[red-knot] Treat empty intersection as 'object', fix intersection simplification (#13880)

## Summary

- Properly treat the empty intersection as being of type `object`.
- Consequently, change the simplification method to explicitly add
`Never` to the positive side of the intersection when collapsing a type
such as `int & str` to `Never`, as opposed to just clearing both the
positive and the negative side.
- Minor code improvement in `bindings_ty`: use `peekable()` to check
whether the iterator over constraints is empty, instead of handling
first and subsequent elements separately.

fixes #13870

## Test Plan

- New unit tests for `IntersectionBuilder` to make sure the empty
intersection represents `object`.
- Markdown-based regression test for the original issue in #13870

crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_nested.md

@@ -1,5 +1,7 @@
 # Narrowing for nested conditionals
 
+## Multiple negative contributions
+
 ```py
 def int_instance() -> int: ...
 
@@ -11,3 +13,14 @@ if x != 1:
         if x != 3:
             reveal_type(x)  # revealed: int & ~Literal[1] & ~Literal[2] & ~Literal[3]
 ```
+
+## Multiple negative contributions with simplification
+
+```py
+x = 1 if flag1 else 2 if flag2 else 3
+
+if x != 1:
+    reveal_type(x)  # revealed: Literal[2, 3]
+    if x != 2:
+        reveal_type(x)  # revealed: Literal[3]
+```

crates/red_knot_python_semantic/src/types.rs

@@ -148,18 +148,18 @@ fn bindings_ty<'db>(
              binding,
              constraints,
          }| {
-            let mut constraint_tys =
-                constraints.filter_map(|constraint| narrowing_constraint(db, constraint, binding));
+            let mut constraint_tys = constraints
+                .filter_map(|constraint| narrowing_constraint(db, constraint, binding))
+                .peekable();
+
             let binding_ty = binding_ty(db, binding);
-            if let Some(first_constraint_ty) = constraint_tys.next() {
-                let mut builder = IntersectionBuilder::new(db);
-                builder = builder
-                    .add_positive(binding_ty)
-                    .add_positive(first_constraint_ty);
-                for constraint_ty in constraint_tys {
-                    builder = builder.add_positive(constraint_ty);
-                }
-                builder.build()
+            if constraint_tys.peek().is_some() {
+                constraint_tys
+                    .fold(
+                        IntersectionBuilder::new(db).add_positive(binding_ty),
+                        IntersectionBuilder::add_positive,
+                    )
+                    .build()
             } else {
                 binding_ty
             }

crates/red_knot_python_semantic/src/types/builder.rs

@@ -253,6 +253,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                 // A & B = Never    if A and B are disjoint
                 if new_positive.is_disjoint_from(db, *existing_positive) {
                     *self = Self::new();
+                    self.positive.insert(Type::Never);
                     return;
                 }
             }
@@ -265,6 +266,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                 // S & ~T = Never    if S <: T
                 if new_positive.is_subtype_of(db, *existing_negative) {
                     *self = Self::new();
+                    self.positive.insert(Type::Never);
                     return;
                 }
                 // A & ~B = A    if A and B are disjoint
@@ -328,6 +330,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                     // S & ~T = Never    if S <: T
                     if existing_positive.is_subtype_of(db, new_negative) {
                         *self = Self::new();
+                        self.positive.insert(Type::Never);
                         return;
                     }
                     // A & ~B = A    if A and B are disjoint
@@ -351,7 +354,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
     fn build(mut self, db: &'db dyn Db) -> Type<'db> {
         self.simplify_unbound();
         match (self.positive.len(), self.negative.len()) {
-            (0, 0) => Type::Never,
+            (0, 0) => KnownClass::Object.to_instance(db),
             (1, 0) => self.positive[0],
             _ => {
                 self.positive.shrink_to_fit();
@@ -523,6 +526,15 @@ mod tests {
         assert_eq!(intersection.neg_vec(&db), &[t0]);
     }
 
+    #[test]
+    fn build_intersection_empty_intersection_equals_object() {
+        let db = setup_db();
+
+        let ty = IntersectionBuilder::new(&db).build();
+
+        assert_eq!(ty, KnownClass::Object.to_instance(&db));
+    }
+
     #[test]
     fn build_intersection_flatten_positive() {
         let db = setup_db();