Fix Type Comparison Edge Cases

mypy/checker.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import functools
 import itertools
 from collections import defaultdict
 from collections.abc import Iterable, Iterator, Mapping, Sequence, Set as AbstractSet
@@ -47,7 +48,12 @@
 from mypy.expandtype import expand_type
 from mypy.literals import Key, extract_var_from_literal_hash, literal, literal_hash
 from mypy.maptype import map_instance_to_supertype
-from mypy.meet import is_overlapping_erased_types, is_overlapping_types, meet_types
+from mypy.meet import (
+    is_overlapping_erased_types,
+    is_overlapping_types,
+    meet_types,
+    narrow_declared_type,
+)
 from mypy.message_registry import ErrorMessage
 from mypy.messages import (
     SUGGESTED_TEST_FIXTURES,
@@ -6237,65 +6243,89 @@ def is_type_call(expr: CallExpr) -> bool:
 
         # exprs that are being passed into type
         exprs_in_type_calls: list[Expression] = []
-        # type that is being compared to type(expr)
-        type_being_compared: list[TypeRange] | None = None
-        # whether the type being compared to is final
+        # all the types that an expression will have if the overall expression is truthy
+        target_types: list[list[TypeRange]] = []
+        # only a single type can be used when passed directly (eg "str")
+        fixed_type: Type | None = None
+        # is this single type final?
         is_final = False
 
+        def update_fixed_type(new_fixed_type: Type, new_is_final: bool) -> bool:
+            """Returns if the update succeeds"""
+            nonlocal fixed_type, is_final
+            if update := (fixed_type is None or (is_same_type(new_fixed_type, fixed_type))):
+                fixed_type = new_fixed_type
+                is_final = new_is_final
+            return update
+
         for index in expr_indices:
             expr = node.operands[index]
+            proper_type = get_proper_type(self.lookup_type(expr))
 
             if isinstance(expr, CallExpr) and is_type_call(expr):
-                exprs_in_type_calls.append(expr.args[0])
-            else:
-                current_type = self.get_isinstance_type(expr)
-                if current_type is None:
-                    continue
-                if type_being_compared is not None:
-                    # It doesn't really make sense to have several types being
-                    # compared to the output of type (like type(x) == int == str)
-                    # because whether that's true is solely dependent on what the
-                    # types being compared are, so we don't try to narrow types any
-                    # further because we can't really get any information about the
-                    # type of x from that check
-                    return {}, {}
-                else:
-                    if isinstance(expr, RefExpr) and isinstance(expr.node, TypeInfo):
-                        is_final = expr.node.is_final
-                    type_being_compared = current_type
+                arg = expr.args[0]
+                exprs_in_type_calls.append(arg)
+            elif (
+                isinstance(expr, OpExpr)
+                or isinstance(proper_type, TupleType)
+                or is_named_instance(proper_type, "builtins.tuple")
+            ):
+                # not valid for type comparisons, but allowed for isinstance checks
+                fixed_type = UninhabitedType()
+                continue
+
+            type_range = self.get_isinstance_type(expr)
+            if type_range is not None:
+                target_types.append(type_range)
+                if (
+                    isinstance(expr, RefExpr)
+                    and isinstance(expr.node, TypeInfo)
+                    and len(type_range) == 1
+                ):
+                    if not update_fixed_type(
+                        Instance(
+                            expr.node,
+                            [AnyType(TypeOfAny.special_form)] * len(expr.node.defn.type_vars),
+                        ),
+                        expr.node.is_final,
+                    ):
+                        return None, {}
 
