Trying to compile simple source file located in directory named "a!":

emcc.bat a!\a.cpp
emcc: error: a\a.cpp: No such file or directory ("a\a.cpp" was expected to be an input file, based on the commandline arguments provided)

Same with "!" in filename:

emcc.bat !!!a.cpp
emcc: error: a.cpp: No such file or directory ("a.cpp" was expected to be an input file, based on the commandline arguments provided)

Seems it just strips ! char from paths/filenames.