[Bug fortran/81615] save-temps and gfortran produces *.f90 files instead of *.i or *i90 files

[sgk at troutmask dot apl.washington.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81615

--- Comment #15 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Sat, Dec 10, 2022 at 01:47:44AM +0000, barrowes at alum dot mit.edu wrote:
> 
> Thanks for engaging, and thanks for the suggestion. I might be able to do this
> over the winter. Could you give me a hint as to where to look. Which files.
> 

I suspect that you should start by understand gcc/fortran/lang-specs.h.
That's where most of the the filename suffixes are defined.  In particular,
this chuck of code

/* Identical to gcc.cc (cpp_options), but omitting %(cpp_unique_options)
   and -fpch-preprocess on -save-temps.  */
#define CPP_ONLY_OPTIONS    "%1 %{m*} %{f*} %{g*:%{!g0:%{g*} \
                             %{!fno-working-directory:-fworking-directory}}} \
                             %{std*&ansi&trigraphs} %{W*&pedantic*} %{w} \
                             %{O*} %{undef}"