It seems that something changed with QSemaphore in Qt 6.10.0 for WASM:

• Since 6.10.0 ,QSemaphore is automatically included in my CPP file without an explicit include statement.
  So I assume that it is now included in another header file and indirectly included in my project
• Even if the QSemaphore class seems to be available the tryAcquire method is not defined and I get the "undefined symbol" error at linking time.

  undefined symbol: QSemaphore::tryAcquire(int)

It seems to me that there is a problem there because I would expect that either QSemaphore is not available at all or the methods are defined.

Unfortunately I could not find anything about this change in the release notes or in the documentation.

Note that everything was working fine with Qt 6.9.3 and previous: I was implementing my own dummy QSemaphore class to be used when compiling for WASM and using the original QSemaphore when compiling for Desktop.
But now this doesn't work any longer.