[ty] Fix incorrect inference of enum.auto() for enums with non-int mixins, and imprecise inference of enum.auto() for single-member enums (#20541)

Co-authored-by: Alex Waygood <alex.waygood@gmail.com>

crates/ty_python_semantic/resources/mdtest/enums.md

@@ -320,6 +320,11 @@ reveal_type(enum_members(Answer))
 
 reveal_type(Answer.YES.value)  # revealed: Literal[1]
 reveal_type(Answer.NO.value)  # revealed: Literal[2]
+
+class SingleMember(Enum):
+    SINGLE = auto()
+
+reveal_type(SingleMember.SINGLE.value)  # revealed: Literal[1]
 ```
 
 Usages of `auto()` can be combined with manual value assignments:
@@ -348,6 +353,11 @@ class Answer(StrEnum):
 
 reveal_type(Answer.YES.value)  # revealed: Literal["yes"]
 reveal_type(Answer.NO.value)  # revealed: Literal["no"]
+
+class SingleMember(StrEnum):
+    SINGLE = auto()
+
+reveal_type(SingleMember.SINGLE.value)  # revealed: Literal["single"]
 ```
 
 Using `auto()` with `IntEnum` also works as expected:
@@ -363,6 +373,52 @@ reveal_type(Answer.YES.value)  # revealed: Literal[1]
 reveal_type(Answer.NO.value)  # revealed: Literal[2]
 ```
 
+As does using `auto()` for other enums that use `int` as a mixin:
+
+```py
+from enum import Enum, auto
+
+class Answer(int, Enum):
+    YES = auto()
+    NO = auto()
+
+reveal_type(Answer.YES.value)  # revealed: Literal[1]
+reveal_type(Answer.NO.value)  # revealed: Literal[2]
+```
+
+It's [hard to predict](https://github.com/astral-sh/ruff/pull/20541#discussion_r2381878613) what the
+effect of using `auto()` will be for an arbitrary non-integer mixin, so for anything that isn't a
+`StrEnum` and has a non-`int` mixin, we simply fallback to typeshed's annotation of `Any` for the
+`value` property:
+
+```python
+from enum import Enum, auto
+
+class A(str, Enum):
+    X = auto()
+    Y = auto()
+
+reveal_type(A.X.value)  # revealed: Any
+
+class B(bytes, Enum):
+    X = auto()
+    Y = auto()
+
+reveal_type(B.X.value)  # revealed: Any
+
+class C(tuple, Enum):
+    X = auto()
+    Y = auto()
+
+reveal_type(C.X.value)  # revealed: Any
+
+class D(float, Enum):
+    X = auto()
+    Y = auto()
+
+reveal_type(D.X.value)  # revealed: Any
+```
+
 Combining aliases with `auto()`:
 
 ```py

crates/ty_python_semantic/src/types.rs

@@ -4365,6 +4365,16 @@ impl<'db> Type<'db> {
                 Place::bound(todo_type!("ParamSpecArgs / ParamSpecKwargs")).into()
             }
 
+            Type::NominalInstance(instance)
+                if matches!(name_str, "value" | "_value_")
+                    && is_single_member_enum(db, instance.class(db).class_literal(db).0) =>
+            {
+                enum_metadata(db, instance.class(db).class_literal(db).0)
+                    .and_then(|metadata| metadata.members.get_index(0).map(|(_, v)| *v))
+                    .map_or(Place::Undefined, Place::bound)
+                    .into()
+            }
+
             Type::NominalInstance(..)
             | Type::ProtocolInstance(..)
             | Type::BooleanLiteral(..)

crates/ty_python_semantic/src/types/enums.rs

@@ -6,7 +6,7 @@ use crate::{
     place::{Place, PlaceAndQualifiers, place_from_bindings, place_from_declarations},
     semantic_index::{place_table, use_def_map},
     types::{
-        ClassLiteral, DynamicType, EnumLiteralType, KnownClass, MemberLookupPolicy,
+        ClassBase, ClassLiteral, DynamicType, EnumLiteralType, KnownClass, MemberLookupPolicy,
         StringLiteralType, Type, TypeQualifiers,
     },
 };
@@ -68,9 +68,6 @@ pub(crate) fn enum_metadata<'db>(
         return None;
     }
 
-    let is_str_enum =
-        Type::ClassLiteral(class).is_subtype_of(db, KnownClass::StrEnum.to_subclass_of(db));
-
     let scope_id = class.body_scope(db);
     let use_def_map = use_def_map(db, scope_id);
     let table = place_table(db, scope_id);
@@ -141,14 +138,48 @@ pub(crate) fn enum_metadata<'db>(
                             // enum.auto
                             Some(KnownClass::Auto) => {
                                 auto_counter += 1;
-                                Some(if is_str_enum {
+
+                                // `StrEnum`s have different `auto()` behaviour to enums inheriting from `(str, Enum)`
+                                let auto_value_ty = if Type::ClassLiteral(class)
+                                    .is_subtype_of(db, KnownClass::StrEnum.to_subclass_of(db))
+                                {
                                     Type::StringLiteral(StringLiteralType::new(
                                         db,
                                         name.to_lowercase().as_str(),
                                     ))
                                 } else {
-                                    Type::IntLiteral(auto_counter)
-                                })
+                                    let custom_mixins: smallvec::SmallVec<[Option<KnownClass>; 1]> =
+                                        class
+                                            .iter_mro(db, None)
+                                            .skip(1)
+                                            .filter_map(ClassBase::into_class)
+                                            .filter(|class| {
+                                                !Type::from(*class).is_subtype_of(
+                                                    db,
+                                                    KnownClass::Enum.to_subclass_of(db),
+                                                )
+                                            })
+                                            .map(|class| class.known(db))
+                                            .filter(|class| {
+                                                !matches!(class, Some(KnownClass::Object))
+                                            })
+                                            .collect();
+
+                                    // `IntEnum`s have the same `auto()` behaviour to enums inheriting from `(int, Enum)`,
+                                    // and `IntEnum`s also have `int` in their MROs, so both cases are handled here.
+                                    //
+                                    // In general, the `auto()` behaviour for enums with non-`int` mixins is hard to predict,
+                                    // so we fall back to `Any` in those cases.
+                                    if matches!(
+                                        custom_mixins.as_slice(),
+                                        [] | [Some(KnownClass::Int)]
+                                    ) {
+                                        Type::IntLiteral(auto_counter)
+                                    } else {
+                                        Type::any()
+                                    }
+                                };
+                                Some(auto_value_ty)
                             }
 
                             _ => None,