Attached an example simply created from project template. Place a breakpoint in the first line in Main function, run the code in debug mode with CDB, press F10 to continue, QtC will freeze when it reach the code line that has a comment in Japanese.

If you terminate the cdb process from task manager, QtC will go back to normal and you can launch debugger again. If you remove the comment, QtC will not freeze. 

Qt Version used, 5.12.9 and 5.15.1

CDB version, 10.0.17763.132  64b version 

Windows version, Windows 10

Compiler, MSVC 2019