Narrow type variable bounds in binder (#19183)

Fixes #5720
Fixes #8556
Fixes #9778
Fixes #10003
Fixes #10817
Fixes #11163
Fixes #11664
Fixes #12882
Fixes #13426
Fixes #13462
Fixes #14941
Fixes #15151
Fixes #19166

This handles a (surprisingly) common edge case. The charges in
`bind_self()` and `bind_self_fast()` are tricky. I got a few "Redundant
cast" errors there, which seemed good, but then I realized that
attribute access etc. on a type variable go through slow `PyObject`
paths, so I am actually forcing `CallableType` instead of
`F(bound=CallableType)` there, since these are performance-critical
functions.

Author: ilevkivskyi

mypy/checkmember.py

@@ -1484,19 +1484,20 @@ def bind_self_fast(method: F, original_type: Type | None = None) -> F:
         items = [bind_self_fast(c, original_type) for c in method.items]
         return cast(F, Overloaded(items))
     assert isinstance(method, CallableType)
-    if not method.arg_types:
+    func: CallableType = method
+    if not func.arg_types:
         # Invalid method, return something.
-        return cast(F, method)
-    if method.arg_kinds[0] in (ARG_STAR, ARG_STAR2):
+        return method
+    if func.arg_kinds[0] in (ARG_STAR, ARG_STAR2):
         # See typeops.py for details.
-        return cast(F, method)
+        return method
     original_type = get_proper_type(original_type)
     if isinstance(original_type, CallableType) and original_type.is_type_obj():
         original_type = TypeType.make_normalized(original_type.ret_type)
-    res = method.copy_modified(
-        arg_types=method.arg_types[1:],
-        arg_kinds=method.arg_kinds[1:],
-        arg_names=method.arg_names[1:],
+    res = func.copy_modified(
+        arg_types=func.arg_types[1:],
+        arg_kinds=func.arg_kinds[1:],
+        arg_names=func.arg_names[1:],
         bound_args=[original_type],
     )
     return cast(F, res)

mypy/expandtype.py

@@ -122,7 +122,7 @@ def freshen_function_type_vars(callee: F) -> F:
     """Substitute fresh type variables for generic function type variables."""
     if isinstance(callee, CallableType):
         if not callee.is_generic():
-            return cast(F, callee)
+            return callee
         tvs = []
         tvmap: dict[TypeVarId, Type] = {}
         for v in callee.variables:

mypy/join.py

@@ -298,7 +298,9 @@ def visit_erased_type(self, t: ErasedType) -> ProperType:
 
     def visit_type_var(self, t: TypeVarType) -> ProperType:
         if isinstance(self.s, TypeVarType) and self.s.id == t.id:
-            return self.s
+            if self.s.upper_bound == t.upper_bound:
+                return self.s
+            return self.s.copy_modified(upper_bound=join_types(self.s.upper_bound, t.upper_bound))
         else:
             return self.default(self.s)
 

mypy/meet.py

@@ -50,6 +50,7 @@
     find_unpack_in_list,
     get_proper_type,
     get_proper_types,
+    has_type_vars,
     is_named_instance,
     split_with_prefix_and_suffix,
 )
@@ -149,6 +150,14 @@ def narrow_declared_type(declared: Type, narrowed: Type) -> Type:
         return make_simplified_union(
             [narrow_declared_type(declared, x) for x in narrowed.relevant_items()]
         )
+    elif (
+        isinstance(declared, TypeVarType)
+        and not has_type_vars(original_narrowed)
+        and is_subtype(original_narrowed, declared.upper_bound)
+    ):
+        # We put this branch early to get T(bound=Union[A, B]) instead of
+        # Union[T(bound=A), T(bound=B)] that will be confusing for users.
+        return declared.copy_modified(upper_bound=original_narrowed)
     elif not is_overlapping_types(declared, narrowed, prohibit_none_typevar_overlap=True):
         if state.strict_optional:
             return UninhabitedType()
@@ -777,7 +786,9 @@ def visit_erased_type(self, t: ErasedType) -> ProperType:
 
     def visit_type_var(self, t: TypeVarType) -> ProperType:
         if isinstance(self.s, TypeVarType) and self.s.id == t.id:
-            return self.s
+            if self.s.upper_bound == t.upper_bound:
+                return self.s
+            return self.s.copy_modified(upper_bound=self.meet(self.s.upper_bound, t.upper_bound))
         else:
             return self.default(self.s)
 

mypy/subtypes.py

@@ -632,7 +632,14 @@ def visit_instance(self, left: Instance) -> bool:
     def visit_type_var(self, left: TypeVarType) -> bool:
         right = self.right
         if isinstance(right, TypeVarType) and left.id == right.id:
-            return True
+            # Fast path for most common case.
+            if left.upper_bound == right.upper_bound:
+                return True
+            # Corner case for self-types in classes generic in type vars
+            # with value restrictions.
+            if left.id.is_self():
+                return True
+            return self._is_subtype(left.upper_bound, right.upper_bound)
         if left.values and self._is_subtype(UnionType.make_union(left.values), right):
             return True
         return self._is_subtype(left.upper_bound, self.right)

mypy/typeops.py

@@ -415,10 +415,10 @@ class B(A): pass
             ]
         return cast(F, Overloaded(items))
     assert isinstance(method, CallableType)
-    func = method
+    func: CallableType = method
     if not func.arg_types:
         # Invalid method, return something.
-        return cast(F, func)
+        return method
     if func.arg_kinds[0] in (ARG_STAR, ARG_STAR2):
         # The signature is of the form 'def foo(*args, ...)'.
         # In this case we shouldn't drop the first arg,
@@ -427,7 +427,7 @@ class B(A): pass
 
