gh-99344, gh-99379, gh-99382: Fix issues in substitution of ParamSpec and TypeVarTuple (GH-99412)

* Fix substitution of TypeVarTuple and ParamSpec together in user generics.

* Fix substitution of ParamSpec followed by TypeVarTuple in generic aliases.

* Check the number of arguments in substitution in user generics containing a
  TypeVarTuple and one or more TypeVar.
(cherry picked from commit 8f2fb7dfe7)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/typing.py

@@ -284,25 +284,6 @@ def _unpack_args(args):
             newargs.append(arg)
     return newargs
 
-def _prepare_paramspec_params(cls, params):
-    """Prepares the parameters for a Generic containing ParamSpec
-    variables (internal helper).
-    """
-    # Special case where Z[[int, str, bool]] == Z[int, str, bool] in PEP 612.
-    if (len(cls.__parameters__) == 1
-            and params and not _is_param_expr(params[0])):
-        assert isinstance(cls.__parameters__[0], ParamSpec)
-        return (params,)
-    else:
-        _check_generic(cls, params, len(cls.__parameters__))
-        _params = []
-        # Convert lists to tuples to help other libraries cache the results.
-        for p, tvar in zip(params, cls.__parameters__):
-            if isinstance(tvar, ParamSpec) and isinstance(p, list):
-                p = tuple(p)
-            _params.append(p)
-        return tuple(_params)
-
 def _deduplicate(params):
     # Weed out strict duplicates, preserving the first of each occurrence.
     all_params = set(params)
@@ -1226,7 +1207,18 @@ def __typing_subst__(self, arg):
         return arg
 
     def __typing_prepare_subst__(self, alias, args):
-        return _prepare_paramspec_params(alias, args)
+        params = alias.__parameters__
+        i = params.index(self)
+        if i >= len(args):
+            raise TypeError(f"Too few arguments for {alias}")
+        # Special case where Z[[int, str, bool]] == Z[int, str, bool] in PEP 612.
+        if len(params) == 1 and not _is_param_expr(args[0]):
+            assert i == 0
+            args = (args,)
+        # Convert lists to tuples to help other libraries cache the results.
+        elif isinstance(args[i], list):
+            args = (*args[:i], tuple(args[i]), *args[i+1:])
+        return args
 
 def _is_dunder(attr):
     return attr.startswith('__') and attr.endswith('__')
@@ -1789,23 +1781,13 @@ def __class_getitem__(cls, params):
         if not isinstance(params, tuple):
             params = (params,)
 
-        if not params:
-            # We're only ok with `params` being empty if the class's only type
-            # parameter is a `TypeVarTuple` (which can contain zero types).
-            class_params = getattr(cls, "__parameters__", None)
-            only_class_parameter_is_typevartuple = (
-                    class_params is not None
-                    and len(class_params) == 1
-                    and isinstance(class_params[0], TypeVarTuple)
-            )
-            if not only_class_parameter_is_typevartuple:
-                raise TypeError(
-                    f"Parameter list to {cls.__qualname__}[...] cannot be empty"
-                )
-
         params = tuple(_type_convert(p) for p in params)
         if cls in (Generic, Protocol):
             # Generic and Protocol can only be subscripted with unique type variables.
+            if not params:
+                raise TypeError(
+                    f"Parameter list to {cls.__qualname__}[...] cannot be empty"
+                )
             if not all(_is_typevar_like(p) for p in params):
                 raise TypeError(
                     f"Parameters to {cls.__name__}[...] must all be type variables "
@@ -1815,13 +1797,20 @@ def __class_getitem__(cls, params):
                     f"Parameters to {cls.__name__}[...] must all be unique")
         else:
             # Subscripting a regular Generic subclass.
-            if any(isinstance(t, ParamSpec) for t in cls.__parameters__):
-                params = _prepare_paramspec_params(cls, params)
-            elif not any(isinstance(p, TypeVarTuple) for p in cls.__parameters__):
-                # We only run this if there are no TypeVarTuples, because we
-                # don't check variadic generic arity at runtime (to reduce
-                # complexity of typing.py).
-                _check_generic(cls, params, len(cls.__parameters__))
+            for param in cls.__parameters__:
+                prepare = getattr(param, '__typing_prepare_subst__', None)
+                if prepare is not None:
+                    params = prepare(cls, params)
+            _check_generic(cls, params, len(cls.__parameters__))
+
+            new_args = []
+            for param, new_arg in zip(cls.__parameters__, params):
+                if isinstance(param, TypeVarTuple):
+                    new_args.extend(new_arg)
+                else:
+                    new_args.append(new_arg)
+            params = tuple(new_args)
+
         return _GenericAlias(cls, params,
                              _paramspec_tvars=True)