Keep TypeVar arguments when narrowing generic subclasses with `isin…

…stance` and `issubclass`.

mypy/checker.py

@@ -7454,7 +7454,10 @@ def conditional_types(
                 ]
             )
             remaining_type = restrict_subtype_away(current_type, proposed_precise_type)
-            return proposed_type, remaining_type
+            proposed_type_with_data = _transfer_type_var_args_from_current_to_proposed(
+                current_type, proposed_type
+            )
+            return proposed_type_with_data, remaining_type
     else:
         # An isinstance check, but we don't understand the type
         return current_type, default
@@ -7478,6 +7481,116 @@ def conditional_types_to_typemaps(
     return cast(Tuple[TypeMap, TypeMap], tuple(maps))
 
 
+def _transfer_type_var_args_from_current_to_proposed(current: Type, proposed: Type) -> Type:
+    """Check if the current type is among the bases of the proposed type.  If so, try to transfer
+    the type variable arguments of the current type's instance to a copy of the proposed type's
+    instance.  This increases information when narrowing generic classes so that, for example,
+    Sequence[int] is narrowed to List[int] instead of List[Any]."""
+
+    def _get_instance_path_from_current_to_proposed(
+        this: Instance, target: TypeInfo
+    ) -> list[Instance] | None:
+        """Search for the current type among the bases of the proposed type and return the
+        "instance path" from the current to proposed type.  Or None, if the current type is not a
+        nominal super type. At most one path is returned, which means there is no special handling
+        of (inconsistent) multiple inheritance."""
+        if target == this.type:
+            return [this]
+        for base in this.type.bases:
+            path = _get_instance_path_from_current_to_proposed(base, target)
+            if path is not None:
+                path.append(this)
+                return path
+        return None
+
+    # Handle "tuple of Instance" cases, e.g. `isinstance(x, (A, B))`:
+    proposed = get_proper_type(proposed)
+    if isinstance(proposed, UnionType):
+        items = [
+            _transfer_type_var_args_from_current_to_proposed(current, item)
+            for item in flatten_nested_unions(proposed.items)
+        ]
+        return make_simplified_union(items)
+
+    # Otherwise handle only Instances:
+    if not isinstance(proposed, Instance):
+        return proposed
+
+    # Handle union cases like `a: A[int] | A[str]; isinstance(a, B)`:
+    current = get_proper_type(current)
+    if isinstance(current, UnionType):
+        items = [
+            _transfer_type_var_args_from_current_to_proposed(item, proposed)
+            for item in flatten_nested_unions(current.items)
+        ]
+        return make_simplified_union(items)
+
+    # Here comes the main logic:
+    if isinstance(current, Instance):
+
+        # Only consider nominal subtyping:
+        instances = _get_instance_path_from_current_to_proposed(proposed, current.type)
+        if instances is None:
+            return proposed
+        assert len(instances) > 0  # shortest case: proposed type is current type
+
+        # Make a list of the proposed type's type variable arguments that allows to replace each
+        # `Any` with one type variable argument or multiple type variable tuple arguments of the
+        # current type:
+        proposed_args: list[Type | tuple[Type, ...]] = list(proposed.args)
+
+        # Try to transfer each type variable argument from the current to the base type separately:
+        for pos1, typevar1 in enumerate(instances[0].args):
+            if isinstance(typevar1, UnpackType):
+                typevar1 = typevar1.type
+            if not isinstance(typevar1, (TypeVarType, TypeVarTupleType)):
+                continue
+            # Find the position of the intermediate types' and finally the proposed type's
+            # related type variable (if not available, `pos2` becomes `None`):
+            for instance in instances[1:]:
+                pos2: int | None = None
+                for pos2, typevar2 in enumerate(instance.type.defn.type_vars):
+                    if typevar1 == typevar2:
+                        if instance.type.has_type_var_tuple_type:
+                            assert (prefix := instance.type.type_var_tuple_prefix) is not None
+                            if pos2 > prefix:
+                                pos2 += len(instance.args) - len(instance.type.defn.type_vars)
+                        typevar1 = instance.args[pos2]
+                        if isinstance(typevar1, UnpackType):
+                            typevar1 = typevar1.type
+                        break
+                else:
+                    pos2 = None
+                    break
+
+            # Transfer the current type's type variable argument or type variable tuple arguments:
+            if pos2 is not None:
+                if current.type.has_type_var_tuple_type:
+                    assert (prefix := current.type.type_var_tuple_prefix) is not None
+                    assert (suffix := current.type.type_var_tuple_suffix) is not None
+                    if pos1 < prefix:
+                        proposed_args[pos2] = current.args[pos1]
+                    elif pos1 == prefix:
+                        proposed_args[pos2] = current.args[prefix:len(current.args) - suffix]
+                    else:
+                        middle = len(current.args) - prefix - suffix
+                        proposed_args[pos2] = current.args[pos1 + middle - 1]
+                else:
+                    proposed_args[pos2] = current.args[pos1]
+
+        # Combine all type variable and type variable tuple arguments to a flat list:
+        flattened_proposed_args: list[Type] = []
+        for arg in proposed_args:
+            if isinstance(arg, tuple):
+                flattened_proposed_args.extend(arg)
+            else:
+                flattened_proposed_args.append(arg)
+
+        return proposed.copy_modified(args=flattened_proposed_args)
+
+    return proposed
+
+
 def gen_unique_name(base: str, table: SymbolTable) -> str:
     """Generate a name that does not appear in table by appending numbers to base."""
     if base not in table:

