patch 8.2.3650: Vim9: for loop variable can be a list member

Problem:    Vim9: for loop variable can be a list member.
Solution:   Check for valid variable name. (closes #9179)

src/testdir/test_vim9_script.vim

@@ -2865,11 +2865,35 @@ def Test_for_loop_fails()
       endfor
   END
   CheckDefExecAndScriptFailure(lines, 'E1012: Type mismatch; expected job but got string', 2)
+
+  lines =<< trim END
+      var i = 0
+      for i in [1, 2, 3]
+        echo i
+      endfor
+  END
+  CheckDefExecAndScriptFailure2(lines, 'E1017:', 'E1041:')
+
+  lines =<< trim END
+      var l = [0]
+      for l[0] in [1, 2, 3]
+        echo l[0]
+      endfor
+  END
+  CheckDefExecAndScriptFailure2(lines, 'E461:', 'E1017:')
+
+  lines =<< trim END
+      var d = {x: 0}
+      for d.x in [1, 2, 3]
+        echo d.x
+      endfor
+  END
+  CheckDefExecAndScriptFailure2(lines, 'E461:', 'E1017:')
 enddef
 
 def Test_for_loop_script_var()
   # cannot use s:var in a :def function
-  CheckDefFailure(['for s:var in range(3)', 'echo 3'], 'E1101:')
+  CheckDefFailure(['for s:var in range(3)', 'echo 3'], 'E461:')
 
   # can use s:var in Vim9 script, with or without s:
   var lines =<< trim END