[ty] implement auto() for StrEnum (#20524)

## Summary
see discussion here:
https://github.com/astral-sh/ty/issues/876#issuecomment-3310130167

https://docs.python.org/3/library/enum.html#enum.StrEnum

> Note Using
[auto](https://docs.python.org/3/library/enum.html#enum.auto) with
[StrEnum](https://docs.python.org/3/library/enum.html#enum.StrEnum)
results in the lower-cased member name as the value.

## Test Plan
- new mdtest
- also, added a test to assert the (already correct) behavior for
`IntEnum`

---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/ty_python_semantic/src/types/class.rs

@@ -3664,6 +3664,7 @@ pub enum KnownClass {
     Auto,
     Member,
     Nonmember,
+    StrEnum,
     // abc
     ABCMeta,
     // Types
@@ -3804,6 +3805,7 @@ impl KnownClass {
             | Self::Auto
             | Self::Member
             | Self::Nonmember
+            | Self::StrEnum
             | Self::ABCMeta
             | Self::Iterable
             | Self::Iterator
@@ -3864,6 +3866,7 @@ impl KnownClass {
             | KnownClass::Auto
             | KnownClass::Member
             | KnownClass::Nonmember
+            | KnownClass::StrEnum
             | KnownClass::ABCMeta
             | KnownClass::GenericAlias
             | KnownClass::ModuleType
@@ -3944,6 +3947,7 @@ impl KnownClass {
             | KnownClass::Auto
             | KnownClass::Member
             | KnownClass::Nonmember
+            | KnownClass::StrEnum
             | KnownClass::ABCMeta
             | KnownClass::GenericAlias
             | KnownClass::ModuleType
@@ -4024,6 +4028,7 @@ impl KnownClass {
             | KnownClass::Auto
             | KnownClass::Member
             | KnownClass::Nonmember
+            | KnownClass::StrEnum
             | KnownClass::ABCMeta
             | KnownClass::GenericAlias
             | KnownClass::ModuleType
@@ -4142,6 +4147,7 @@ impl KnownClass {
             | Self::Auto
             | Self::Member
             | Self::Nonmember
+            | Self::StrEnum
             | Self::ABCMeta
             | Self::Super
             | Self::StdlibAlias
@@ -4226,6 +4232,7 @@ impl KnownClass {
             Self::Auto => "auto",
             Self::Member => "member",
             Self::Nonmember => "nonmember",
+            Self::StrEnum => "StrEnum",
             Self::ABCMeta => "ABCMeta",
             Self::Super => "super",
             Self::Iterable => "Iterable",
@@ -4462,9 +4469,12 @@ impl KnownClass {
             | Self::Property => KnownModule::Builtins,
             Self::VersionInfo => KnownModule::Sys,
             Self::ABCMeta => KnownModule::Abc,
-            Self::Enum | Self::EnumType | Self::Auto | Self::Member | Self::Nonmember => {
-                KnownModule::Enum
-            }
+            Self::Enum
+            | Self::EnumType
+            | Self::Auto
+            | Self::Member
+            | Self::Nonmember
+            | Self::StrEnum => KnownModule::Enum,
             Self::GenericAlias
             | Self::ModuleType
             | Self::FunctionType
@@ -4591,6 +4601,7 @@ impl KnownClass {
             | Self::Auto
             | Self::Member
             | Self::Nonmember
+            | Self::StrEnum
             | Self::ABCMeta
             | Self::Super
             | Self::NewType
@@ -4675,6 +4686,7 @@ impl KnownClass {
             | Self::Auto
             | Self::Member
             | Self::Nonmember
+            | Self::StrEnum
             | Self::ABCMeta
             | Self::Super
             | Self::UnionType
@@ -4762,6 +4774,9 @@ impl KnownClass {
             "EnumType" if Program::get(db).python_version(db) >= PythonVersion::PY311 => {
                 Self::EnumType
             }
+            "StrEnum" if Program::get(db).python_version(db) >= PythonVersion::PY311 => {
+                Self::StrEnum
+            }
             "auto" => Self::Auto,
             "member" => Self::Member,
             "nonmember" => Self::Nonmember,
@@ -4835,6 +4850,7 @@ impl KnownClass {
             | Self::Auto
             | Self::Member
             | Self::Nonmember
+            | Self::StrEnum
             | Self::ABCMeta
             | Self::Super
             | Self::NotImplementedType
@@ -5387,7 +5403,9 @@ mod tests {
                     }
                     KnownClass::GenericAlias => PythonVersion::PY39,
                     KnownClass::KwOnly => PythonVersion::PY310,
-                    KnownClass::Member | KnownClass::Nonmember => PythonVersion::PY311,
+                    KnownClass::Member | KnownClass::Nonmember | KnownClass::StrEnum => {
+                        PythonVersion::PY311
+                    }
                     _ => PythonVersion::PY37,
                 };
                 (class, version_added)

crates/ty_python_semantic/src/types/enums.rs

@@ -6,8 +6,8 @@ use crate::{
     place::{Place, PlaceAndQualifiers, place_from_bindings, place_from_declarations},
     semantic_index::{place_table, use_def_map},
     types::{
-        ClassLiteral, DynamicType, EnumLiteralType, KnownClass, MemberLookupPolicy, Type,
-        TypeQualifiers,
+        ClassLiteral, DynamicType, EnumLiteralType, KnownClass, MemberLookupPolicy,
+        StringLiteralType, Type, TypeQualifiers,
     },
 };
 
@@ -77,12 +77,14 @@ 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);
 
     let mut enum_values: FxHashMap<Type<'db>, Name> = FxHashMap::default();
-    // TODO: handle `StrEnum` which uses lowercase names as values when using `auto()`.
     let mut auto_counter = 0;
 
     let ignored_names: Option<Vec<&str>> = if let Some(ignore) = table.symbol_id("_ignore_") {
@@ -148,7 +150,14 @@ pub(crate) fn enum_metadata<'db>(
                             // enum.auto
                             Some(KnownClass::Auto) => {
                                 auto_counter += 1;
-                                Some(Type::IntLiteral(auto_counter))
+                                Some(if is_str_enum {
+                                    Type::StringLiteral(StringLiteralType::new(
+                                        db,
+                                        name.to_lowercase().as_str(),
+                                    ))
+                                } else {
+                                    Type::IntLiteral(auto_counter)
+                                })
                             }
 
                             _ => None,