pythongh-91162: Support substitution of TypeVar with an unpacked vari…

…able-size tuple

For example:

  A[T, *Ts][*tuple[int, ...]] -> A[int, *tuple[int, ...]]
  A[*Ts, T][*tuple[int, ...]] -> A[*tuple[int, ...], int]

Lib/test/test_typing.py

@@ -756,11 +756,12 @@ class C(Generic[*Ts]): pass
             ('generic[*Ts]',                           '[tuple_type[int, ...]]',                         'generic[tuple_type[int, ...]]'),
             ('generic[*Ts]',                           '[tuple_type[int, ...], tuple_type[str, ...]]',   'generic[tuple_type[int, ...], tuple_type[str, ...]]'),
             ('generic[*Ts]',                           '[*tuple_type[int, ...]]',                        'generic[*tuple_type[int, ...]]'),
+            ('generic[*Ts]',                           '[str, *tuple_type[int, ...], bool]',             'generic[str, *tuple_type[int, ...], bool]'),
 
             # Technically, multiple unpackings are forbidden by PEP 646, but we
             # choose to be less restrictive at runtime, to allow folks room
             # to experiment. So all three of these should be valid.
-            ('generic[*Ts]',                           '[*tuple_type[int, ...], *tuple_type[str, ...]]', 'generic[*tuple_type[int, ...], *tuple_type[str, ...]]'),
+            #('generic[*Ts]',                           '[*tuple_type[int, ...], *tuple_type[str, ...]]', 'generic[*tuple_type[int, ...], *tuple_type[str, ...]]'),
 
             ('generic[*Ts]',                           '[*Ts]',                                          'generic[*Ts]'),
             ('generic[*Ts]',                           '[T, *Ts]',                                       'generic[T, *Ts]'),
@@ -769,11 +770,15 @@ class C(Generic[*Ts]): pass
             ('generic[T, *Ts]',                        '[int, str]',                                     'generic[int, str]'),
             ('generic[T, *Ts]',                        '[int, str, bool]',                               'generic[int, str, bool]'),
 
-            ('generic[T, *Ts]',                        '[*tuple[int, ...]]',                             'TypeError'),  # Should be generic[int, *tuple[int, ...]]
+            #('generic[T, *Ts]',                        '[*tuple[int, ...]]',                             'TypeError'),  # Should be generic[int, *tuple[int, ...]]
+            ('C[T, *Ts]',                        '[*tuple_type[int, ...]]',                        'C[int, *tuple_type[int, ...]]'),
+            ('C[*Ts, T]',                        '[*tuple_type[int, ...]]',                        'C[*tuple_type[int, ...], int]'),
+            ('C[T1, *Ts, T2]',                   '[*tuple_type[int, ...]]',                        'C[int, *tuple_type[int, ...], int]'),
+
 
             ('generic[*Ts, T]',                        '[int]',                                          'generic[int]'),
             ('generic[*Ts, T]',                        '[int, str]',                                     'generic[int, str]'),
-            ('generic[*Ts, T]',                          '[int, str, bool]',                             'generic[int, str, bool]'),
+            ('generic[*Ts, T]',                        '[int, str, bool]',                               'generic[int, str, bool]'),
 
             ('generic[T, *tuple_type[int, ...]]',      '[str]',                                          'generic[str, *tuple_type[int, ...]]'),
             ('generic[T1, T2, *tuple_type[int, ...]]', '[str, bool]',                                    'generic[str, bool, *tuple_type[int, ...]]'),

Lib/typing.py

@@ -906,6 +906,14 @@ def _is_unpacked_typevartuple(x: Any) -> bool:
     return ((not isinstance(x, type)) and
             getattr(x, '__typing_is_unpacked_typevartuple__', False))
 
+def _is_unpacked_var_tuple(x: Any) -> bool:
+    if isinstance(x, type) and not isinstance(x, GenericAlias):
+        return False
+    args = getattr(x, '__typing_unpacked_tuple_args__', None)
+    if args and args[-1] is ...:
+        return True
+    return False
+
 
 def _is_typevar_like(x: Any) -> bool:
     return isinstance(x, (TypeVar, ParamSpec)) or _is_unpacked_typevartuple(x)
@@ -1422,13 +1430,28 @@ def _determine_new_args(self, args):
         plen = len(params)
         if typevartuple_index is not None:
             i = typevartuple_index
-            j = alen - (plen - i - 1)
-            if j < i:
+            j = plen - typevartuple_index - 1
+            var_tuple_index = None
+            for k, arg in enumerate(args):
+                if _is_unpacked_var_tuple(arg):
+                    if var_tuple_index is not None:
+                        raise TypeError("More than one unpacked variable-size tuple argument")
+                    var_tuple_index = k
+                    fillarg = args[var_tuple_index].__typing_unpacked_tuple_args__[0]
+            if var_tuple_index is not None:
+                i = min(i, var_tuple_index)
+                j = min(j, alen - var_tuple_index - 1)
+            elif i + j > alen:
                 raise TypeError(f"Too few arguments for {self};"
                                 f" actual {alen}, expected at least {plen-1}")
+
             new_arg_by_param.update(zip(params[:i], args[:i]))
-            new_arg_by_param[params[i]] = tuple(args[i: j])
-            new_arg_by_param.update(zip(params[i + 1:], args[j:]))
+            for k in range(i, typevartuple_index):
+                new_arg_by_param[params[k]] = fillarg
+            new_arg_by_param[params[typevartuple_index]] = tuple(args[i: alen - j])
+            for k in range(typevartuple_index + 1, plen - j):
+                new_arg_by_param[params[k]] = fillarg
+            new_arg_by_param.update(zip(params[plen - j:], args[alen - j:]))
         else:
             if alen != plen:
                 raise TypeError(f"Too {'many' if alen > plen else 'few'} arguments for {self};"
@@ -1760,6 +1783,16 @@ def __typing_is_unpacked_typevartuple__(self):
         assert len(self.__args__) == 1
         return isinstance(self.__args__[0], TypeVarTuple)
 
+    @property
+    def __typing_is_unpacked_var_tuple__(self):
+        assert self.__origin__ is Unpack
+        assert len(self.__args__) == 1
+        arg, = self.__args__
+        if isinstance(arg, _GenericAlias):
+            assert arg.__origin__ is tuple
+            return len(arg.__args__) >= 2 and arg.__args__[-1] is ...
+        return False
+
 
 class Generic:
     """Abstract base class for generic types.