Fix crash with PartialTypes and the enum plugin

Fixes python#12109.

The original issue reported that the bug had to do with the use of the
`--follow-imports=skip` flag. However, it turned out this was a red
herring after closer inspection: I was able to trigger a more minimal
repro both with and without this flag:

```python
from enum import Enum

class Foo(Enum):
    a = []  # E: Need type annotation for "a" (hint: "a: List[<type>] = ...")
    b = None

    def check(self) -> None:
        reveal_type(Foo.a.value)  # N: Revealed type is "<partial list[?]>"
        reveal_type(Foo.b.value)  # N: Revealed type is "<partial None>"
```

The first two `reveal_types` demonstrate the crux of the bug: the enum
plugin does not correctly handle and convert partial types into regular
types when inferring the type of the `.value` field.

This can then cause any number of downstream problems. For example,
suppose we modify `def check(...)` so it runs `reveal_type(self.value)`.
Doing this will trigger a crash in mypy because it makes the enum plugin
eventually try running `is_equivalent(...)` on the two partial types.
But `is_equivalent` does not support partial types, so we crash.

I opted to solve this problem by:

1. Making the enum plugin explicitly call the `fixup_partial_types`
   function on all field types. This prevents the code from crashing.

2. Modifies mypy so that Final vars are never marked as being
   PartialTypes. Without this, `reveal_type(Foo.b.value)` would
   report a type of `Union[Any, None]` instead of just `None`.
   (Note that all enum fields are implicitly final).

mypy/checker.py

@@ -159,6 +159,7 @@
     erase_to_bound,
     erase_to_union_or_bound,
     false_only,
+    fixup_partial_type,
     function_type,
     get_type_vars,
     is_literal_type_like,
@@ -2738,8 +2739,8 @@ def check_assignment(
                         # None initializers preserve the partial None type.
                         return
 
-                    if is_valid_inferred_type(rvalue_type):
-                        var = lvalue_type.var
+                    var = lvalue_type.var
+                    if is_valid_inferred_type(rvalue_type, is_lvalue_final=var.is_final):
                         partial_types = self.find_partial_types(var)
                         if partial_types is not None:
                             if not self.current_node_deferred:
@@ -3687,7 +3688,10 @@ def infer_variable_type(
         """Infer the type of initialized variables from initializer type."""
         if isinstance(init_type, DeletedType):
             self.msg.deleted_as_rvalue(init_type, context)
-        elif not is_valid_inferred_type(init_type) and not self.no_partial_types:
+        elif (
+            not is_valid_inferred_type(init_type, is_lvalue_final=name.is_final)
+            and not self.no_partial_types
+        ):
             # We cannot use the type of the initialization expression for full type
             # inference (it's not specific enough), but we might be able to give
             # partial type which will be made more specific later. A partial type
@@ -6089,7 +6093,7 @@ def enter_partial_types(
                         self.msg.need_annotation_for_var(var, context, self.options.python_version)
                         self.partial_reported.add(var)
                     if var.type:
-                        fixed = self.fixup_partial_type(var.type)
+                        fixed = fixup_partial_type(var.type)
                         var.invalid_partial_type = fixed != var.type
                         var.type = fixed
 
@@ -6120,20 +6124,7 @@ def handle_partial_var_type(
                 else:
                     # Defer the node -- we might get a better type in the outer scope
                     self.handle_cannot_determine_type(node.name, context)
-            return self.fixup_partial_type(typ)
-
-    def fixup_partial_type(self, typ: Type) -> Type:
-        """Convert a partial type that we couldn't resolve into something concrete.
-
-        This means, for None we make it Optional[Any], and for anything else we
-        fill in all of the type arguments with Any.
-        """
-        if not isinstance(typ, PartialType):
-            return typ
-        if typ.type is None:
-            return UnionType.make_union([AnyType(TypeOfAny.unannotated), NoneType()])
-        else:
-            return Instance(typ.type, [AnyType(TypeOfAny.unannotated)] * len(typ.type.type_vars))
+            return fixup_partial_type(typ)
 
     def is_defined_in_base_class(self, var: Var) -> bool:
         if var.info:
@@ -6981,20 +6972,27 @@ def infer_operator_assignment_method(typ: Type, operator: str) -> tuple[bool, st
     return False, method
 
 
-def is_valid_inferred_type(typ: Type) -> bool:
-    """Is an inferred type valid?
+def is_valid_inferred_type(typ: Type, is_lvalue_final: bool = False) -> bool:
+    """Is an inferred type valid and needs no further refinement?
 
-    Examples of invalid types include the None type or List[<uninhabited>].
+    Examples of invalid types include the None type (when we are not assigning
+    None to a final lvalue) or List[<uninhabited>].
 
