Fix special cases where type context is a union (#2715)

This helps with the new definition of `dict.get` in typeshed.

The fix feels a little ad-hoc, but it seems to fix a real-world issue
and doesn't seem to break anything.

Fixes #2703.

mypy/constraints.py

@@ -2,10 +2,12 @@
 
 from typing import Iterable, List, Optional
 
+from mypy import experiments
 from mypy.types import (
     CallableType, Type, TypeVisitor, UnboundType, AnyType, Void, NoneTyp, TypeVarType,
     Instance, TupleType, TypedDictType, UnionType, Overloaded, ErasedType, PartialType,
-    DeletedType, UninhabitedType, TypeType, TypeVarId, is_named_instance
+    DeletedType, UninhabitedType, TypeType, TypeVarId, TypeQuery, ALL_TYPES_STRATEGY,
+    is_named_instance
 )
 from mypy.maptype import map_instance_to_supertype
 from mypy import nodes
@@ -149,13 +151,23 @@ def infer_constraints(template: Type, actual: Type,
     # be a supertype of the potential subtype, some item of the Union
     # must be a supertype of it.
     if direction == SUBTYPE_OF and isinstance(actual, UnionType):
+        # If some of items is not a complete type, disregard that.
+        items = simplify_away_incomplete_types(actual.items)
+        # We infer constraints eagerly -- try to find constraints for a type
+        # variable if possible. This seems to help with some real-world
+        # use cases.
         return any_constraints(
             [infer_constraints_if_possible(template, a_item, direction)
-             for a_item in actual.items])
+             for a_item in items],
+            eager=True)
     if direction == SUPERTYPE_OF and isinstance(template, UnionType):
+        # 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.
         return any_constraints(
             [infer_constraints_if_possible(t_item, actual, direction)
-             for t_item in template.items])
+             for t_item in template.items],
+            eager=False)
 
     # Remaining cases are handled by ConstraintBuilderVisitor.
     return template.accept(ConstraintBuilderVisitor(actual, direction))
@@ -177,12 +189,18 @@ def infer_constraints_if_possible(template: Type, actual: Type,
     return infer_constraints(template, actual, direction)
 
 
-def any_constraints(options: List[Optional[List[Constraint]]]) -> List[Constraint]:
-    """Deduce what we can from a collection of constraint lists given that
-    at least one of the lists must be satisfied. A None element in the
-    list of options represents an unsatisfiable constraint and is ignored.
+def any_constraints(options: List[Optional[List[Constraint]]], eager: bool) -> List[Constraint]:
+    """Deduce what we can from a collection of constraint lists.
+
+    It's a given that at least one of the lists must be satisfied. A
+    None element in the list of options represents an unsatisfiable
+    constraint and is ignored.  Ignore empty constraint lists if eager
+    is true -- they are always trivially satisfiable.
     """
-    valid_options = [option for option in options if option is not None]
+    if eager:
+        valid_options = [option for option in options if option]
+    else:
+        valid_options = [option for option in options if option is not None]
     if len(valid_options) == 1:
         return valid_options[0]
     # Otherwise, there are either no valid options or multiple valid options.
@@ -196,6 +214,34 @@ def any_constraints(options: List[Optional[List[Constraint]]]) -> List[Constrain
     # every option, combine the bounds with meet/join.
 
 
+def simplify_away_incomplete_types(types: List[Type]) -> List[Type]:
+    complete = [typ for typ in types if is_complete_type(typ)]
+    if complete:
+        return complete
+    else:
+        return types
+
+
+def is_complete_type(typ: Type) -> bool:
+    """Is a type complete?
+
+    A complete doesn't have uninhabited type components or (when not in strict
+    optional mode) None components.
+    """
+    return typ.accept(CompleteTypeVisitor())
+
+
+class CompleteTypeVisitor(TypeQuery):
+    def __init__(self) -> None:
+        super().__init__(default=True, strategy=ALL_TYPES_STRATEGY)
+
+    def visit_none_type(self, t: NoneTyp) -> bool:
+        return experiments.STRICT_OPTIONAL
+
+    def visit_uninhabited_type(self, t: UninhabitedType) -> bool:
+        return False
+
+
 class ConstraintBuilderVisitor(TypeVisitor[List[Constraint]]):
     """Visitor class for inferring type constraints."""
 

test-data/unit/check-inference-context.test

@@ -826,3 +826,42 @@ class A(Generic[T]):
     pass
 reveal_type(A()) # E: Revealed type is '__main__.A[builtins.None]'
 b = reveal_type(A())  # type: A[int] # E: Revealed type is '__main__.A[builtins.int]'
+
+[case testUnionWithGenericTypeItemContext]
+from typing import TypeVar, Union, List
+
+T = TypeVar('T')
+
+def f(x: Union[T, List[int]]) -> Union[T, List[int]]: pass
+reveal_type(f(1)) # E: Revealed type is 'Union[builtins.int*, builtins.list[builtins.int]]'
+reveal_type(f([])) # E: Revealed type is 'builtins.list[builtins.int]'
+reveal_type(f(None)) # E: Revealed type is 'builtins.list[builtins.int]'
+[builtins fixtures/list.pyi]
+
+[case testUnionWithGenericTypeItemContextAndStrictOptional]
+# flags: --strict-optional
+from typing import TypeVar, Union, List
+
+T = TypeVar('T')
+
+def f(x: Union[T, List[int]]) -> Union[T, List[int]]: pass
+reveal_type(f(1)) # E: Revealed type is 'Union[builtins.int*, builtins.list[builtins.int]]'
+reveal_type(f([])) # E: Revealed type is 'builtins.list[builtins.int]'
+reveal_type(f(None)) # E: Revealed type is 'Union[builtins.None, builtins.list[builtins.int]]'
+[builtins fixtures/list.pyi]
+
+[case testUnionWithGenericTypeItemContextInMethod]
+from typing import TypeVar, Union, List, Generic
+
+T = TypeVar('T')
+S = TypeVar('S')
+
+class C(Generic[T]):
+    def f(self, x: Union[T, S]) -> Union[T, S]: pass
+
+c = C[List[int]]()
+reveal_type(c.f('')) # E: Revealed type is 'Union[builtins.list[builtins.int], builtins.str*]'
+reveal_type(c.f([1])) # E: Revealed type is 'builtins.list[builtins.int]'
+reveal_type(c.f([])) # E: Revealed type is 'builtins.list[builtins.int]'
+reveal_type(c.f(None)) # E: Revealed type is 'builtins.list[builtins.int]'
+[builtins fixtures/list.pyi]