[red-knot] don't remove negations when simplifying constrained typevars

crates/red_knot_python_semantic/resources/mdtest/generics/pep695.md

@@ -488,7 +488,7 @@ from knot_extensions import Not
 
 def remove_constraint[T: (int, str, bool)](t: T) -> None:
     def _(x: Intersection[T, Not[int]]) -> None:
-        reveal_type(x)  # revealed: str
+        reveal_type(x)  # revealed: str & ~int
 
     def _(x: Intersection[T, Not[str]]) -> None:
         # With OneOf this would be OneOf[int, bool]
@@ -521,14 +521,14 @@ def f[T: (P, Q)](t: T) -> None:
         reveal_type(t)  # revealed: P
         p: P = t
     else:
-        reveal_type(t)  # revealed: Q
+        reveal_type(t)  # revealed: Q & ~P
         q: Q = t
 
     if isinstance(t, Q):
         reveal_type(t)  # revealed: Q
         q: Q = t
     else:
-        reveal_type(t)  # revealed: P
+        reveal_type(t)  # revealed: P & ~Q
         p: P = t
 
 def g[T: (P, Q, R)](t: T) -> None:
@@ -539,7 +539,7 @@ def g[T: (P, Q, R)](t: T) -> None:
         reveal_type(t)  # revealed: Q & ~P
         q: Q = t
     else:
-        reveal_type(t)  # revealed: R
+        reveal_type(t)  # revealed: R & ~P & ~Q
         r: R = t
 
     if isinstance(t, P):
@@ -555,4 +555,16 @@ def g[T: (P, Q, R)](t: T) -> None:
         reveal_type(t)  # revealed: Never
 ```
 
+If the constraints are disjoint, simplification does eliminate the redundant negative:
+
+```py
+def h[T: (P, None)](t: T) -> None:
+    if t is None:
+        reveal_type(t)  # revealed: None
+        p: None = t
+    else:
+        reveal_type(t)  # revealed: P
+        p: P = t
+```
+
 [pep 695]: https://peps.python.org/pep-0695/

crates/red_knot_python_semantic/src/types/builder.rs

@@ -499,7 +499,6 @@ impl<'db> InnerIntersectionBuilder<'db> {
     fn simplify_constrained_typevars(&mut self, db: &'db dyn Db) {
         let mut to_add = SmallVec::<[Type<'db>; 1]>::new();
         let mut positive_to_remove = SmallVec::<[usize; 1]>::new();
-        let mut negative_to_remove = Vec::new();
 
         for (typevar_index, ty) in self.positive.iter().enumerate() {
             let Type::TypeVar(typevar) = ty else {
@@ -567,20 +566,13 @@ impl<'db> InnerIntersectionBuilder<'db> {
             // replace the typevar itself with the remaining positive constraint.
             to_add.push(remaining_constraint);
             positive_to_remove.push(typevar_index);
-            negative_to_remove.extend(to_remove);
         }
 
         // We don't need to sort the positive list, since we only append to it in increasing order.
         for index in positive_to_remove.into_iter().rev() {
             self.positive.swap_remove_index(index);
         }
 
-        negative_to_remove.sort_unstable();
-        negative_to_remove.dedup();
-        for index in negative_to_remove.into_iter().rev() {
-            self.negative.swap_remove_index(index);
-        }
-
         for remaining_constraint in to_add {
             self.add_positive(db, remaining_constraint);
         }