Compiler dependencies are not being passed to dependencies if package doesn't depend on the language

[white238]

Steps to reproduce

I ran into an issue where I need to enforce the same compiler chain on all packages built by Spack. Spack lost the use of gfortran on my dependencies and went with just using flang, which I did not want. The important take away is the high level package does not depend on fortran but packages under it do. The quick fix was to add depends_on("fortran", type="build") to the high level package but that is not ideal.

Here is a minimal thing to show what is wrong:

Compiler toolchain:

  toolchains:
    clang_19:
    - spec: '%cxx=llvm'
      when: '%cxx'
    - spec: '%fortran=gcc'
      when: '%fortran'

Minimal Packages:

class TopLevel(CMakePackage):
    depends_on("lowlevel")
    depends_on("cxx", type="build")

class LowLevel(CMakePackage):
    depends_on("fortran", type="build")

Minimal concretization of showing the wrong fortran compiler:

==> Concretized 1 spec:
-   cjd6jhd  serac@develop fflags=-fPIC ~asan~cuda+devtools+enzyme~ipo+openmp~petsc+profiling+raja~rocm~shared~slepc+strumpack+sundials+tribol+umpire build_system=cmake build_type=Release dev_path=/serac/repo generator=make arch=linux-rhel8-ivybridge %c,cxx=clang@19.1.7
-   tnt5ej5      ^adiak@0.4.1 fflags=-fPIC ~ipo+mpi~shared build_system=cmake build_type=Release commit=7ac997111785bee6d9391664b1d18ebc2b3c557b generator=make arch=linux-rhel8-ivybridge %c,cxx,fortran=clang@19.1.7

@scheibelp

Error message

No response

Information on your system

• Spack: 1.1.0.dev0 (b4b1c5f)
• Builtin repo: spack/spack-packages@9c24df9
• Python: 3.9.12
• Platform: linux-rhel8-sapphirerapids

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output

[alalazo]

This is a known limitation, see #49696 for a proposal to overcome it

[white238]

#51135