     When not doing strict Optional checking, all types containing None are
     invalid.  When doing strict Optional checking, only None and types that are
     incompletely defined (i.e. contain UninhabitedType) are invalid.
     """
-    if isinstance(get_proper_type(typ), (NoneType, UninhabitedType)):
-        # With strict Optional checking, we *may* eventually infer NoneType when
-        # the initializer is None, but we only do that if we can't infer a
-        # specific Optional type.  This resolution happens in
-        # leave_partial_types when we pop a partial types scope.
+    proper_type = get_proper_type(typ)
+    if isinstance(proper_type, NoneType):
+        # If the lvalue is final, we may immediately infer NoneType when the
+        # initializer is None.
+        #
+        # If not, we want to defer making this decision. The final inferred
+        # type could either be NoneType or an Optional type, depending on
+        # the context. This resolution happens in leave_partial_types when
+        # we pop a partial types scope.
+        return is_lvalue_final
+    elif isinstance(proper_type, UninhabitedType):
         return False
     return not typ.accept(NothingSeeker())
 

mypy/checkexpr.py

@@ -111,6 +111,7 @@
     custom_special_method,
     erase_to_union_or_bound,
     false_only,
+    fixup_partial_type,
     function_type,
     is_literal_type_like,
     make_simplified_union,
@@ -2925,7 +2926,7 @@ def find_partial_type_ref_fast_path(self, expr: Expression) -> Type | None:
         if isinstance(expr.node, Var):
             result = self.analyze_var_ref(expr.node, expr)
             if isinstance(result, PartialType) and result.type is not None:
-                self.chk.store_type(expr, self.chk.fixup_partial_type(result))
+                self.chk.store_type(expr, fixup_partial_type(result))
                 return result
         return None
 

mypy/plugins/enums.py

@@ -19,7 +19,7 @@
 from mypy.nodes import TypeInfo
 from mypy.semanal_enum import ENUM_BASES
 from mypy.subtypes import is_equivalent
-from mypy.typeops import make_simplified_union
+from mypy.typeops import fixup_partial_type, make_simplified_union
 from mypy.types import CallableType, Instance, LiteralType, ProperType, Type, get_proper_type
 
 ENUM_NAME_ACCESS: Final = {f"{prefix}.name" for prefix in ENUM_BASES} | {
@@ -77,6 +77,7 @@ def _infer_value_type_with_auto_fallback(
     """
     if proper_type is None:
         return None
+    proper_type = get_proper_type(fixup_partial_type(proper_type))
     if not (isinstance(proper_type, Instance) and proper_type.type.fullname == "enum.auto"):
         return proper_type
     assert isinstance(ctx.type, Instance), "An incorrect ctx.type was passed."

mypy/typeops.py

@@ -41,6 +41,7 @@
     Overloaded,
     Parameters,
     ParamSpecType,
+    PartialType,
     ProperType,
     TupleType,
     Type,
@@ -1016,3 +1017,17 @@ def try_getting_instance_fallback(typ: Type) -> Instance | None:
     elif isinstance(typ, TypeVarType):
         return try_getting_instance_fallback(typ.upper_bound)
     return None
+
+
+def fixup_partial_type(typ: Type) -> Type:
+    """Convert a partial type that we couldn't resolve into something concrete.
+
+    This means, for None we make it Optional[Any], and for anything else we
+    fill in all of the type arguments with Any.
+    """
+    if not isinstance(typ, PartialType):
+        return typ
+    if typ.type is None:
+        return UnionType.make_union([AnyType(TypeOfAny.unannotated), NoneType()])
+    else:
+        return Instance(typ.type, [AnyType(TypeOfAny.unannotated)] * len(typ.type.type_vars))

test-data/unit/check-enum.test

@@ -2100,3 +2100,30 @@ class Some:
 class A(Some, Enum):
     __labels__ = {1: "1"}
 [builtins fixtures/dict.pyi]
+
+[case testEnumWithPartialTypes]
+from enum import Enum
+
+class Mixed(Enum):
+    a = []  # E: Need type annotation for "a" (hint: "a: List[<type>] = ...")
+    b = None
+
+    def check(self) -> None:
+        reveal_type(Mixed.a.value)  # N: Revealed type is "builtins.list[Any]"
+        reveal_type(Mixed.b.value)  # N: Revealed type is "None"
+
+        # Inferring Any here instead of a union seems to be a deliberate
+        # choice; see the testEnumValueInhomogenous case above.
+        reveal_type(self.value)  # N: Revealed type is "Any"
+
+        for field in Mixed:
+            reveal_type(field.value)  # N: Revealed type is "Any"
+            if field.value is None:
+                pass
+
+class AllPartialList(Enum):
+    a = []  # E: Need type annotation for "a" (hint: "a: List[<type>] = ...")
+    b = []  # E: Need type annotation for "b" (hint: "b: List[<type>] = ...")
+
+    def check(self) -> None:
+        reveal_type(self.value)  # N: Revealed type is "builtins.list[Any]"