It should work by modifying the qstring header file - simply remove the problematic ctor.
That's not a solution, that's a kludge. Changing the definition of a class means you now have a mismatch between Qt (which was built using the unmodified header) and application code (which was built with a different definition for QChar).

The /Zc:wchar_t vs. /Zc:wchar_t- issue is a fundamental difference and will affect every piece of code compiled and linked into the project, and will likely cause run-time errors as well. The correct approach would be to discover the source of the problem (which isn't in the Qt source code) and fix it there. Trust me, I've been there. You cannot mix and match the two language options. CHoose one or the other, and build -everything- to that choice.