Tolerate compiler tokens in external specs

lib/spack/spack/solver/asp.py

@@ -2272,13 +2272,30 @@ def external_packages(self):
                     warnings.warn(f"cannot use the external spec {spec}: needs a concrete version")
                     continue
 
+                def external_requirement(input_spec, requirements):
+                    result = []
+                    for asp_fn in requirements:
+                        if asp_fn.args[0] == "depends_on":
+                            continue
+                        if asp_fn.args[1] != input_spec.name:
+                            continue
+                        result.append(asp_fn)
+                    return result
+
                 def external_imposition(input_spec, requirements):
-                    return requirements + [
-                        fn.attr("external_conditions_hold", input_spec.name, local_idx)
-                    ]
+                    result = []
+                    for asp_fn in requirements:
+                        if asp_fn.args[0] in ("depends_on", "build_requirement"):
+                            continue
+                        if asp_fn.args[1] != input_spec.name:
+                            continue
+                        result.append(asp_fn)
+                    result.append(fn.attr("external_conditions_hold", input_spec.name, local_idx))
+                    return result
 
                 try:
                     context = ConditionContext()
+                    context.transform_required = external_requirement
                     context.transform_imposed = external_imposition
                     self.condition(spec, spec, msg=msg, context=context)
                 except (spack.error.SpecError, RuntimeError) as e:

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

@@ -3196,3 +3196,21 @@ def test_compiler_can_depend_on_themselves_to_build(config, mock_packages):
 
     gcc_used_to_build = s.dependencies(name="gcc", virtuals=("c",))
     assert len(gcc_used_to_build) == 1 and gcc_used_to_build[0].satisfies("gcc@9.4.0")
+
+
+def test_compiler_attribute_is_tolerated_in_externals(mutable_config, mock_packages, tmp_path):
+    """Tests that we don't error out if an external specifies a compiler, even though externals
+    don't have dependencies.
+    """
+    packages_yaml = syaml.load_config(
+        f"""
+packages:
+  cmake:
+    externals:
+    - spec: "cmake@3.27.4 %gcc@14.1.0"
+      prefix: {tmp_path}
+"""
+    )
+    mutable_config.set("packages", packages_yaml["packages"])
+    s = spack.concretize.concretize_one("cmake")
+    assert s.external and s.external_path == str(tmp_path)