patch 8.2.3646: using <sfile> in a function gives an unexpected result

Problem:    Using <sfile> in a function gives an unexpected result.
Solution:   Give an error in a Vim9 function. (issue #9189)

src/errors.h

@@ -684,3 +684,5 @@ EXTERN char e_ascii_code_not_in_range[]
 	INIT(= N_("E1243: ASCII code not in 32-127 range"));
 EXTERN char e_bad_color_string_str[]
 	INIT(= N_("E1244: Bad color string: %s"));
+EXTERN char e_cannot_expand_sfile_in_vim9_function[]
+	INIT(= N_("E1245: Cannot expand <sfile> in a Vim9 function"));

src/scriptfile.c

@@ -135,6 +135,20 @@ estack_sfile(estack_arg_T which UNUSED)
 	return vim_strsave(entry->es_name);
     }
 #ifdef FEAT_EVAL
+    // expand('<sfile>') works in a function for backwards compatibility, but
+    // may give an unexpected result.  Disallow it in Vim 9 script.
+    if (which == ESTACK_SFILE && in_vim9script())
+    {
+	int  save_emsg_off = emsg_off;
+
+	if (emsg_off == 1)
+	    // f_expand() silences errors but we do want this one
+	    emsg_off = 0;
+	emsg(_(e_cannot_expand_sfile_in_vim9_function));
+	emsg_off = save_emsg_off;
+	return NULL;
+    }
+
     // Give information about each stack entry up to the root.
     // For a function we compose the call stack, as it was done in the past:
     //   "function One[123]..Two[456]..Three"

src/testdir/test_vim9_builtin.vim

@@ -915,6 +915,14 @@ def Test_expand()
   CheckDefAndScriptFailure2(['expand("a", 2)'], 'E1013: Argument 2: type mismatch, expected bool but got number', 'E1212: Bool required for argument 2')
   CheckDefAndScriptFailure2(['expand("a", true, 2)'], 'E1013: Argument 3: type mismatch, expected bool but got number', 'E1212: Bool required for argument 3')
   expand('')->assert_equal('')
+
+  var caught = false
+  try
+    echo expand("<sfile>")
+  catch /E1245:/
+    caught = true
+  endtry
+  assert_true(caught)
 enddef
 
 def Test_expandcmd()

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3646,
 /**/
     3645,
 /**/