Bug#1032275: gcc-12-cross: gfortran-12-ARCH is missing Provides: virtual packages

[Dima Kogan <dkogan@debian.org>]

Package: gfortran-12-aarch64-linux-gnu
Severity: normal
X-Debbugs-Cc: debian-cross@lists.debian.org, Dima Kogan <dkogan@debian.org>
Control: affects 983600

Hi. This is the underlying cause of

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=983600

Installing libopenmpi-dev:foreign is impossible because it depends on
some virtual gfortran packages that the cross-compiler is not providing.
I see this:

  # dpkg --print-architecture
  amd64

  # dpkg --print-foreign-architectures
  arm64

  # apt install libopenmpi-dev:arm64
  ...
  The following packages have unmet dependencies:
   libopenmpi-dev:arm64 : Depends: gfortran-12:arm64 but it is not going to be installed or
                                   gfortran-mod-15:arm64

  # apt show libopenmpi-dev:arm64
  Package: libopenmpi-dev:arm64
  Depends: gfortran-12 | gfortran-mod-15, ...

So to install libopenmpi-dev:arm64 we need gfortran-mod-15. This is
provided by the native compiler:

  # apt show gfortran-12 | grep Provides
  Provides: fortran95-compiler, gfortran-mod-15

But not by the cross compiler:

  # apt show gfortran-12-aarch64-linux-gnu | grep Provides
  ---- nothing printed ----

Should the cross-compiler Provide this? Or is libopenmpi-dev wrong to
Depend on it?

Thanks