Fix type variable clash in nested positions and in attributes (#14095)

Addresses the non-crash part of #10244 (and similar situations).

The `freshen_function_type_vars()` use in `checkmember.py` was
inconsistent:
* It needs to be applied to attributes too, not just methods
* It needs to be a visitor, since generic callable can appear in a
nested position

The downsides are ~2% performance regression, and people will see more
large ids in `reveal_type()` (since refreshing functions uses a global
unique counter). But since this is a correctness issue that can cause
really bizarre error messages, I think it is totally worth it.

mypy/checkmember.py

@@ -6,7 +6,11 @@
 
 from mypy import meet, message_registry, subtypes
 from mypy.erasetype import erase_typevars
-from mypy.expandtype import expand_self_type, expand_type_by_instance, freshen_function_type_vars
+from mypy.expandtype import (
+    expand_self_type,
+    expand_type_by_instance,
+    freshen_all_functions_type_vars,
+)
 from mypy.maptype import map_instance_to_supertype
 from mypy.messages import MessageBuilder
 from mypy.nodes import (
@@ -66,6 +70,7 @@
     get_proper_type,
     has_type_vars,
 )
+from mypy.typetraverser import TypeTraverserVisitor
 
 if TYPE_CHECKING:  # import for forward declaration only
     import mypy.checker
@@ -311,7 +316,7 @@ def analyze_instance_member_access(
         if mx.is_lvalue:
             mx.msg.cant_assign_to_method(mx.context)
         signature = function_type(method, mx.named_type("builtins.function"))
-        signature = freshen_function_type_vars(signature)
+        signature = freshen_all_functions_type_vars(signature)
         if name == "__new__" or method.is_static:
             # __new__ is special and behaves like a static method -- don't strip
             # the first argument.
@@ -329,7 +334,7 @@ def analyze_instance_member_access(
         # Since generic static methods should not be allowed.
         typ = map_instance_to_supertype(typ, method.info)
         member_type = expand_type_by_instance(signature, typ)
-        freeze_type_vars(member_type)
+        freeze_all_type_vars(member_type)
         return member_type
     else:
         # Not a method.
@@ -727,11 +732,13 @@ def analyze_var(
             mx.msg.read_only_property(name, itype.type, mx.context)
         if mx.is_lvalue and var.is_classvar:
             mx.msg.cant_assign_to_classvar(name, mx.context)
+        t = freshen_all_functions_type_vars(typ)
         if not (mx.is_self or mx.is_super) or supported_self_type(
             get_proper_type(mx.original_type)
         ):
-            typ = expand_self_type(var, typ, mx.original_type)
-        t = get_proper_type(expand_type_by_instance(typ, itype))
+            t = expand_self_type(var, t, mx.original_type)
+        t = get_proper_type(expand_type_by_instance(t, itype))
+        freeze_all_type_vars(t)
         result: Type = t
         typ = get_proper_type(typ)
         if (
@@ -759,13 +766,13 @@ def analyze_var(
                 # In `x.f`, when checking `x` against A1 we assume x is compatible with A
                 # and similarly for B1 when checking against B
                 dispatched_type = meet.meet_types(mx.original_type, itype)
-                signature = freshen_function_type_vars(functype)
+                signature = freshen_all_functions_type_vars(functype)
                 signature = check_self_arg(
                     signature, dispatched_type, var.is_classmethod, mx.context, name, mx.msg
                 )
                 signature = bind_self(signature, mx.self_type, var.is_classmethod)
                 expanded_signature = expand_type_by_instance(signature, itype)
-                freeze_type_vars(expanded_signature)
+                freeze_all_type_vars(expanded_signature)
                 if var.is_property:
                     # A property cannot have an overloaded type => the cast is fine.
                     assert isinstance(expanded_signature, CallableType)
@@ -788,16 +795,14 @@ def analyze_var(
     return result
 
 
-def freeze_type_vars(member_type: Type) -> None:
-    if not isinstance(member_type, ProperType):
-        return
-    if isinstance(member_type, CallableType):
-        for v in member_type.variables:
+def freeze_all_type_vars(member_type: Type) -> None:
+    member_type.accept(FreezeTypeVarsVisitor())
+
+
+class FreezeTypeVarsVisitor(TypeTraverserVisitor):
+    def visit_callable_type(self, t: CallableType) -> None:
+        for v in t.variables:
             v.id.meta_level = 0
-    if isinstance(member_type, Overloaded):
-        for it in member_type.items:
-            for v in it.variables:
-                v.id.meta_level = 0
 
 
 def lookup_member_var_or_accessor(info: TypeInfo, name: str, is_lvalue: bool) -> SymbolNode | None:
@@ -1131,11 +1136,11 @@ class B(A[str]): pass
     if isinstance(t, CallableType):
         tvars = original_vars if original_vars is not None else []
         if is_classmethod:
-            t = freshen_function_type_vars(t)
+            t = freshen_all_functions_type_vars(t)
             t = bind_self(t, original_type, is_classmethod=True)
             assert isuper is not None
             t = cast(CallableType, expand_type_by_instance(t, isuper))
-            freeze_type_vars(t)
+            freeze_all_type_vars(t)
         return t.copy_modified(variables=list(tvars) + list(t.variables))
     elif isinstance(t, Overloaded):
         return Overloaded(

mypy/expandtype.py

@@ -3,6 +3,7 @@
 from typing import Iterable, Mapping, Sequence, TypeVar, cast, overload
 
 from mypy.nodes import ARG_STAR, Var
+from mypy.type_visitor import TypeTranslator
 from mypy.types import (
     AnyType,
     CallableType,
@@ -130,6 +131,26 @@ def freshen_function_type_vars(callee: F) -> F:
         return cast(F, fresh_overload)
 
