bpo-46032: Check types in singledispatch's register() at declaration time (GH-30050)

The registry() method of functools.singledispatch() functions checks now
the first argument or the first parameter annotation and raises a TypeError if it is
not supported. Previously unsupported "types" were ignored (e.g. typing.List[int])
or caused an error at calling time (e.g. list[int]).

Lib/functools.py

@@ -740,6 +740,7 @@ def _compose_mro(cls, types):
     # Remove entries which are already present in the __mro__ or unrelated.
     def is_related(typ):
         return (typ not in bases and hasattr(typ, '__mro__')
+                                 and not isinstance(typ, GenericAlias)
                                  and issubclass(cls, typ))
     types = [n for n in types if is_related(n)]
     # Remove entries which are strict bases of other entries (they will end up
@@ -841,9 +842,13 @@ def singledispatch(func):
         from typing import get_origin, Union
         return get_origin(cls) in {Union, types.UnionType}
 
-    def _is_valid_union_type(cls):
+    def _is_valid_dispatch_type(cls):
+        if isinstance(cls, type) and not isinstance(cls, GenericAlias):
+            return True
         from typing import get_args
-        return _is_union_type(cls) and all(isinstance(arg, type) for arg in get_args(cls))
+        return (_is_union_type(cls) and
+                all(isinstance(arg, type) and not isinstance(arg, GenericAlias)
+                    for arg in get_args(cls)))
 
     def register(cls, func=None):
         """generic_func.register(cls, func) -> func
@@ -852,9 +857,15 @@ def singledispatch(func):
 
         """
         nonlocal cache_token
-        if func is None:
-            if isinstance(cls, type) or _is_valid_union_type(cls):
+        if _is_valid_dispatch_type(cls):
+            if func is None:
                 return lambda f: register(cls, f)
+        else:
+            if func is not None:
+                raise TypeError(
+                    f"Invalid first argument to `register()`. "
+                    f"{cls!r} is not a class or union type."
+                )
             ann = getattr(cls, '__annotations__', {})
             if not ann:
                 raise TypeError(
@@ -867,7 +878,7 @@ def singledispatch(func):
             # only import typing if annotation parsing is necessary
             from typing import get_type_hints
             argname, cls = next(iter(get_type_hints(func).items()))
-            if not isinstance(cls, type) and not _is_valid_union_type(cls):
+            if not _is_valid_dispatch_type(cls):
                 if _is_union_type(cls):
                     raise TypeError(
                         f"Invalid annotation for {argname!r}. "