[ty] Support custom __new__ in enums for value type inference

crates/ty_python_semantic/resources/mdtest/enums.md

@@ -1118,6 +1118,142 @@ class MyEnum[T](MyEnumBase):
     A = 1
 ```
 
+## Custom `__new__`
+
+When an enum defines a custom `__new__` method, the raw assignment values are unpacked as arguments
+to `__new__`, and `_value_` is explicitly set inside the method body. The member's `.value` type
+falls back to `Any`.
+
+### Custom `__new__` with tuple values
+
+```py
+from enum import Enum
+
+class Planet(Enum):
+    def __new__(cls, mass: float, radius: float) -> "Planet":
+        obj = object.__new__(cls)
+        obj._value_ = mass
+        return obj
+    MERCURY = (3.303e23, 2.4397e6)
+    VENUS = (4.869e24, 6.0518e6)
+    EARTH = (5.976e24, 6.37814e6)
+
+# Without a `_value_` annotation, values fall back to `Any`
+reveal_type(Planet.MERCURY.value)  # revealed: Any
+reveal_type(Planet.VENUS.value)  # revealed: Any
+reveal_type(Planet.EARTH.value)  # revealed: Any
+
+# `.name` still works correctly
+reveal_type(Planet.MERCURY.name)  # revealed: Literal["MERCURY"]
+```
+
+### Custom `__init__` without `__new__`
+
+Custom `__init__` does **not** change `_value_` — the raw assignment is the value.
+
+```py
+from enum import Enum
+
+class Mood(Enum):
+    def __init__(self, value: int):
+        self._mood_level = value
+    HAPPY = 1
+    SAD = 2
+    NEUTRAL = 3
+
+reveal_type(Mood.HAPPY.value)  # revealed: Literal[1]
+reveal_type(Mood.SAD.value)  # revealed: Literal[2]
+```
+
+### Inherited custom `__new__` from a base class
+
+```py
+from enum import Enum
+
+class AutoNameEnum(Enum):
+    def __new__(cls, display_name: str) -> "AutoNameEnum":
+        obj = object.__new__(cls)
+        obj._value_ = display_name.lower()
+        return obj
+
+class Color(AutoNameEnum):
+    RED = "Red"
+    GREEN = "Green"
+    BLUE = "Blue"
+
+reveal_type(Color.RED.value)  # revealed: Any
+reveal_type(Color.GREEN.value)  # revealed: Any
+```
+
+### Standard enums are not affected
+
+```py
+from enum import Enum
+
+class Color(Enum):
+    RED = 1
+    GREEN = 2
+    BLUE = 3
+
+reveal_type(Color.RED.value)  # revealed: Literal[1]
+reveal_type(Color.GREEN.value)  # revealed: Literal[2]
+reveal_type(Color.BLUE.value)  # revealed: Literal[3]
+```
+
+### `IntEnum` not affected by `int.__new__`
+
+```py
+from enum import IntEnum
+
+class Status(IntEnum):
+    ACTIVE = 1
+    INACTIVE = 2
+
+reveal_type(Status.ACTIVE.value)  # revealed: Literal[1]
+reveal_type(Status.INACTIVE.value)  # revealed: Literal[2]
+```
+
+### Member detection still works with custom `__new__`
+
+```py
+from enum import Enum
+from typing_extensions import reveal_type
+
+class Planet(Enum):
+    def __new__(cls, mass: float, radius: float) -> "Planet":
+        obj = object.__new__(cls)
+        obj._value_ = mass
+        return obj
+    MERCURY = (3.303e23, 2.4397e6)
+    VENUS = (4.869e24, 6.0518e6)
+
+reveal_type(Planet.MERCURY)  # revealed: Literal[Planet.MERCURY]
+reveal_type(Planet.VENUS)  # revealed: Literal[Planet.VENUS]
+```
+
+### Exhaustiveness with custom `__new__`
+
+```py
+from enum import Enum
+from typing_extensions import assert_never
+
+class Planet(Enum):
+    def __new__(cls, mass: float, radius: float) -> "Planet":
+        obj = object.__new__(cls)
+        obj._value_ = mass
+        return obj
+    MERCURY = (3.303e23, 2.4397e6)
+    VENUS = (4.869e24, 6.0518e6)
+
+def check(planet: Planet) -> str:
+    if planet is Planet.MERCURY:
+        return "Mercury"
+    elif planet is Planet.VENUS:
+        return "Venus"
+    else:
+        assert_never(planet)
+```
+
 ## References
 
 - Typing spec: <https://typing.python.org/en/latest/spec/enums.html>

crates/ty_python_semantic/src/types/enums.rs

@@ -79,6 +79,15 @@ pub(crate) fn enum_metadata<'db>(
     let use_def_map = use_def_map(db, scope_id);
     let table = place_table(db, scope_id);
 
+    // When an enum has a custom `__new__`, the raw assignment type doesn't represent the
+    // member's value — `__new__` unpacks the arguments and explicitly sets `_value_`.
