SL-16207: Update llstring.h handling of different string types.

In llpreprocessor.h, consider the case of clang on Windows: #define
LL_WCHAR_T_NATIVE there as well as for the Microsoft compiler with /Zc:wchar_t
switch.

In stdtypes.h, inject a LLWCHAR_IS_WCHAR_T symbol to allow the preprocessor to
make decisions about when the types are identical.

llstring.h's conversion logic deals with three types of wide strings
(LLWString, std::wstring and utf16string) based on three types of wide char
(llwchar, wchar_t and U16, respectively). Sometimes they're three distinct
types, sometimes wchar_t is identical to llwchar and sometimes wchar_t is
identical to U16. Rationalize the three cases using ll_convert_u16_alias() and
new ll_convert_wstr_alias() macros.

stringize.h was directly calling wstring_to_utf8str() and utf8str_to_wstring(),
which was producing errors with VS 2019 clang since there isn't actually a
wstring_to_utf8str(std::wstring) overload. Use ll_convert<std::string>()
instead, since that redirects to the relevant ll_convert_wide_to_string()
function. (And now you see why we've been trying to migrate to the uniform
ll_convert<target>() wrapper!) Similarly, call ll_convert<std::wstring>()
instead of a two-step conversion from utf8str_to_wstring(), producing LLWString,
then a character-by-character copy from LLWString to std::wstring. That
isn't even correct: on Windows, we should be encoding from UTF32 to UTF16.

1 files changed, 7 insertions, 0 deletions

diff --git a/indra/llcommon/stdtypes.h b/indra/llcommon/stdtypes.h
index 887f6ab733..b07805b628 100644
--- a/indra/llcommon/stdtypes.h
+++ b/indra/llcommon/stdtypes.h
@@ -42,10 +42,17 @@ typedef unsigned int			U32;
 // Windows wchar_t is 16-bit, whichever way /Zc:wchar_t is set. In effect,
 // Windows wchar_t is always a typedef, either for unsigned short or __wchar_t.
 // (__wchar_t, available either way, is Microsoft's native 2-byte wchar_t type.)
+// The version of clang available with VS 2019 also defines wchar_t as __wchar_t
+// which is also 16 bits.
 // In any case, llwchar should be a UTF-32 type.
 typedef U32				llwchar;
 #else
 typedef wchar_t				llwchar;
+// What we'd actually want is a simple module-scope 'if constexpr' to test
+// std::is_same<wchar_t, llwchar>::value and use that to define, or not
+// define, string conversion specializations. Since we don't have that, we'll
+// have to rely on #if instead. Sorry, Dr. Stroustrup.
+#define LLWCHAR_IS_WCHAR_T 1
 #endif
 
 #if LL_WINDOWS