fix regression, add more tests

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/type_properties/is_equivalent_to.md

@@ -138,6 +138,11 @@ static_assert(is_equivalent_to(Any, Any | Intersection[Any, str]))
 static_assert(is_equivalent_to(Any, Intersection[str, Any] | Any))
 static_assert(is_equivalent_to(Any, Any | Intersection[Any, Not[None]]))
 static_assert(is_equivalent_to(Any, Intersection[Not[None], Any] | Any))
+
+static_assert(is_equivalent_to(Any, Unknown | Intersection[Unknown, str]))
+static_assert(is_equivalent_to(Any, Intersection[str, Unknown] | Unknown))
+static_assert(is_equivalent_to(Any, Unknown | Intersection[Unknown, Not[None]]))
+static_assert(is_equivalent_to(Any, Intersection[Not[None], Unknown] | Unknown))
 ```
 
 ## Tuples

crates/ty_python_semantic/resources/mdtest/union_types.md

@@ -306,3 +306,28 @@ def _(c: BC, d: BD):
     reveal_type(c)  # revealed: Literal[b""]
     reveal_type(d)  # revealed: Literal[b""]
 ```
+
+## Unions of tuples
+
+A union of a fixed-length tuple and a variable-length tuple must be collapsed to the variable-length
+element, never to the fixed-length element (`tuple[()] | tuple[Any, ...]` -> `tuple[Any, ...]`, not
+`tuple[()]`).
+
+```py
+from typing import Any
+
+def f(
+    a: tuple[()] | tuple[int, ...],
+    b: tuple[int, ...] | tuple[()],
+    c: tuple[int] | tuple[str, ...],
+    d: tuple[str, ...] | tuple[int],
+    e: tuple[()] | tuple[Any, ...],
+    f: tuple[Any, ...] | tuple[()],
+):
+    reveal_type(a)  # revealed: tuple[int, ...]
+    reveal_type(b)  # revealed: tuple[int, ...]
+    reveal_type(c)  # revealed: tuple[int] | tuple[str, ...]
+    reveal_type(d)  # revealed: tuple[str, ...] | tuple[int]
+    reveal_type(e)  # revealed: tuple[Any, ...]
+    reveal_type(f)  # revealed: tuple[Any, ...]
+```

crates/ty_python_semantic/src/types/tuple.rs

@@ -757,7 +757,7 @@ impl<'db> VariableLengthTuple<Type<'db>> {
                 // (or any other dynamic type), then the `...` is the _gradual choice_ of all
                 // possible lengths. This means that `tuple[Any, ...]` can match any tuple of any
                 // length.
-                if relation == TypeRelation::Subtyping || !matches!(self.variable, Type::Dynamic(_))
+                if !relation.is_assignability() || !matches!(self.variable, Type::Dynamic(_))
                 {
                     return ConstraintSet::from(false);
                 }