-P doesn't produce .i files in ICC

hi, i can't produce the preprocessed files with -P on my project compiling with ICC: https://github.com/riclas/rstm

in order to compile it with -P just change the line:
set(rstm_init_cxx_flags -Wall)
to: set(rstm_init_cxx_flags -Wall -P)
in file: cmake/UserOverrides.cmake
then follow the readme instructions, but with CC=icc and CXX=icpc

I've tried with a simple "hello world" and the .i file is produced, so this must be something related to the other compiler options of my project... ?

with GCC and -P -save-temps it creates the appropriate .ii files.