 
+T = TypeVar("T", bound=Type)
+
+
+def freshen_all_functions_type_vars(t: T) -> T:
+    result = t.accept(FreshenCallableVisitor())
+    assert isinstance(result, type(t))
+    return result
+
+
+class FreshenCallableVisitor(TypeTranslator):
+    def visit_callable_type(self, t: CallableType) -> Type:
+        result = super().visit_callable_type(t)
+        assert isinstance(result, ProperType) and isinstance(result, CallableType)
+        return freshen_function_type_vars(result)
+
+    def visit_type_alias_type(self, t: TypeAliasType) -> Type:
+        # Same as for ExpandTypeVisitor
+        return t.copy_modified(args=[arg.accept(self) for arg in t.args])
+
+
 class ExpandTypeVisitor(TypeVisitor[Type]):
     """Visitor that substitutes type variables with values."""
 

mypy/typestate.py

@@ -10,7 +10,7 @@
 
 from mypy.nodes import TypeInfo
 from mypy.server.trigger import make_trigger
-from mypy.types import Instance, Type, get_proper_type
+from mypy.types import Instance, Type, TypeVarId, get_proper_type
 
 # Represents that the 'left' instance is a subtype of the 'right' instance
 SubtypeRelationship: _TypeAlias = Tuple[Instance, Instance]
@@ -275,3 +275,4 @@ def reset_global_state() -> None:
     """
     TypeState.reset_all_subtype_caches()
     TypeState.reset_protocol_deps()
+    TypeVarId.next_raw_id = 1

test-data/unit/check-generics.test

@@ -1544,7 +1544,7 @@ class C(Generic[T]):
 reveal_type(C.F(17).foo())      # N: Revealed type is "builtins.int"
 reveal_type(C("").F(17).foo())  # N: Revealed type is "builtins.int"
 reveal_type(C.F)                # N: Revealed type is "def [K] (k: K`1) -> __main__.C.F[K`1]"
-reveal_type(C("").F)            # N: Revealed type is "def [K] (k: K`1) -> __main__.C.F[K`1]"
+reveal_type(C("").F)            # N: Revealed type is "def [K] (k: K`6) -> __main__.C.F[K`6]"
 
 
 -- Callable subtyping with generic functions
@@ -2580,3 +2580,68 @@ class Bar(Foo[AnyStr]):
 [out]
 main:10: error: Argument 1 to "method1" of "Foo" has incompatible type "str"; expected "AnyStr"
 main:10: error: Argument 2 to "method1" of "Foo" has incompatible type "bytes"; expected "AnyStr"
+
+[case testTypeVariableClashVar]
+from typing import Generic, TypeVar, Callable
+
+T = TypeVar("T")
+R = TypeVar("R")
+class C(Generic[R]):
+    x: Callable[[T], R]
+
+def func(x: C[R]) -> R:
+    return x.x(42)  # OK
+
+[case testTypeVariableClashVarTuple]
+from typing import Generic, TypeVar, Callable, Tuple
+
+T = TypeVar("T")
+R = TypeVar("R")
+class C(Generic[R]):
+    x: Callable[[T], Tuple[R, T]]
+
+def func(x: C[R]) -> R:
+    if bool():
+        return x.x(42)[0]  # OK
+    else:
+        return x.x(42)[1]  # E: Incompatible return value type (got "int", expected "R")
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVariableClashMethod]
+from typing import Generic, TypeVar, Callable
+
+T = TypeVar("T")
+R = TypeVar("R")
+class C(Generic[R]):
+    def x(self) -> Callable[[T], R]: ...
+
+def func(x: C[R]) -> R:
+    return x.x()(42)  # OK
+
+[case testTypeVariableClashMethodTuple]
+from typing import Generic, TypeVar, Callable, Tuple
+
+T = TypeVar("T")
+R = TypeVar("R")
+class C(Generic[R]):
+    def x(self) -> Callable[[T], Tuple[R, T]]: ...
+
+def func(x: C[R]) -> R:
+    if bool():
+        return x.x()(42)[0]  # OK
+    else:
+        return x.x()(42)[1]  # E: Incompatible return value type (got "int", expected "R")
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVariableClashVarSelf]
+from typing import Self, TypeVar, Generic, Callable
+
+T = TypeVar("T")
+S = TypeVar("S")
+
+class C(Generic[T]):
+    x: Callable[[S], Self]
+    y: T
+
+def foo(x: C[T]) -> T:
+    return x.x(42).y  # OK

test-data/unit/check-selftype.test

@@ -1674,7 +1674,7 @@ class C:
     def bar(self) -> Self: ...
     foo: Callable[[S, Self], Tuple[Self, S]]
 
-reveal_type(C().foo)  # N: Revealed type is "def [S] (S`-1, __main__.C) -> Tuple[__main__.C, S`-1]"
+reveal_type(C().foo)  # N: Revealed type is "def [S] (S`1, __main__.C) -> Tuple[__main__.C, S`1]"
 reveal_type(C().foo(42, C()))  # N: Revealed type is "Tuple[__main__.C, builtins.int]"
 class This: ...
 [builtins fixtures/tuple.pyi]