patch 8.2.2687: Vim9: cannot use "const" for global variable in :def function

Problem:    Vim9: cannot use "const" for global variable in :def function.
Solution:   Do allow using :const for a global variable. (closes #8030)

src/testdir/test_vim9_assign.vim

@@ -1277,6 +1277,13 @@ def Test_var_declaration()
     g:FLIST[0] = 22
     assert_equal([22], g:FLIST)
 
+    def SetGlobalConst()
+      const g:globConst = 123
+    enddef
+    SetGlobalConst()
+    assert_equal(123, g:globConst)
+    assert_true(islocked('g:globConst'))
+
     const w:FOO: number = 46
     assert_equal(46, w:FOO)
     const w:FOOS = 'wfoos'
@@ -1339,6 +1346,17 @@ def Test_var_declaration_fails()
   CheckScriptFailure(lines, 'E741:')
   unlet g:constvar
 
+  lines =<< trim END
+    vim9script
+    def SetGlobalConst()
+      const g:globConst = 123
+    enddef
+    SetGlobalConst()
+    g:globConst = 234
+  END
+  CheckScriptFailure(lines, 'E741: Value is locked: globConst')
+  unlet g:globConst
+
   lines =<< trim END
     vim9script
     const cdict: dict<string> = {}

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2687,
 /**/
     2686,
 /**/

src/vim9compile.c

@@ -5752,7 +5752,8 @@ compile_lhs(
 				      &lhs->lhs_opt_flags, &lhs->lhs_vimvaridx,
 						 &lhs->lhs_type, cctx) == FAIL)
 	    return FAIL;
-	if (lhs->lhs_dest != dest_local)
+	if (lhs->lhs_dest != dest_local
+				 && cmdidx != CMD_const && cmdidx != CMD_final)
 	{
 	    // Specific kind of variable recognized.
 	    declare_error = is_decl;
@@ -6519,6 +6520,7 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	else
 	{
 	    if (is_decl && cmdidx == CMD_const && (lhs.lhs_dest == dest_script
+						|| lhs.lhs_dest == dest_global
 						|| lhs.lhs_dest == dest_local))
 		// ":const var": lock the value, but not referenced variables
 		generate_LOCKCONST(cctx);

src/vim9execute.c

@@ -963,9 +963,12 @@ check_for_number(typval_T *tv)
 store_var(char_u *name, typval_T *tv)
 {
     funccal_entry_T entry;
+    int		    flags = ASSIGN_DECL;
 
+    if (tv->v_lock)
+	flags |= ASSIGN_CONST;
     save_funccal(&entry);
-    set_var_const(name, NULL, tv, FALSE, ASSIGN_DECL, 0);
+    set_var_const(name, NULL, tv, FALSE, flags, 0);
     restore_funccal();
 }