gh-105499: Merge typing.Union and types.UnionType (#105511)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Ken Jin <kenjin@python.org>
Co-authored-by: Carl Meyer <carl@oddbird.net>

Lib/functools.py

@@ -926,16 +926,11 @@ def singledispatch(func):
             dispatch_cache[cls] = impl
         return impl
 
-    def _is_union_type(cls):
-        from typing import get_origin, Union
-        return get_origin(cls) in {Union, UnionType}
-
     def _is_valid_dispatch_type(cls):
         if isinstance(cls, type):
             return True
-        from typing import get_args
-        return (_is_union_type(cls) and
-                all(isinstance(arg, type) for arg in get_args(cls)))
+        return (isinstance(cls, UnionType) and
+                all(isinstance(arg, type) for arg in cls.__args__))
 
     def register(cls, func=None):
         """generic_func.register(cls, func) -> func
@@ -967,7 +962,7 @@ def singledispatch(func):
             from annotationlib import Format, ForwardRef
             argname, cls = next(iter(get_type_hints(func, format=Format.FORWARDREF).items()))
             if not _is_valid_dispatch_type(cls):
-                if _is_union_type(cls):
+                if isinstance(cls, UnionType):
                     raise TypeError(
                         f"Invalid annotation for {argname!r}. "
                         f"{cls!r} not all arguments are classes."
@@ -983,10 +978,8 @@ def singledispatch(func):
                         f"{cls!r} is not a class."
                     )
 
-        if _is_union_type(cls):
-            from typing import get_args
-
-            for arg in get_args(cls):
+        if isinstance(cls, UnionType):
+            for arg in cls.__args__:
                 registry[arg] = func
         else:
             registry[cls] = func