Fix union inference of generic class and its generic type

Author: roberfi

mypy/constraints.py

@@ -390,10 +390,24 @@ def _infer_constraints(
         # When the template is a union, we are okay with leaving some
         # type variables indeterminate. This helps with some special
         # cases, though this isn't very principled.
+
+        def _is_item_being_overlaped_by_other(item: Type) -> bool:
+            # It returns true if the item is an argument of other item
+            # that is subtype of the actual type
+            return any(
+                isinstance(p_type := get_proper_type(item_to_compare), Instance)
+                and mypy.subtypes.is_subtype(actual, erase_typevars(p_type))
+                and item in p_type.args
+                for item_to_compare in template.items
+                if item is not item_to_compare
+            )
+
         result = any_constraints(
             [
                 infer_constraints_if_possible(t_item, actual, direction)
-                for t_item in template.items
+                for t_item in [
+                    item for item in template.items if not _is_item_being_overlaped_by_other(item)
+                ]
             ],
             eager=False,
         )

test-data/unit/check-inference.test

@@ -873,13 +873,7 @@ def g(x: Union[T, List[T]]) -> List[T]: pass
 def h(x: List[str]) -> None: pass
 g('a')() # E: "List[str]" not callable
 
-# The next line is a case where there are multiple ways to satisfy a constraint
-# involving a Union. Either T = List[str] or T = str would turn out to be valid,
-# but mypy doesn't know how to branch on these two options (and potentially have
-# to backtrack later) and defaults to T = Never. The result is an
-# awkward error message. Either a better error message, or simply accepting the
-# call, would be preferable here.
-g(['a']) # E: Argument 1 to "g" has incompatible type "List[str]"; expected "List[Never]"
+g(['a'])
 
 h(g(['a']))
 
@@ -891,6 +885,28 @@ i(b, a, b)
 i(a, b, b) # E: Argument 1 to "i" has incompatible type "List[int]"; expected "List[str]"
 [builtins fixtures/list.pyi]
 
+[case testUnionInferenceOfGenericClassAndItsGenericType]
+from typing import Generic, TypeVar, Union
+
+T = TypeVar('T')
+
+class GenericClass(Generic[T]):
+    def __init__(self, value: T) -> None:
+        self.value = value
+
+def method_with_union(arg: Union[GenericClass[T], T]) -> GenericClass[T]:
+    if not isinstance(arg, GenericClass):
+        arg = GenericClass(arg)
+    return arg
+
+result_1 = method_with_union(GenericClass("test"))
+reveal_type(result_1) # N: Revealed type is "__main__.GenericClass[builtins.str]"
+
+result_2 = method_with_union("test")
+reveal_type(result_2) # N: Revealed type is "__main__.GenericClass[builtins.str]"
+
+[builtins fixtures/isinstance.pyi]
+
 [case testCallableListJoinInference]
 from typing import Any, Callable