2 files changed, 47 insertions, 2 deletions
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index 619464727b..96225c0691 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -116,7 +116,8 @@ def extend_with_default(validator_class):
     def set_defaults(validator, properties, instance, schema):
         for property, subschema in iteritems(properties):
             if "default" in subschema:
-                instance.setdefault(property, subschema["default"])
+                instance.setdefault(
+                    property, copy.deepcopy(subschema["default"]))
         for err in validate_properties(
                 validator, properties, instance, schema):
             yield err
@@ -127,7 +128,7 @@ def extend_with_default(validator_class):
                 if isinstance(instance, dict):
                     for key, val in iteritems(instance):
                         if re.match(property, key) and val is None:
-                            instance[key] = subschema["default"]
+                            instance[key] = copy.deepcopy(subschema["default"])
 
         for err in validate_pattern_properties(
                 validator, properties, instance, schema):
diff --git a/lib/spack/spack/test/config.py b/lib/spack/spack/test/config.py
index f3a745ad17..5188af5465 100644
--- a/lib/spack/spack/test/config.py
+++ b/lib/spack/spack/test/config.py
@@ -167,6 +167,50 @@ config_override_list = {
         'build_stage:': ['patha', 'pathb']}}
 
 
+packages_merge_low = {
+    'packages': {
+        'foo': {
+            'variants': ['+v1']
+        },
+        'bar': {
+            'variants': ['+v2']
+        }
+    }
+}
+
+packages_merge_high = {
+    'packages': {
+        'foo': {
+            'version': ['a']
+        },
+        'bar': {
+            'version': ['b'],
+            'variants': ['+v3']
+        },
+        'baz': {
+            'version': ['c']
+        }
+    }
+}
+
+
+@pytest.mark.regression('7924')
+def test_merge_with_defaults(config, write_config_file):
+    """This ensures that specified preferences merge with defaults as
+       expected. Originally all defaults were initialized with the
+       exact same object, which led to aliasing problems. Therefore
+       the test configs used here leave 'version' blank for multiple
+       packages in 'packages_merge_low'.
+    """
+    write_config_file('packages', packages_merge_low, 'low')
+    write_config_file('packages', packages_merge_high, 'high')
+    cfg = spack.config.get_config('packages')
+
+    assert cfg['foo']['version'] == ['a']
+    assert cfg['bar']['version'] == ['b']
+    assert cfg['baz']['version'] == ['c']
+
+
 def check_compiler_config(comps, *compiler_names):
     """Check that named compilers in comps match Spack's config."""
     config = spack.config.get_config('compilers')