Steps to reproduce :

• create a manual compiler definition with a C++ and a C compiler, in a custom directory (/opt/gcc for example)
• create a kit using these compilers
• open a project, add this kit, set it to compile
• close qt creator
• rename the directory containing the kit (to mark it as temporary unavailable)
• open qt creator, open the project. Your compiler definition is no longer available, and the kit is broken. close qtcreator
• rename back the directory to /opt/gcc, so that compiler is available again
• open qt creator : compiler is still not available, completely disappeared. Kit is in error.

Real use case is compiler on a nfs mount. If the nfs mount becomes unavailable for some reason, qt loses the compiler definition.

QtCreator should not automatically remove compilers if they have the AutoDetect property set to false.