patch 9.1.0665: Locked variable can be changed in a :for loop

Problem:  Locked variable can be changed in a :for loop.
Solution: Always do a full permission check on the first loop iteration
          where ASSIGN_DECL is not set (zeertzjq).

related: #12470
fixes: #15450
closes: #15454

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/evalvars.c

@@ -4100,7 +4100,7 @@ set_var_const(
 
 	    // Modifying a final variable with a List value using the "+="
 	    // operator is allowed.  For other types, it is not allowed.
-	    if (((flags & ASSIGN_FOR_LOOP) == 0
+	    if ((((flags & ASSIGN_FOR_LOOP) == 0 || (flags & ASSIGN_DECL) == 0)
 			&& ((flags & ASSIGN_COMPOUND_OP) == 0
 			    || !type_inplace_modifiable))
 				 ? var_check_permission(di, name) == FAIL

src/testdir/test_eval_stuff.vim

@@ -2,6 +2,7 @@
 
 source view_util.vim
 source shared.vim
+import './vim9.vim' as v9
 
 function s:foo() abort
   try
@@ -126,7 +127,31 @@ func Test_for_invalid()
   call assert_fails("for x in 99", 'E1098:')
   call assert_fails("for x in function('winnr')", 'E1098:')
   call assert_fails("for x in {'a': 9}", 'E1098:')
-  call assert_fails("for v:maxcol in range(1)", 'E46:')
+
+  let lines =<< trim END
+    for v:maxcol in range(5)
+    endfor
+  END
+
+  let save_v_maxcol = v:maxcol
+  call v9.CheckLegacyAndVim9Failure(lines, 'E46:')
+  call assert_equal(save_v_maxcol, v:maxcol)
+
+  let lines =<< trim END
+    for g:constvar in range(5)
+    endfor
+  END
+
+  const g:constvar = 10
+  call v9.CheckLegacyAndVim9Failure(lines, 'E741:')
+  call assert_equal(10, g:constvar)
+  unlet g:constvar
+
+  let g:constvar = 10
+  lockvar 0 g:constvar
+  call v9.CheckLegacyAndVim9Failure(lines, 'E1122:')
+  call assert_equal(10, g:constvar)
+  unlet g:constvar
 
   if 0
     /1/5/2/s/\n

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    665,
 /**/
     664,
 /**/