improve error msg when handling OmegaConfBaseException in config_loader

hydra/_internal/config_loader_impl.py

@@ -13,7 +13,6 @@
     ConfigAttributeError,
     ConfigKeyError,
     OmegaConfBaseException,
-    ValidationError,
 )
 
 from hydra._internal.config_repository import (
@@ -517,9 +516,9 @@ def _compose_config_from_defaults_list(
                 loaded = self._load_single_config(default=default, repo=repo)
                 try:
                     cfg.merge_with(loaded.config)
-                except ValidationError as e:
+                except OmegaConfBaseException as e:
                     raise ConfigCompositionException(
-                        f"In '{default.config_path}': Validation error while composing config:\n{e}"
+                        f"In '{default.config_path}': {type(e).__name__} raised while composing config:\n{e}"
                     ).with_traceback(sys.exc_info()[2])
 
         # # remove remaining defaults lists from all nodes.

hydra/test_utils/configs/schema_key_error.yaml

@@ -0,0 +1,5 @@
+defaults:
+  - base_mysql
+  - _self_
+
+foo: not_in_schema

hydra/test_utils/configs/schema_validation_error.yaml

@@ -0,0 +1,5 @@
+defaults:
+  - base_mysql
+  - _self_
+
+port: not_an_int

news/1697.feature

@@ -0,0 +1 @@
+Improve error message with more context when an omegaconf exception occurs during the config merge step.

tests/test_config_loader.py

@@ -312,6 +312,56 @@ def test_load_config_file_with_schema_validation(
             },
         }
 
+    def test_load_config_with_validation_error(
+        self, hydra_restore_singletons: Any, path: str
+    ) -> None:
+
+        ConfigStore.instance().store(
+            name="base_mysql", node=MySQLConfig, provider="this_test"
+        )
+        config_loader = ConfigLoaderImpl(
+            config_search_path=create_config_search_path(path)
+        )
+
+        msg = dedent(
+            """\
+            In 'schema_validation_error': ValidationError raised while composing config:
+            Value 'not_an_int' could not be converted to Integer
+                full_key: port
+                object_type=MySQLConfig"""
+        )
+        with raises(ConfigCompositionException, match=re.escape(msg)):
+            config_loader.load_configuration(
+                config_name="schema_validation_error",
+                overrides=[],
+                run_mode=RunMode.RUN,
+            )
+
+    def test_load_config_with_key_error(
+        self, hydra_restore_singletons: Any, path: str
+    ) -> None:
+
+        ConfigStore.instance().store(
+            name="base_mysql", node=MySQLConfig, provider="this_test"
+        )
+        config_loader = ConfigLoaderImpl(
+            config_search_path=create_config_search_path(path)
+        )
+
+        msg = dedent(
+            """\
+            In 'schema_key_error': ConfigKeyError raised while composing config:
+            Key 'foo' not in 'MySQLConfig'
+                full_key: foo
+                object_type=MySQLConfig"""
+        )
+        with raises(ConfigCompositionException, match=re.escape(msg)):
+            config_loader.load_configuration(
+                config_name="schema_key_error",
+                overrides=[],
+                run_mode=RunMode.RUN,
+            )
+
     def test_assign_null(self, hydra_restore_singletons: Any, path: str) -> None:
         config_loader = ConfigLoaderImpl(
             config_search_path=create_config_search_path(path)