patch 8.2.1682: Vim9: const works in an unexpected way

Problem:    Vim9: const works in an unexpected way.
Solution:   ":const" only disallows changing the variable, not the value.
            Make "list[0] = 9" work at the script level.

Author: brammool

src/evalvars.c

@@ -2945,6 +2945,7 @@ set_var_const(
     char_u	*varname;
     hashtab_T	*ht;
     int		is_script_local;
+    int		vim9script = in_vim9script();
 
     ht = find_var_ht(name, &varname);
     if (ht == NULL || *varname == NUL)
@@ -2954,7 +2955,7 @@ set_var_const(
     }
     is_script_local = ht == get_script_local_ht();
 
-    if (in_vim9script()
+    if (vim9script
 	    && !is_script_local
 	    && (flags & LET_NO_COMMAND) == 0
 	    && name[1] == ':')
@@ -2992,7 +2993,7 @@ set_var_const(
 		goto failed;
 	    }
 
-	    if (is_script_local && in_vim9script())
+	    if (is_script_local && vim9script)
 	    {
 		if ((flags & LET_NO_COMMAND) == 0)
 		{
@@ -3088,7 +3089,7 @@ set_var_const(
 	if (flags & LET_IS_CONST)
 	    di->di_flags |= DI_FLAGS_LOCK;
 
-	if (is_script_local && in_vim9script())
+	if (is_script_local && vim9script)
 	{
 	    scriptitem_T *si = SCRIPT_ITEM(current_sctx.sc_sid);
 
@@ -3123,7 +3124,8 @@ set_var_const(
 	init_tv(tv);
     }
 
-    if (flags & LET_IS_CONST)
+    // ":const var = val" locks the value, but not in Vim9 script
+    if ((flags & LET_IS_CONST) && !vim9script)
 	// Like :lockvar! name: lock the value and what it contains, but only
 	// if the reference count is up to one.  That locks only literal
 	// values.

src/testdir/test_vim9_script.vim

@@ -821,8 +821,15 @@ enddef
 def Test_const()
   CheckDefFailure(['const var = 234', 'var = 99'], 'E1018:')
   CheckDefFailure(['const one = 234', 'let one = 99'], 'E1017:')
+  CheckDefFailure(['const list = [1, 2]', 'let list = [3, 4]'], 'E1017:')
   CheckDefFailure(['const two'], 'E1021:')
   CheckDefFailure(['const &option'], 'E996:')
+
+  let lines =<< trim END
+    const list = [1, 2, 3]
+    list[0] = 4
+  END
+  CheckDefAndScriptSuccess(lines)
 enddef
 
 def Test_range_no_colon()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1682,
 /**/
     1681,
 /**/

src/vim9compile.c

@@ -4800,11 +4800,6 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 			semsg(_(e_variable_already_declared), name);
 			goto theend;
 		    }
-		    else if (lvar->lv_const)
-		    {
-			semsg(_(e_cannot_assign_to_constant), name);
-			goto theend;
-		    }
 		}
 		else
 		{
@@ -4960,6 +4955,11 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	    semsg(_(e_cannot_assign_to_argument), name);
 	    goto theend;
 	}
+	if (!is_decl && lvar != NULL && lvar->lv_const && !has_index)
+	{
+	    semsg(_(e_cannot_assign_to_constant), name);
+	    goto theend;
+	}
 
 	if (!heredoc)
 	{