concretize: fix UnboundLocalError due to import within a function (#16809)

lib/spack/spack/spec.py

@@ -2134,6 +2134,8 @@ def concretize(self, tests=False):
         consistent with requirements of its packages. See flatten() and
         normalize() for more details on this.
         """
+        import spack.concretize
+
         if not self.name:
             raise spack.error.SpecError(
                 "Attempting to concretize anonymous spec")
@@ -2145,7 +2147,6 @@ def concretize(self, tests=False):
         force = False
 
         user_spec_deps = self.flat_dependencies(copy=False)
-        import spack.concretize
         concretizer = spack.concretize.Concretizer(self.copy())
         while changed:
             changes = (self.normalize(force, tests=tests,

lib/spack/spack/test/concretize.py

@@ -633,3 +633,8 @@ def test_compiler_version_matches_any_entry_in_compilers_yaml(self):
         s = Spec('mpileaks %gcc@4.5:')
         s.concretize()
         assert str(s.compiler.version) == '4.5.0'
+
+    def test_concretize_anonymous(self):
+        with pytest.raises(spack.error.SpecError):
+            s = Spec('+variant')
+            s.concretize()