         if not exprs_in_type_calls:
             return {}, {}
 
-        if_maps: list[TypeMap] = []
-        else_maps: list[TypeMap] = []
+        if_maps = []
+        else_maps = []
         for expr in exprs_in_type_calls:
-            current_if_type, current_else_type = self.conditional_types_with_intersection(
-                self.lookup_type(expr), type_being_compared, expr
-            )
-            current_if_map, current_else_map = conditional_types_to_typemaps(
-                expr, current_if_type, current_else_type
-            )
-            if_maps.append(current_if_map)
-            else_maps.append(current_else_map)
+            expr_type: Type = get_proper_type(self.lookup_type(expr))
+            for type_range in target_types:
+                restriction, _ = self.conditional_types_with_intersection(
+                    expr_type, type_range, expr
+                )
+                if restriction is not None:
+                    narrowed_type = get_proper_type(narrow_declared_type(expr_type, restriction))
+                    # Cannot be guaranteed that this is unreachable, so use fallback type.
+                    if isinstance(narrowed_type, UninhabitedType):
+                        expr_type = restriction
+                    else:
+                        expr_type = narrowed_type
+                _, else_map = conditional_types_to_typemaps(
+                    expr,
+                    *self.conditional_types_with_intersection(
+                        (self.lookup_type(expr)), (type_range), expr
+                    ),
+                )
+                else_maps.append(else_map)
 
-        def combine_maps(list_maps: list[TypeMap]) -> TypeMap:
-            """Combine all typemaps in list_maps into one typemap"""
-            if all(m is None for m in list_maps):
-                return None
-            result_map = {}
-            for d in list_maps:
-                if d is not None:
-                    result_map.update(d)
-            return result_map
-
-        if_map = combine_maps(if_maps)
-        # type(x) == T is only true when x has the same type as T, meaning
-        # that it can be false if x is an instance of a subclass of T. That means
-        # we can't do any narrowing in the else case unless T is final, in which
-        # case T can't be subclassed
+            if fixed_type and expr_type is not None:
+                expr_type = narrow_declared_type(expr_type, fixed_type)
+
+            if_map, _ = conditional_types_to_typemaps(expr, expr_type, None)
+            if_maps.append(if_map)
+
+        if_map = functools.reduce(and_conditional_maps, if_maps)
         if is_final:
-            else_map = combine_maps(else_maps)
+            else_map = functools.reduce(or_conditional_maps, else_maps)
         else:
             else_map = {}
         return if_map, else_map
@@ -7039,7 +7069,6 @@ def refine_away_none_in_comparison(
         if_map, else_map = {}, {}
 
