This is a cautionary tale. I did this myself and it took several days until I discovered the mistake. I wanted the Intel Compiler to handle .f95 as well as .f08 files from Visual Studio so I went into Tools/Options/Intel Visual Fortran/General and added these extensions to the Sources for free format files.
My projects compiled ok, until a Clean or Rebuild was issued (.obj & .mod files removed) and then it was a global disaster. Nothing worked anymore. My code would not link with my modules that were in .f90 files. When I opened .f90 files the Compile button that I had added to the UI was greyed out. I even removed and re-installed VS and Fortran.
In checking with the options again, I discovered my mistake: I had added the extension like ".f90,.f95,.f08" instead of ".f90;.f95;.f08". See the difference? Subtle but crutiial. I cought it when I expanded the node in the options and instead of putting the extensions in separate lines, or had them all in one line.
== Facepalm ==
