solver: test that we can build llvm%gcc

lib/spack/spack/solver/concretize.lp

@@ -748,6 +748,7 @@ do_not_impose(EffectID, node(X, Package))
    explicitly_requested_root(PossibleProvider),
    not self_build_requirement(PossibleProvider, ProviderNode),
    not explicitly_requested_root(ProviderNode),
+   not language(Virtual),
    internal_error("If a root can provide a virtual, it must be the provider").
 
 % A package cannot be the actual provider for a virtual if it does not

lib/spack/spack/test/concretization/core.py

@@ -3214,3 +3214,13 @@ def test_compiler_attribute_is_tolerated_in_externals(mutable_config, mock_packa
     mutable_config.set("packages", packages_yaml["packages"])
     s = spack.concretize.concretize_one("cmake")
     assert s.external and s.external_path == str(tmp_path)
+
+
+def test_compiler_can_be_built_with_other_compilers(config, mock_packages):
+    """Tests that a compiler can be built also with another compiler."""
+    s = Spec("llvm@18 +clang %gcc").concretized()
+    print(s.tree())
+    assert s.satisfies("llvm@18")
+
+    c_compiler = s.dependencies(virtuals=("c",))
+    assert len(c_compiler) == 1 and c_compiler[0].satisfies("gcc@10")

lib/spack/spack/test/data/config/packages.yaml

@@ -80,7 +80,6 @@ packages:
             cxx: /path/bin/g++-10
             fortran: /path/bin/gfortran-10
   llvm:
-    buildable: false
     externals:
       - spec: "llvm@15.0.0 +clang os={linux_os.name}{linux_os.version} target={target}"
         prefix: /path

lib/spack/spack/test/package_class.py

@@ -45,7 +45,7 @@ def mpileaks_possible_deps(mock_packages, mpi_names, compiler_names):
         "intel-parallel-studio": set(),
         "libdwarf": set(["libelf"] + compiler_names),
         "libelf": set(compiler_names),
-        "llvm": set(),
+        "llvm": {"gcc", "llvm"},
         "mpich": set(compiler_names),
         "mpich2": set(compiler_names),
         "mpileaks": set(["callpath"] + mpi_names + compiler_names),

var/spack/repos/builtin.mock/packages/llvm/package.py

@@ -19,6 +19,8 @@ class Llvm(Package, CompilerPackage):
 
     provides("c", "cxx", when="+clang")
 
+    depends_on("c")
+
     c_names = ["clang"]
     cxx_names = ["clang++"]
     fortran_names = ["flang"]