x40003).

Differential Revision: https://reviews.llvm.org/D60950


CaseyCarterApr 22 2019, 12:08 PM
Differential Revision
D60950: [libc++][test] Update some wstring_convert tests for MSVC quirks