[ty] Reachability and narrowing for enum methods (#21130)

## Summary

Adds proper type narrowing and reachability analysis for matching on
non-inferable type variables bound to enums. For example:

```py
from enum import Enum

class Answer(Enum):
    NO = 0
    YES = 1

    def is_yes(self) -> bool:  # no error here!
        match self:
            case Answer.YES:
                return True
            case Answer.NO:
                return False
```

closes https://github.com/astral-sh/ty/issues/1404

## Test Plan

Added regression tests

crates/ty_python_semantic/src/semantic_index/reachability_constraints.rs

@@ -802,10 +802,27 @@ impl ReachabilityConstraints {
     fn analyze_single_pattern_predicate(db: &dyn Db, predicate: PatternPredicate) -> Truthiness {
         let subject_ty = infer_expression_type(db, predicate.subject(db), TypeContext::default());
 
-        let narrowed_subject_ty = IntersectionBuilder::new(db)
+        let narrowed_subject = IntersectionBuilder::new(db)
             .add_positive(subject_ty)
-            .add_negative(type_excluded_by_previous_patterns(db, predicate))
+            .add_negative(type_excluded_by_previous_patterns(db, predicate));
+
+        let narrowed_subject_ty = narrowed_subject.clone().build();
+
+        // Consider a case where we match on a subject type of `Self` with an upper bound of `Answer`,
+        // where `Answer` is a {YES, NO} enum. After a previous pattern matching on `NO`, the narrowed
+        // subject type is `Self & ~Literal[NO]`. This type is *not* equivalent to `Literal[YES]`,
+        // because `Self` could also specialize to `Literal[NO]` or `Never`, making the intersection
+        // empty. However, if the current pattern matches on `YES`, the *next* narrowed subject type
+        // will be `Self & ~Literal[NO] & ~Literal[YES]`, which *is* always equivalent to `Never`. This
+        // means that subsequent patterns can never match. And we know that if we reach this point,
+        // the current pattern will have to match. We return `AlwaysTrue` here, since the call to
+        // `analyze_single_pattern_predicate_kind` below would return `Ambiguous` in this case.
+        let next_narrowed_subject_ty = narrowed_subject
+            .add_negative(pattern_kind_to_type(db, predicate.kind(db)))
             .build();
+        if !narrowed_subject_ty.is_never() && next_narrowed_subject_ty.is_never() {
+            return Truthiness::AlwaysTrue;
+        }
 
         let truthiness = Self::analyze_single_pattern_predicate_kind(
             db,

crates/ty_python_semantic/src/types/builder.rs

@@ -44,6 +44,7 @@ use crate::types::{
     TypeVarBoundOrConstraints, UnionType,
 };
 use crate::{Db, FxOrderSet};
+use rustc_hash::FxHashSet;
 use smallvec::SmallVec;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -422,9 +423,9 @@ impl<'db> UnionBuilder<'db> {
                     .iter()
                     .filter_map(UnionElement::to_type_element)
                     .filter_map(Type::as_enum_literal)
-                    .map(|literal| literal.name(self.db).clone())
-                    .chain(std::iter::once(enum_member_to_add.name(self.db).clone()))
-                    .collect::<FxOrderSet<_>>();
+                    .map(|literal| literal.name(self.db))
+                    .chain(std::iter::once(enum_member_to_add.name(self.db)))
+                    .collect::<FxHashSet<_>>();
 
                 let all_members_are_in_union = metadata
                     .members
@@ -780,6 +781,37 @@ impl<'db> IntersectionBuilder<'db> {
                     seen_aliases,
                 )
             }
+            Type::EnumLiteral(enum_literal) => {
+                let enum_class = enum_literal.enum_class(self.db);
+                let metadata =
+                    enum_metadata(self.db, enum_class).expect("Class of enum literal is an enum");
+
+                let enum_members_in_negative_part = self
+                    .intersections
+                    .iter()
+                    .flat_map(|intersection| &intersection.negative)
+                    .filter_map(|ty| ty.as_enum_literal())
+                    .filter(|lit| lit.enum_class(self.db) == enum_class)
+                    .map(|lit| lit.name(self.db))
+                    .chain(std::iter::once(enum_literal.name(self.db)))
+                    .collect::<FxHashSet<_>>();
+
+                let all_members_are_in_negative_part = metadata
+                    .members
+                    .keys()
+                    .all(|name| enum_members_in_negative_part.contains(name));
+
+                if all_members_are_in_negative_part {
+                    for inner in &mut self.intersections {
+                        inner.add_negative(self.db, enum_literal.enum_class_instance(self.db));
+                    }
+                } else {
+                    for inner in &mut self.intersections {
+                        inner.add_negative(self.db, ty);
+                    }
+                }
+                self
+            }
             _ => {
                 for inner in &mut self.intersections {
                     inner.add_negative(self.db, ty);