         if not non_optional_types or (len(non_optional_types) != len(chain_indices)):
-
             # Narrow e.g. `Optional[A] == "x"` or `Optional[A] is "x"` to `A` (which may be
             # convenient but is strictly not type-safe):
             for i in narrowable_operand_indices:
@@ -7961,35 +7990,41 @@ def get_isinstance_type(self, expr: Expression) -> list[TypeRange] | None:
                 return None
             return left + right
         all_types = get_proper_types(flatten_types(self.lookup_type(expr)))
-        types: list[TypeRange] = []
+        type_ranges: list[TypeRange] = []
         for typ in all_types:
-            if isinstance(typ, FunctionLike) and typ.is_type_obj():
-                # If a type is generic, `isinstance` can only narrow its variables to Any.
-                any_parameterized = fill_typevars_with_any(typ.type_object())
-                # Tuples may have unattended type variables among their items
-                if isinstance(any_parameterized, TupleType):
-                    erased_type = erase_typevars(any_parameterized)
-                else:
-                    erased_type = any_parameterized
-                types.append(TypeRange(erased_type, is_upper_bound=False))
-            elif isinstance(typ, TypeType):
-                # Type[A] means "any type that is a subtype of A" rather than "precisely type A"
-                # we indicate this by setting is_upper_bound flag
-                is_upper_bound = True
-                if isinstance(typ.item, NoneType):
-                    # except for Type[None], because "'NoneType' is not an acceptable base type"
-                    is_upper_bound = False
-                types.append(TypeRange(typ.item, is_upper_bound=is_upper_bound))
-            elif isinstance(typ, Instance) and typ.type.fullname == "builtins.type":
-                object_type = Instance(typ.type.mro[-1], [])
-                types.append(TypeRange(object_type, is_upper_bound=True))
-            elif isinstance(typ, Instance) and typ.type.fullname == "types.UnionType" and typ.args:
-                types.append(TypeRange(UnionType(typ.args), is_upper_bound=False))
-            elif isinstance(typ, AnyType):
-                types.append(TypeRange(typ, is_upper_bound=False))
-            else:  # we didn't see an actual type, but rather a variable with unknown value
+            type_range = self.isinstance_type_range(typ)
+            if type_range is None:
                 return None
-        return types
+            type_ranges.append(type_range)
+        return type_ranges
+
+    def isinstance_type_range(self, typ: ProperType) -> TypeRange | None:
+        if isinstance(typ, FunctionLike) and typ.is_type_obj():
+            # If a type is generic, `isinstance` can only narrow its variables to Any.
+            any_parameterized = fill_typevars_with_any(typ.type_object())
+            # Tuples may have unattended type variables among their items
+            if isinstance(any_parameterized, TupleType):
+                erased_type = erase_typevars(any_parameterized)
+            else:
+                erased_type = any_parameterized
+            return TypeRange(erased_type, is_upper_bound=False)
+        elif isinstance(typ, TypeType):
+            # Type[A] means "any type that is a subtype of A" rather than "precisely type A"
+            # we indicate this by setting is_upper_bound flag
+            is_upper_bound = True
+            if isinstance(typ.item, NoneType):
+                # except for Type[None], because "'NoneType' is not an acceptable base type"
+                is_upper_bound = False
+            return TypeRange(typ.item, is_upper_bound=is_upper_bound)
+        elif isinstance(typ, Instance) and typ.type.fullname == "builtins.type":
+            object_type = Instance(typ.type.mro[-1], [])
+            return TypeRange(object_type, is_upper_bound=True)
+        elif isinstance(typ, Instance) and typ.type.fullname == "types.UnionType" and typ.args:
+            return TypeRange(UnionType(typ.args), is_upper_bound=False)
+        elif isinstance(typ, AnyType):
+            return TypeRange(typ, is_upper_bound=False)
+        else:  # we didn't see an actual type, but rather a variable with unknown value
+            return None
 
     def is_literal_enum(self, n: Expression) -> bool:
         """Returns true if this expression (with the given type context) is an Enum literal.

mypy/subtypes.py

@@ -8,6 +8,7 @@
 import mypy.applytype
 import mypy.constraints
 import mypy.typeops
+from mypy.checker_shared import TypeRange
 from mypy.checker_state import checker_state
 from mypy.erasetype import erase_type
 from mypy.expandtype import (
@@ -255,6 +256,28 @@ def is_equivalent(
     )
 
 
+def is_same_type_ranges(
+    a: list[TypeRange],
+    b: list[TypeRange],
+    ignore_promotions: bool = True,
+    subtype_context: SubtypeContext | None = None,
+) -> bool:
+    return len(a) == len(b) and all(
+        is_same_type_range(x, y, ignore_promotions, subtype_context) for x, y in zip(a, b)
+    )
+
+
+def is_same_type_range(
+    a: TypeRange,
+    b: TypeRange,
+    ignore_promotions: bool = True,
+    subtype_context: SubtypeContext | None = None,
+) -> bool:
+    return a.is_upper_bound == b.is_upper_bound and is_same_type(
+        a.item, b.item, ignore_promotions, subtype_context
+    )
+
+
 def is_same_type(
     a: Type, b: Type, ignore_promotions: bool = True, subtype_context: SubtypeContext | None = None
 ) -> bool:

test-data/unit/check-isinstance.test

@@ -2716,23 +2716,81 @@ if type(x) == type(y) == int:
     reveal_type(y) # N: Revealed type is "builtins.int"
     reveal_type(x) # N: Revealed type is "builtins.int"
 