         # In the case of **kwargs we should probably emit an error, but
         # for now we simply skip it, to avoid crashes down the line.
-        return cast(F, func)
+        return method
     self_param_type = get_proper_type(func.arg_types[0])
 
     variables: Sequence[TypeVarLikeType]

mypy/types.py

@@ -461,6 +461,11 @@ def __init__(self, type_guard: Type) -> None:
     def __repr__(self) -> str:
         return f"TypeGuard({self.type_guard})"
 
+    # This may hide some real bugs, but it is convenient for various "synthetic"
+    # visitors, similar to RequiredType and ReadOnlyType below.
+    def accept(self, visitor: TypeVisitor[T]) -> T:
+        return self.type_guard.accept(visitor)
+
 
 class RequiredType(Type):
     """Required[T] or NotRequired[T]. Only usable at top-level of a TypedDict definition."""

mypyc/test-data/run-classes.test

@@ -2983,3 +2983,31 @@ class B(native.A):
 
 b: B = B.make()
 assert(B.count == 2)
+
+[case testTypeVarNarrowing]
+from typing import TypeVar
+
+class B:
+    def __init__(self, x: int) -> None:
+        self.x = x
+class C(B):
+    def __init__(self, x: int, y: str) -> None:
+        self.x = x
+        self.y = y
+
+T = TypeVar("T", bound=B)
+def f(x: T) -> T:
+    if isinstance(x, C):
+        print("C", x.y)
+        return x
+    print("B", x.x)
+    return x
+
+[file driver.py]
+from native import f, B, C
+
+f(B(1))
+f(C(1, "yes"))
+[out]
+B 1
+C yes

test-data/unit/check-classes.test

@@ -6891,24 +6891,21 @@ reveal_type(i.x)  # N: Revealed type is "builtins.int"
 [builtins fixtures/isinstancelist.pyi]
 
 [case testIsInstanceTypeTypeVar]
-from typing import Type, TypeVar, Generic
+from typing import Type, TypeVar, Generic, ClassVar
 
 class Base: ...
-class Sub(Base): ...
+class Sub(Base):
+    other: ClassVar[int]
 
 T = TypeVar('T', bound=Base)
 
 class C(Generic[T]):
     def meth(self, cls: Type[T]) -> None:
         if not issubclass(cls, Sub):
             return
-        reveal_type(cls)  # N: Revealed type is "type[__main__.Sub]"
-    def other(self, cls: Type[T]) -> None:
-        if not issubclass(cls, Sub):
-            return
-        reveal_type(cls)  # N: Revealed type is "type[__main__.Sub]"
-
-[builtins fixtures/isinstancelist.pyi]
+        reveal_type(cls)  # N: Revealed type is "type[T`1]"
+        reveal_type(cls.other)  # N: Revealed type is "builtins.int"
+[builtins fixtures/isinstance.pyi]
 
 [case testIsInstanceTypeSubclass]
 from typing import Type, Optional
@@ -7602,7 +7599,7 @@ class C1:
 class C2(Generic[TypeT]):
     def method(self, other: TypeT) -> int:
         if issubclass(other, Base):
-            reveal_type(other)  # N: Revealed type is "type[__main__.Base]"
+            reveal_type(other)  # N: Revealed type is "TypeT`1"
             return other.field
         return 0
 

test-data/unit/check-isinstance.test

@@ -1821,19 +1821,23 @@ if issubclass(fm, Baz):
 from typing import TypeVar
 
 class A: pass
-class B(A): pass
+class B(A):
+    attr: int
 
 T = TypeVar('T', bound=A)
 
 def f(x: T) -> None:
     if isinstance(x, B):
-        reveal_type(x) # N: Revealed type is "__main__.B"
+        reveal_type(x) # N: Revealed type is "T`-1"
+        reveal_type(x.attr)  # N: Revealed type is "builtins.int"
     else:
         reveal_type(x) # N: Revealed type is "T`-1"
+        x.attr  # E: "T" has no attribute "attr"
     reveal_type(x) # N: Revealed type is "T`-1"
+    x.attr  # E: "T" has no attribute "attr"
 [builtins fixtures/isinstance.pyi]
 
-[case testIsinstanceAndNegativeNarrowTypeVariableWithUnionBound]
+[case testIsinstanceAndNegativeNarrowTypeVariableWithUnionBound1]
 from typing import Union, TypeVar
 
 class A:
@@ -1845,9 +1849,11 @@ T = TypeVar("T", bound=Union[A, B])
 
 def f(x: T) -> T:
     if isinstance(x, A):
-        reveal_type(x)      # N: Revealed type is "__main__.A"
+        reveal_type(x)      # N: Revealed type is "T`-1"
         x.a
-        x.b                 # E: "A" has no attribute "b"
+        x.b                 # E: "T" has no attribute "b"
+        if bool():
+            return x
     else:
         reveal_type(x)      # N: Revealed type is "T`-1"
         x.a                 # E: "T" has no attribute "a"
@@ -1857,6 +1863,24 @@ def f(x: T) -> T:
     return x
 [builtins fixtures/isinstance.pyi]
 
+[case testIsinstanceAndNegativeNarrowTypeVariableWithUnionBound2]
+from typing import Union, TypeVar
+
+class A:
+    a: int
+class B:
+    b: int
+
+T = TypeVar("T", bound=Union[A, B])
+
+def f(x: T) -> T:
+    if isinstance(x, A):
+        return x
+    x.a # E: "T" has no attribute "a"
+    x.b # OK
+    return x
+[builtins fixtures/isinstance.pyi]
+
 [case testIsinstanceAndTypeType]
 from typing import Type
 def f(x: Type[int]) -> None: