diff --git a/mypy/constraints.py b/mypy/constraints.py
index d88b722aa1ce..531de8b198e1 100644
--- a/mypy/constraints.py
+++ b/mypy/constraints.py
@@ -276,7 +276,11 @@ def infer_constraints_for_callable(
def infer_constraints(
- template: Type, actual: Type, direction: int, skip_neg_op: bool = False
+ template: Type,
+ actual: Type,
+ direction: int,
+ skip_neg_op: bool = False,
+ can_have_union_overlapping: bool = True,
) -> list[Constraint]:
"""Infer type constraints.
@@ -316,11 +320,15 @@ def infer_constraints(
res = _infer_constraints(template, actual, direction, skip_neg_op)
type_state.inferring.pop()
return res
- return _infer_constraints(template, actual, direction, skip_neg_op)
+ return _infer_constraints(template, actual, direction, skip_neg_op, can_have_union_overlapping)
def _infer_constraints(
- template: Type, actual: Type, direction: int, skip_neg_op: bool
+ template: Type,
+ actual: Type,
+ direction: int,
+ skip_neg_op: bool,
+ can_have_union_overlapping: bool = True,
) -> list[Constraint]:
orig_template = template
template = get_proper_type(template)
@@ -383,13 +391,46 @@ def _infer_constraints(
return res
if direction == SUPERTYPE_OF and isinstance(actual, UnionType):
res = []
+
+ def _can_have_overlapping(_item: Type, _actual: UnionType) -> bool:
+ # There is a special overlapping case, where we have a Union of where two types
+ # are the same, but one of them contains the other.
+ # For example, we have Union[Sequence[T], Sequence[Sequence[T]]]
+ # In this case, only the second one can have overlapping because it contains the other.
+ # So, in case of list[list[int]], second one would be chosen.
+ if isinstance(p_item := get_proper_type(_item), Instance) and p_item.args:
+ other_items = [o_item for o_item in _actual.items if o_item is not a_item]
+
+ if len(other_items) == 1 and other_items[0] in p_item.args:
+ return True
+
+ if len(other_items) > 1:
+ union_args = [
+ p_arg
+ for arg in p_item.args
+ if isinstance(p_arg := get_proper_type(arg), UnionType)
+ ]
+
+ for union_arg in union_args:
+ if all(o_item in union_arg.items for o_item in other_items):
+ return True
+
+ return False
+
for a_item in actual.items:
# `orig_template` has to be preserved intact in case it's recursive.
# If we unwrapped ``type[...]`` previously, wrap the item back again,
# as ``type[...]`` can't be removed from `orig_template`.
if type_type_unwrapped:
a_item = TypeType.make_normalized(a_item)
- res.extend(infer_constraints(orig_template, a_item, direction))
+ res.extend(
+ infer_constraints(
+ orig_template,
+ a_item,
+ direction,
+ can_have_union_overlapping=_can_have_overlapping(a_item, actual),
+ )
+ )
return res
# Now the potential subtype is known not to be a Union or a type
@@ -410,10 +451,30 @@ 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_overlapping_actual_type(_item: Type) -> bool:
+ # Overlapping occurs when we have a Union where two types are
+ # compatible and the more generic one is chosen.
+ # For example, in Union[T, Sequence[T]], we have to choose
+ # Sequence[T] if actual type is list[int].
+ # This returns true if the item is an argument of other item
+ # that is subtype of the actual type
+ return any(
+ isinstance(p_item_to_compare := get_proper_type(item_to_compare), Instance)
+ and mypy.subtypes.is_subtype(actual, erase_typevars(p_item_to_compare))
+ and _item in p_item_to_compare.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 (can_have_union_overlapping and _is_item_overlapping_actual_type(item))
+ ]
],
eager=False,
)
diff --git a/test-data/unit/check-inference.test b/test-data/unit/check-inference.test
index cb0b11bf013c..0dc4d8ec5840 100644
--- a/test-data/unit/check-inference.test
+++ b/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 +885,53 @@ 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 testInferenceOfUnionOfGenericClassAndItsGenericType]
+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 testInferenceOfUnionOfSequenceOfAnyAndSequenceOfSequence]
+from typing import Sequence, Iterable, TypeVar, Union
+
+T = TypeVar("T")
+S = TypeVar("S")
+
+def sub_method(value: Union[S, Iterable[S]]) -> Iterable[S]:
+ pass
+
+def method(value: Union[Sequence[T], Sequence[Sequence[T]]]) -> None:
+ reveal_type(sub_method(value)) # N: Revealed type is "typing.Iterable[typing.Sequence[T`-1]]"
+
+[case testInferenceOfUnionOfUnionWithSequenceAndSequenceOfThatUnion]
+from typing import Sequence, Iterable, TypeVar, Union
+
+T = Union[str, Sequence[int]]
+S = TypeVar("S", bound=T)
+
+def sub_method(value: Union[S, Iterable[S]]) -> Iterable[S]:
+ pass
+
+def method(value: Union[T, Sequence[T]]) -> None:
+ reveal_type(sub_method(value)) # N: Revealed type is "typing.Iterable[Union[builtins.str, typing.Sequence[builtins.int]]]"
+
+
[case testCallableListJoinInference]
from typing import Any, Callable