+    // Fall back to `Any` for the member's value type.
+    let custom_new_value_ty = if has_custom_enum_new(db, class) {
+        Some(Type::any())
+    } else {
+        None
+    };
+
     let mut enum_values: FxHashMap<Type<'db>, Name> = FxHashMap::default();
     let mut auto_counter = 0;
 
@@ -276,7 +285,8 @@ pub(crate) fn enum_metadata<'db>(
                 }
             }
 
-            Some((name.clone(), value_ty))
+            let final_value_ty = custom_new_value_ty.unwrap_or(value_ty);
+            Some((name.clone(), final_value_ty))
         })
         .collect::<FxIndexMap<_, _>>();
 
@@ -348,3 +358,53 @@ pub(crate) fn try_unwrap_nonmember_value<'db>(db: &'db dyn Db, ty: Type<'db>) ->
         _ => None,
     }
 }
+
+/// Returns `true` if the enum class (or a class in its MRO) defines a custom `__new__` method.
+///
+/// When an enum has a custom `__new__`, the assigned tuple values are unpacked as arguments to
+/// `__new__`, and `_value_` is explicitly set inside the method body. This means we can't infer
+/// the member's value type from the raw assignment — we fall back to `Any`.
+fn has_custom_enum_new<'db>(db: &'db dyn Db, class: StaticClassLiteral<'db>) -> bool {
+    // Check the enum class itself
+    if has_own_dunder_new(db, class) {
+        return true;
+    }
+
+    // Walk the MRO (skipping the class itself) looking for a custom `__new__`
+    class
+        .iter_mro(db, None)
+        .skip(1)
+        .filter_map(ClassBase::into_class)
+        .any(|mro_class| {
+            let Some(static_class) = mro_class.class_literal(db).as_static() else {
+                return false;
+            };
+
+            // Skip classes defined in vendored typeshed (e.g. `object.__new__`,
+            // `int.__new__`, `IntEnum.__new__`, `IntFlag.__new__`). These `__new__`
+            // definitions exist for typing purposes and don't represent custom value
+            // transformations. We specifically check for vendored paths rather than all
+            // stub files, because a user-provided `.pyi` stub for a library with a
+            // custom `__new__` should still be recognized.
+            if static_class
+                .body_scope(db)
+                .file(db)
+                .path(db)
+                .is_vendored_path()
+            {
+                return false;
+            }
+
+            has_own_dunder_new(db, static_class)
+        })
+}
+
+/// Returns `true` if the class defines `__new__` directly in its own body scope.
+fn has_own_dunder_new<'db>(db: &'db dyn Db, class: StaticClassLiteral<'db>) -> bool {
+    let scope = class.body_scope(db);
+    let table = place_table(db, scope);
+    table.symbol_id("__new__").is_some_and(|symbol_id| {
+        let bindings = use_def_map(db, scope).reachable_symbol_bindings(symbol_id);
+        matches!(place_from_bindings(db, bindings).place, Place::Defined(_))
+    })
+}