'QChar::QChar(ushort) noexcept': member function already defined or declared

[d_stranz]

Not exactly. The change you suggested only comes into play when someone -uses- the header containing the QChar constructor. In that case library still contains code with an inconsistent definition of wchar_t. The change to the Qt sources affects not only the use of the header but the library build itself so that everything is now internally and externally consistent.

In any case, if the size and layout of wchar_t and ushort are the same regardless of the compiler flag, it probably doesn't make any difference in fact. I do know that if you try to mix stdlib code (particularly std:: wstring) with QString code where Qt was compiled with a different version of the flag from the code that uses wstring, you get link-time errors because the mangled names of the functions do not match.

[ChristianEhrlicher]

It's an inline ctor, you won't find any code inside the Qt library for it...
/edit: therefore it will even work when you compile Qt with /Zc:wchar_t- and your lib with /Zc:wchar_t+ or the other way round.

[d_stranz]

OK, I guess because this is a non-virtual method, it is resolved statically at compile-time and thus has no effect on either the class layout or the vtable. If the library is compiled with the code in place, it will generate a reference to a method that is never called externally by a caller using code compiled with the constructor commented out.

It is still hard for me to believe you can mix and match code from Qt, an app, and third-party libraries using different values for the flag and not end up getting into trouble somewhere.