https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101399
--- Comment #2 from urbanjost at comcast dot net --- I agree tabs are not part of the standard, but even by default it is a very common extension ( I tried it with gfortran, ifort, nvfortran just today). I not not use the extension normally myself, but it came up in another context today and is an odd inconsistency. I ran 200 000, lines of Fortran through the ULS command unexpand(1) to create many source files with tabs in them and had no other problem with gfortran with tabs. I also took a small program with no long lines in it and replaced all spaces with tabs and had no problem compiling it with gfortran. This is not a high priority for me but seems like an odd inconsistency that would not be hard to rectify.
