G4VGFlashSensitiveDetector.hh relies on G4VSensitiveDetector.hh already being included before - this is a missing include that results in a compiler error. /Users/nevay/physics/packages/geant4.10.02.p01-install/include/Geant4/G4VGFlashSensitiveDetector.hh:82:13: error: 'G4VSensitiveDetector' is an incomplete type = dynamic_cast<G4VSensitiveDetector *>(this); From a C++ point of view, the error is correct. G4VSensitiveDetector.hh should be included at the top of G4VGFlashSensitiveDetector.hh I believe to allow the dynamic cast. I'm on Mac OSX 10.11.4, Apple LLVM version 7.3.0 (clang-703.0.29), Geant4.10.2.p01. I found this when alphabetising the includes.
Thank you for reporting, this is indeed surprising that no compilers complained about this missing include. This has been fixed in: gflash-V10-02-00 It is mentioned here that the code involved is rather old, and will be reviewed.