test-data/unit/check-narrowing.test

@@ -2089,3 +2089,149 @@ if isinstance(x, (Z, NoneType)):  # E: Subclass of "X" and "Z" cannot exist: "Z"
     reveal_type(x)  # E: Statement is unreachable
 
 [builtins fixtures/isinstance.pyi]
+
+[case testKeepTypeVarArgsWhenNarrowingGenericsWithIsInstance]
+from typing import Generic, Sequence, Tuple, TypeVar, Union
+
+s: Sequence[str]
+if isinstance(s, tuple):
+    reveal_type(s)  # N: Revealed type is "builtins.tuple[builtins.str, ...]"
+else:
+    reveal_type(s)  # N: Revealed type is "typing.Sequence[builtins.str]"
+if isinstance(s, list):
+    reveal_type(s)  # N: Revealed type is "builtins.list[builtins.str]"
+else:
+    reveal_type(s)  # N: Revealed type is "typing.Sequence[builtins.str]"
+
+t1: Tuple[str, int]
+if isinstance(t1, tuple):
+    reveal_type(t1)  # N: Revealed type is "Tuple[builtins.str, builtins.int]"
+else:
+    reveal_type(t1)
+
+t2: Tuple[str, ...]
+if isinstance(t2, tuple):
+    reveal_type(t2)  # N: Revealed type is "builtins.tuple[builtins.str, ...]"
+else:
+    reveal_type(t2)
+
+T1 = TypeVar("T1")
+T2 = TypeVar("T2")
+class A(Generic[T1]): ...
+class B(A[T1], Generic[T1, T2]):...
+a: A[str]
+if isinstance(a, B):
+    reveal_type(a)  # N: Revealed type is "__main__.B[builtins.str, Any]"
+else:
+    reveal_type(a)  # N: Revealed type is "__main__.A[builtins.str]"
+class C(A[str], Generic[T1]):...
+if isinstance(a, C):
+    reveal_type(a)  # N: Revealed type is "__main__.C[Any]"
+else:
+    reveal_type(a)  # N: Revealed type is "__main__.A[builtins.str]"
+
+class AA(Generic[T1]): ...
+class BB(A[T1], AA[T1], Generic[T1, T2]):...
+aa: Union[A[int], Union[AA[str], AA[int]]]
+if isinstance(aa, BB):
+    reveal_type(aa)  # N: Revealed type is "Union[__main__.BB[builtins.int, Any], __main__.BB[builtins.str, Any]]"
+else:
+    reveal_type(aa)  # N: Revealed type is "Union[__main__.A[builtins.int], __main__.AA[builtins.str], __main__.AA[builtins.int]]"
+
+T3 = TypeVar("T3")
+T4 = TypeVar("T4")
+T5 = TypeVar("T5")
+T6 = TypeVar("T6")
+T7 = TypeVar("T7")
+T8 = TypeVar("T8")
+T9 = TypeVar("T9")
+T10 = TypeVar("T10")
+T11 = TypeVar("T11")
+class A1(Generic[T1, T2]): ...
+class A2(Generic[T3, T4]): ...
+class B1(A1[T5, T6]):...
+class B2(A2[T7, T8]):...
+class C1(B1[T9, T10], B2[T11, T9]):...
+a2: A2[str, int]
+if isinstance(a2, C1):
+    reveal_type(a2)  # N: Revealed type is "__main__.C1[builtins.int, Any, builtins.str]"
+else:
+    reveal_type(a2)  # N: Revealed type is "__main__.A2[builtins.str, builtins.int]"
+[builtins fixtures/tuple.pyi]
+
+[case testKeepTypeVarArgsWhenNarrowingGenericsWithIsInstanceAndTuples]
+from typing import Generic, TypeVar, Union
+
+T1 = TypeVar("T1")
+T2 = TypeVar("T2")
+class A(Generic[T1]): ...
