patch 8.2.4312: no error for using :vim9script in a :def function

Problem:    No error for using :vim9script in a :def function.
Solution:   Give an error when compiling.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 28dd78b..4fa71d9 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -1227,6 +1227,7 @@
   v9.CheckScriptFailure(['vim9script', 'const str = "asdf"', 'str = "xxx"'], 'E46:')
 
   assert_fails('vim9script', 'E1038:')
+  v9.CheckDefFailure(['vim9script'], 'E1038:')
 enddef
 
 def Test_script_var_shadows_function()
diff --git a/src/version.c b/src/version.c
index 2342462..825f12d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4312,
+/**/
     4311,
 /**/
     4310,
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 9dbd119..0bf47e2 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3138,6 +3138,10 @@
 			line = compile_script(line, &cctx);
 		    break;
 
+	    case CMD_vim9script:
+		    emsg(_(e_vim9script_can_only_be_used_in_script));
+		    goto erret;
+
 	    case CMD_global:
 		    if (check_global_and_subst(ea.cmd, p) == FAIL)
 			goto erret;