Display note when having invariants in Union (#6877)

Fixes #5895

Author: Rémi Lapeyre

mypy/messages.py

@@ -628,8 +628,13 @@ def incompatible_argument(self, n: int, m: int, callee: CallableType, arg_type:
             msg = 'Argument {} {}has incompatible type {}; expected {}'.format(
                 arg_label, target, self.quote_type_string(arg_type_str),
                 self.quote_type_string(expected_type_str))
-            if isinstance(arg_type, Instance) and isinstance(expected_type, Instance):
-                notes = append_invariance_notes(notes, arg_type, expected_type)
+            if isinstance(expected_type, UnionType):
+                expected_types = expected_type.items
+            else:
+                expected_types = [expected_type]
+            for type in expected_types:
+                if isinstance(arg_type, Instance) and isinstance(type, Instance):
+                    notes = append_invariance_notes(notes, arg_type, type)
         self.fail(msg, context)
         if notes:
             for note_msg in notes:

test-data/unit/check-unions.test

@@ -992,3 +992,18 @@ def union_test3():
     return x + 1
 
 [builtins fixtures/isinstancelist.pyi]
+
+[case testInvariance]
+from typing import List, Union
+from enum import Enum
+
+class Boop(Enum):
+    FOO = 1
+
+def do_thing_with_enums(enums: Union[List[Enum], Enum]) -> None: ...
+
+boop: List[Boop] = []
+do_thing_with_enums(boop)  # E: Argument 1 to "do_thing_with_enums" has incompatible type "List[Boop]"; expected "Union[List[Enum], Enum]" \
+                           # N: "List" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance \
+                           # N: Consider using "Sequence" instead, which is covariant
+[builtins fixtures/isinstancelist.pyi]