+class B(A[T1], Generic[T1, T2]):...
+class C(A[T2], Generic[T1, T2]):...
+a: Union[A[str], A[int]]
+if isinstance(a, (B, C)):
+    reveal_type(a)  # N: Revealed type is "Union[__main__.B[builtins.str, Any], __main__.C[Any, builtins.str], __main__.B[builtins.int, Any], __main__.C[Any, builtins.int]]"
+else:
+    reveal_type(a)  # N: Revealed type is "Union[__main__.A[builtins.str], __main__.A[builtins.int]]"
+[builtins fixtures/isinstance.pyi]
+
+
+[case testKeepTypeVarArgsWhenNarrowingGenericsWithIsSubclass]
+from typing import Generic, Sequence, Type, TypeVar
+
+T1 = TypeVar("T1")
+T2 = TypeVar("T2")
+class A(Generic[T1]): ...
+class B(A[T1], Generic[T1, T2]):...
+a: Type[A[str]]
+if issubclass(a, B):
+    reveal_type(a)  # N: Revealed type is "Type[__main__.B[builtins.str, Any]]"
+else:
+    reveal_type(a)  # N: Revealed type is "Type[__main__.A[builtins.str]]"
+class C(A[str], Generic[T1]):...
+if issubclass(a, C):
+    reveal_type(a)  # N: Revealed type is "Type[__main__.C[Any]]"
+else:
+    reveal_type(a)  # N: Revealed type is "Type[__main__.A[builtins.str]]"
+[builtins fixtures/isinstance.pyi]
+
+[case testKeepTypeVarTupleArgsWhenNarrowingGenericsWithIsInstance]
+from typing import Generic, Sequence, Tuple, TypeVar
+from typing_extensions import TypeVarTuple, Unpack
+
+TP = TypeVarTuple("TP")
+class A(Generic[Unpack[TP]]): ...
+class B(A[Unpack[TP]]): ...
+a: A[str, int]
+if isinstance(a, B):
+    reveal_type(a)  # N: Revealed type is "__main__.B[builtins.str, builtins.int]"
+else:
+    reveal_type(a)  # N: Revealed type is "__main__.A[builtins.str, builtins.int]"
+
+def f1(a: A[*Tuple[str, ...]]):
+    if isinstance(a, B):
+        reveal_type(a)  # N: Revealed type is "__main__.B[Unpack[builtins.tuple[builtins.str, ...]]]"
+
+T = TypeVar("T")
+def f2(a: A[T, str, T]):
+    if isinstance(a, B):
+        reveal_type(a)  # N: Revealed type is "__main__.B[T`-1, builtins.str, T`-1]"
+
+T1 = TypeVar("T1")
+T2 = TypeVar("T2")
+T3 = TypeVar("T3")
+T4 = TypeVar("T4")
+T5 = TypeVar("T5")
+T6 = TypeVar("T6")
+class C(Generic[T1, Unpack[TP], T2]): ...
+class D(C[T1, Unpack[TP], T2], Generic[T2, T4, T6, Unpack[TP], T5, T3, T1]): ...
+class E(D[T1, T2, float, Unpack[TP], float, T3, T4]): ...
+c: C[int, str, int, str]
+if isinstance(c, E):
+    reveal_type(c)  # N: Revealed type is "__main__.E[builtins.str, Any, builtins.str, builtins.int, Any, builtins.int]"
+else:
+    reveal_type(c)  # N: Revealed type is "__main__.C[builtins.int, builtins.str, builtins.int, builtins.str]"
+
+class F(E[T1, T2, str, int, T3, T4]): ...
+if isinstance(c, F):
+    reveal_type(c)  # N: Revealed type is "__main__.F[builtins.str, Any, Any, builtins.int]"
+[builtins fixtures/tuple.pyi]