bpo-40903: Handle multiple '=' in invalid assignment rules in the PEG parser (GH-20697)



Automerge-Triggered-By: @pablogsal
diff --git a/Grammar/python.gram b/Grammar/python.gram
index 19d9bb3..dd425ef 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -92,7 +92,7 @@
     | a=('(' b=single_target ')' { b }
          | single_subscript_attribute_target) ':' b=expression c=['=' d=annotated_rhs { d }] {
         CHECK_VERSION(6, "Variable annotations syntax is", _Py_AnnAssign(a, b, c, 0, EXTRA)) }
-    | a=(z=star_targets '=' { z })+ b=(yield_expr | star_expressions) tc=[TYPE_COMMENT] {
+    | a=(z=star_targets '=' { z })+ b=(yield_expr | star_expressions) !'=' tc=[TYPE_COMMENT] {
          _Py_Assign(a, b, NEW_TYPE_COMMENT(p, tc), EXTRA) }
     | a=single_target b=augassign c=(yield_expr | star_expressions) {
          _Py_AugAssign(a, b->kind, c, EXTRA) }
@@ -646,10 +646,11 @@
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not tuple) can be annotated") }
     | a=expression ':' expression ['=' annotated_rhs] {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "illegal target for annotation") }
-    | a=star_expressions '=' (yield_expr | star_expressions) {
+    | (star_targets '=')* a=star_expressions '=' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
             _PyPegen_get_invalid_target(a),
             "cannot assign to %s", _PyPegen_get_expr_name(_PyPegen_get_invalid_target(a))) }
+    | (star_targets '=')* a=yield_expr '=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "assignment to yield expression not possible") }
     | a=star_expressions augassign (yield_expr | star_expressions) {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION( 
             a,