[3.9] bpo-40631: Disallow single parenthesized star target (GH-24027) (GH-24068)

(cherry picked from commit 2ea320d)

Automerge-Triggered-By: GH:pablogsal

Author: lysnikolaou

Grammar/python.gram

@@ -563,18 +563,23 @@ star_targets[expr_ty]:
     | a=star_target !',' { a }
     | a=star_target b=(',' c=star_target { c })* [','] {
         _Py_Tuple(CHECK(_PyPegen_seq_insert_in_front(p, a, b)), Store, EXTRA) }
-star_targets_seq[asdl_seq*]: a=','.star_target+ [','] { a }
+star_targets_list_seq[asdl_seq*]: a=','.star_target+ [','] { a }
+star_targets_tuple_seq[asdl_seq*]:
+    | a=star_target b=(',' c=star_target { c })+ [','] { _PyPegen_seq_insert_in_front(p, a, b) }
+    | a=star_target ',' { _PyPegen_singleton_seq(p, a) }
 star_target[expr_ty] (memo):
     | '*' a=(!'*' star_target) {
         _Py_Starred(CHECK(_PyPegen_set_expr_context(p, a, Store)), Store, EXTRA) }
+    | target_with_star_atom
+target_with_star_atom[expr_ty] (memo):
     | a=t_primary '.' b=NAME !t_lookahead { _Py_Attribute(a, b->v.Name.id, Store, EXTRA) }
     | a=t_primary '[' b=slices ']' !t_lookahead { _Py_Subscript(a, b, Store, EXTRA) }
     | star_atom
 star_atom[expr_ty]:
     | a=NAME { _PyPegen_set_expr_context(p, a, Store) }
-    | '(' a=star_target ')' { _PyPegen_set_expr_context(p, a, Store) }
-    | '(' a=[star_targets_seq] ')' { _Py_Tuple(a, Store, EXTRA) }
-    | '[' a=[star_targets_seq] ']' { _Py_List(a, Store, EXTRA) }
+    | '(' a=target_with_star_atom ')' { _PyPegen_set_expr_context(p, a, Store) }
+    | '(' a=[star_targets_tuple_seq] ')' { _Py_Tuple(a, Store, EXTRA) }
+    | '[' a=[star_targets_list_seq] ']' { _Py_List(a, Store, EXTRA) }
 
 single_target[expr_ty]:
     | single_subscript_attribute_target

Lib/test/test_unpack_ex.py

@@ -346,6 +346,31 @@
       ...
     SyntaxError: can't use starred expression here
 
+    >>> (*x),y = 1, 2 # doctest:+ELLIPSIS
+    Traceback (most recent call last):
+      ...
+    SyntaxError: can't use starred expression here
+
+    >>> (((*x))),y = 1, 2 # doctest:+ELLIPSIS
+    Traceback (most recent call last):
+      ...
+    SyntaxError: can't use starred expression here
+
+    >>> z,(*x),y = 1, 2, 4 # doctest:+ELLIPSIS
+    Traceback (most recent call last):
+      ...
+    SyntaxError: can't use starred expression here
+
+    >>> z,(*x) = 1, 2 # doctest:+ELLIPSIS
+    Traceback (most recent call last):
+      ...
+    SyntaxError: can't use starred expression here
+
+    >>> ((*x),y) = 1, 2 # doctest:+ELLIPSIS
+    Traceback (most recent call last):
+      ...
+    SyntaxError: can't use starred expression here
+
 Some size constraints (all fail.)
 
     >>> s = ", ".join("a%d" % i for i in range(1<<8)) + ", *rest = range(1<<8 + 1)"

Misc/NEWS.d/next/Core and Builtins/2020-12-31-20-58-22.bpo-40631.deRMCx.rst

@@ -0,0 +1,2 @@
+Fix regression where a single parenthesized starred expression was a valid
+assignment target.