bpo-40769: Allow extra surrounding parentheses for invalid annotated …

…assignment rule (GH-20387)

Grammar/python.gram

@@ -646,8 +646,12 @@ invalid_named_expression:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
             a, "cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }
 invalid_assignment:
-    | a=list ':' expression { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not list) can be annotated") }
-    | a=tuple ':' expression { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not tuple) can be annotated") }
+    | a=invalid_ann_assign_target ':' expression {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a,
+            "only single target (not %s) can be annotated",
+            _PyPegen_get_expr_name(a)
+        )}
     | a=star_named_expression ',' star_named_expressions* ':' expression {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not tuple) can be annotated") }
     | a=expression ':' expression {
@@ -661,6 +665,10 @@ invalid_assignment:
             "'%s' is an illegal expression for augmented assignment",
             _PyPegen_get_expr_name(a)
         )}
+invalid_ann_assign_target[expr_ty]:
+    | list
+    | tuple
+    | '(' a=invalid_ann_assign_target ')' { a }
 invalid_del_stmt:
     | 'del' a=star_expressions {
         RAISE_SYNTAX_ERROR_INVALID_TARGET(DEL_TARGETS, a) }

Lib/test/test_syntax.py

@@ -733,6 +733,19 @@
 Traceback (most recent call last):
 SyntaxError: trailing comma not allowed without surrounding parentheses
 
+>>> (): int
+Traceback (most recent call last):
+SyntaxError: only single target (not tuple) can be annotated
+>>> []: int
+Traceback (most recent call last):
+SyntaxError: only single target (not list) can be annotated
+>>> (()): int
+Traceback (most recent call last):
+SyntaxError: only single target (not tuple) can be annotated
+>>> ([]): int
+Traceback (most recent call last):
+SyntaxError: only single target (not list) can be annotated
+
 Corner-cases that used to fail to raise the correct error:
 
     >>> def f(*, x=lambda __debug__:0): pass