[red-knot] simplify gradually-equivalent types out of unions and intersections (#17467)

## Summary

If two types are gradually-equivalent, that means they share the same
set of possible materializations. There's no need to keep two such types
in the same union or intersection; we should simplify them.

Fixes https://github.com/astral-sh/ruff/issues/17465

The one downside here is that now we will simplify e.g. `Unknown |
Todo(...)` to just `Unknown`, if `Unknown` was added to the union first.
This is correct from a type perspective (they are equivalent types), but
it can mean we lose visibility into part of the cause for the type
inferring as unknown. I think this is OK, but if we think it's important
to avoid this, I can add a special case to try to preserve `Todo` over
`Unknown`, if we see them both in the same union or intersection.

## Test Plan

Added and updated mdtests.

crates/red_knot_python_semantic/resources/mdtest/attributes.md

@@ -302,7 +302,7 @@ class C:
 
 c_instance = C()
 reveal_type(c_instance.a)  # revealed: Unknown | Literal[1]
-reveal_type(c_instance.b)  # revealed: Unknown | @Todo(starred unpacking)
+reveal_type(c_instance.b)  # revealed: Unknown
 ```
 
 #### Attributes defined in for-loop (unpacking)
@@ -1892,6 +1892,17 @@ reveal_type(B().x)  # revealed: Unknown | Literal[1]
 reveal_type(A().x)  # revealed: Unknown | Literal[1]
 ```
 
+This case additionally tests our union/intersection simplification logic:
+
+```py
+class H:
+    def __init__(self):
+        self.x = 1
+
+    def copy(self, other: "H"):
+        self.x = other.x or self.x
+```
+
 ### Builtin types attributes
 
 This test can probably be removed eventually, but we currently include it because we do not yet

crates/red_knot_python_semantic/resources/mdtest/call/union.md

@@ -201,3 +201,15 @@ def _(literals_2: Literal[0, 1], b: bool, flag: bool):
     # Now union the two:
     reveal_type(bool_and_literals_128 if flag else literals_128_shifted)  # revealed: int
 ```
+
+## Simplifying gradually-equivalent types
+
+If two types are gradually equivalent, we can keep just one of them in a union:
+
+```py
+from typing import Any, Union
+from knot_extensions import Intersection, Not
+
+def _(x: Union[Intersection[Any, Not[int]], Intersection[Any, Not[int]]]):
+    reveal_type(x)  # revealed: Any & ~int
+```

crates/red_knot_python_semantic/resources/mdtest/intersection_types.md

@@ -842,7 +842,7 @@ def unknown(
 
 ### Mixed dynamic types
 
-We currently do not simplify mixed dynamic types, but might consider doing so in the future:
+Gradually-equivalent types can be simplified out of intersections:
 
 ```py
 from typing import Any
@@ -854,10 +854,10 @@ def mixed(
     i3: Intersection[Not[Any], Unknown],
     i4: Intersection[Not[Any], Not[Unknown]],
 ) -> None:
-    reveal_type(i1)  # revealed: Any & Unknown
-    reveal_type(i2)  # revealed: Any & Unknown
-    reveal_type(i3)  # revealed: Any & Unknown
-    reveal_type(i4)  # revealed: Any & Unknown
+    reveal_type(i1)  # revealed: Any
+    reveal_type(i2)  # revealed: Any
+    reveal_type(i3)  # revealed: Any
+    reveal_type(i4)  # revealed: Any
 ```
 
 ## Invalid

crates/red_knot_python_semantic/resources/mdtest/subscript/lists.md

@@ -12,7 +12,7 @@ x = [1, 2, 3]
 reveal_type(x)  # revealed: list
 
 # TODO reveal int
-reveal_type(x[0])  # revealed: Unknown | @Todo(Support for `typing.TypeVar` instances in type expressions)
+reveal_type(x[0])  # revealed: Unknown
 
 # TODO reveal list
 reveal_type(x[0:1])  # revealed: @Todo(specialized non-generic class)

crates/red_knot_python_semantic/resources/mdtest/type_properties/is_gradual_equivalent_to.md

@@ -47,10 +47,7 @@ static_assert(is_gradual_equivalent_to(Intersection[str | int, Not[type[Any]]],
 static_assert(not is_gradual_equivalent_to(str | int, int | str | bytes))
 static_assert(not is_gradual_equivalent_to(str | int | bytes, int | str | dict))
 
-# TODO: No errors
-# error: [static-assert-error]
 static_assert(is_gradual_equivalent_to(Unknown, Unknown | Any))
-# error: [static-assert-error]
 static_assert(is_gradual_equivalent_to(Unknown, Intersection[Unknown, Any]))
 ```
 

crates/red_knot_python_semantic/src/types.rs

@@ -1441,24 +1441,6 @@ impl<'db> Type<'db> {
         }
     }
 
-    /// Returns true if both `self` and `other` are the same gradual form
-    /// (limited to `Any`, `Unknown`, or `Todo`).
-    pub(crate) fn is_same_gradual_form(self, other: Type<'db>) -> bool {
-        matches!(
-            (self, other),
-            (
-                Type::Dynamic(DynamicType::Any),
-                Type::Dynamic(DynamicType::Any)
-            ) | (
-                Type::Dynamic(DynamicType::Unknown),
-                Type::Dynamic(DynamicType::Unknown)
-            ) | (
-                Type::Dynamic(DynamicType::Todo(_)),
-                Type::Dynamic(DynamicType::Todo(_))
-            )
-        )
-    }
-
     /// Returns true if this type and `other` are gradual equivalent.
     ///
     /// > Two gradual types `A` and `B` are equivalent

crates/red_knot_python_semantic/src/types/builder.rs

@@ -278,7 +278,7 @@ impl<'db> UnionBuilder<'db> {
                         break;
                     }
 
-                    if ty.is_same_gradual_form(element_type)
+                    if ty.is_gradual_equivalent_to(self.db, element_type)
                         || ty.is_subtype_of(self.db, element_type)
                         || element_type.is_object(self.db)
                     {
@@ -560,7 +560,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                 for (index, existing_positive) in self.positive.iter().enumerate() {
                     // S & T = S    if S <: T
                     if existing_positive.is_subtype_of(db, new_positive)
-                        || existing_positive.is_same_gradual_form(new_positive)
+                        || existing_positive.is_gradual_equivalent_to(db, new_positive)
                     {
                         return;
                     }
@@ -656,7 +656,9 @@ impl<'db> InnerIntersectionBuilder<'db> {
                 let mut to_remove = SmallVec::<[usize; 1]>::new();
                 for (index, existing_negative) in self.negative.iter().enumerate() {
                     // ~S & ~T = ~T    if S <: T
-                    if existing_negative.is_subtype_of(db, new_negative) {
+                    if existing_negative.is_subtype_of(db, new_negative)
+                        || existing_negative.is_gradual_equivalent_to(db, new_negative)
+                    {
                         to_remove.push(index);
                     }
                     // same rule, reverse order