App fails to start if executable's filename is longer than 128 characters

Somewhere between Qt 5.11.0 and Qt 5.12.5, executables with names longer than 128 characters stopped working: they'll show a generic Windows error dialog instead.

Steps to reproduce the issue:

1. create a new Qt Widgets App with Qt Creator (or open it from filename_length.zip ). Its main.cpp looks like this:

   #include "mainwindow.h"
   
   #include <QApplication>
   
   int main(int argc, char *argv[]) {
     QApplication a(argc, argv);
     MainWindow w;
     w.show();
     return a.exec();
   }
   

2. compile it with Qt 5.12.5.
3. copy Qt5Core, Qt5Gui, Qt5Widgets DLLs next to the executable.
4. make sure the executable starts fine.
5. rename executable to ___________________________________________________________________________________________________________________________x.exe – that is 123 underscores, one "x", and the filename extension. The total length is thus 128 characters.
6. try to start the executable and get a generic Windows error dialog:
   
   The details say:

   Сигнатура проблемы:
     Имя события проблемы:	BEX
     Имя приложения:	________________________________________________________________
     Версия приложения:	0.0.0.0
     Отметка времени приложения:	5dca5cba
     Имя модуля с ошибкой:	igdumdim32.dll
     Версия модуля с ошибкой:	10.18.14.5057
     Отметка времени модуля с ошибкой:	5b75a484
     Смещение исключения:	001599d2
     Код исключения:	c0000417
     Данные исключения:	00000000
     Версия ОС:	6.1.7601.2.1.0.256.48
     Код языка:	1049
     Дополнительные сведения 1:	bb5f
     Дополнительные сведения 2:	bb5ff5b63d0256cfc8d75f4881c993cb
     Дополнительные сведения 3:	fa22
     Дополнительные сведения 4:	fa22a9b38a3fac2a94e9385fb41b7142
   
   Ознакомьтесь с заявлением о конфиденциальности в Интернете:
     http://go.microsoft.com/fwlink/?linkid=104288&clcid=0x0419
   
   Если заявление о конфиденциальности в Интернете недоступно, ознакомьтесь с его локальным вариантом:
     C:\Windows\system32\ru-RU\erofflps.txt
   

   (I'm sorry it's in Russian, but the layout is probably the same across different languages. Exception code c0000417 didn't give me any clues about the source of the problem.)
   Note that the third line says "123 underscores" without the "x" – in other words, the filename was truncated.

7. open main.cpp and comment out the line saying w.show().
8. rename the newly compiled app again, start it, observe there is no error. (Since there is no main window displayed, I ran the app from Git Bash to make sure it is running.)

Further details:

• I can also reproduce this with Qt 5.12.5 64-bit
• I couldn't reproduce this with Qt 5.5.1 32-bit and 5.10.1 32-bit