+z: Any
+if int == type(z) == int:
+    reveal_type(z) # N: Revealed type is "builtins.int"
+
 [case testTypeEqualsCheckUsingIs]
 from typing import Any
 
 y: Any
 if type(y) is int:
     reveal_type(y) # N: Revealed type is "builtins.int"
 
+[case testTypeEqualsCheckUsingImplicitTypes]
+from typing import Any
+
+x: str
+y: Any
+z: object
+if type(y) is type(x):
+    reveal_type(x) # N: Revealed type is "builtins.str"
+    reveal_type(y) # N: Revealed type is "builtins.str"
+
+if type(x) is type(z):
+    reveal_type(x) # N: Revealed type is "builtins.str"
+    reveal_type(z) # N: Revealed type is "builtins.str"
+
+[case testTypeEqualsCheckUsingDifferentSpecializedTypes]
+from collections import defaultdict
+
+x: defaultdict
+y: dict
+z: object
+if type(x) is type(y) is type(z):
+    reveal_type(x) # N: Revealed type is "collections.defaultdict[Any, Any]"
+    reveal_type(y) # N: Revealed type is "collections.defaultdict[Any, Any]"
+    reveal_type(z) # N: Revealed type is "collections.defaultdict[Any, Any]"
+
+[case testUnionTypeEquality]
+from typing import Any, reveal_type
+# flags: --warn-unreachable
+
+x: Any = ()
+if type(x) == (int, str):
+    reveal_type(x) # E: Statement is unreachable
+
+[builtins fixtures/tuple.pyi]
+
+[case testTypeIntersectionWithConcreteTypes]
+class X: x = 1
+class Y: y = 1
+class Z(X, Y): ...
+
+z = Z()
+x: X = z
+y: Y = z
+if type(x) is type(y):
+    reveal_type(x)  # N: Revealed type is "__main__.<subclass of "__main__.X" and "__main__.Y">"
+    reveal_type(y)  # N: Revealed type is "__main__.<subclass of "__main__.Y" and "__main__.X">"
+    x.y + y.x
+
+if isinstance(x, type(y)) and isinstance(y, type(x)):
+    reveal_type(x)  # N: Revealed type is "__main__.<subclass of "__main__.X" and "__main__.Y">"
+    reveal_type(y)  # N: Revealed type is "__main__.<subclass of "__main__.X" and "__main__.Y">"
+    x.y + y.x
+
+[builtins fixtures/isinstance.pyi]
+
 [case testTypeEqualsCheckUsingIsNonOverlapping]
 # flags: --warn-unreachable
 from typing import Union
 
 y: str
-if type(y) is int:  # E: Subclass of "str" and "int" cannot exist: would have incompatible method signatures
-    y # E: Statement is unreachable
+if type(y) is int:
+    y
 else:
     reveal_type(y) # N: Revealed type is "builtins.str"
-[builtins fixtures/isinstance.pyi]
 
 [case testTypeEqualsCheckUsingIsNonOverlappingChild-xfail]
 # flags: --warn-unreachable
@@ -2761,12 +2819,13 @@ else:
 
 [case testTypeEqualsMultipleTypesShouldntNarrow]
 # make sure we don't do any narrowing if there are multiple types being compared
+# flags: --warn-unreachable
 
 from typing import Union
 
 x: Union[int, str]
 if type(x) == int == str:
-    reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.str]"
+    reveal_type(x)  # E: Statement is unreachable
 else:
     reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.str]"
 

test-data/unit/check-narrowing.test

@@ -1262,7 +1262,7 @@ def f(t: Type[C]) -> None:
     if type(t) is M:
         reveal_type(t)  # N: Revealed type is "type[__main__.C]"
     else:
-        reveal_type(t)  # N: Revealed type is "type[__main__.C]"
+       reveal_type(t)  # N: Revealed type is "type[__main__.C]"
     if type(t) is not M:
         reveal_type(t)  # N: Revealed type is "type[__main__.C]"
     else: