Fix crash on nested unions in recursive types (#14007)

Fixes #14000 

This will introduce some minor perf penalty, but only for code that
actually uses recursive types.

mypy/typeops.py

@@ -71,13 +71,13 @@ def is_recursive_pair(s: Type, t: Type) -> bool:
     """
     if isinstance(s, TypeAliasType) and s.is_recursive:
         return (
-            isinstance(get_proper_type(t), Instance)
+            isinstance(get_proper_type(t), (Instance, UnionType))
             or isinstance(t, TypeAliasType)
             and t.is_recursive
         )
     if isinstance(t, TypeAliasType) and t.is_recursive:
         return (
-            isinstance(get_proper_type(s), Instance)
+            isinstance(get_proper_type(s), (Instance, UnionType))
             or isinstance(s, TypeAliasType)
             and s.is_recursive
         )

test-data/unit/check-recursive-types.test

@@ -808,3 +808,21 @@ def test2() -> Tree2:
 def test3() -> Tree3:
     return 42  # E: Incompatible return value type (got "int", expected "Union[str, Tuple[Tree3, Tree3, Tree3]]")
 [builtins fixtures/tuple.pyi]
+
+[case testRecursiveDoubleUnionNoCrash]
+from typing import Tuple, Union, Callable, Sequence
+
+K = Union[int, Tuple[Union[int, K]]]
+L = Union[int, Callable[[], Union[int, L]]]
+M = Union[int, Sequence[Union[int, M]]]
+
+x: K
+x = x
+y: L
+y = y
+z: M
+z = z
+
+x = y  # E: Incompatible types in assignment (expression has type "L", variable has type "K")
+z = x  # OK
+[builtins fixtures/tuple.pyi]