patch 8.2.2926: Vim9: no good error for using :legacy in a :def function

Problem:    Vim9: no good error for using :legacy in a :def function.
Solution:   Give an explicit error where :legacy is not working.
            (closes #8309)

src/errors.h

@@ -417,3 +417,5 @@ EXTERN char e_failed_to_source_defaults[]
 	INIT(= N_("E1187: Failed to source defaults.vim"));
 EXTERN char e_cannot_open_terminal_from_command_line_window[]
 	INIT(= N_("E1188: Cannot open a terminal from the command line window"));
+EXTERN char e_cannot_use_legacy_with_command_str[]
+	INIT(= N_("E1189: Cannot use :legacy with this command: %s"));

src/testdir/test_vim9_func.vim

@@ -2207,6 +2207,15 @@ def Test_legacy_lambda()
   CheckScriptSuccess(lines)
 enddef
 
+def Test_legacy_errors()
+  for cmd in ['if', 'elseif', 'else', 'endif',
+              'for', 'endfor', 'continue', 'break',
+              'while', 'endwhile',
+              'try', 'catch', 'finally', 'endtry']
+    CheckDefFailure(['legacy ' .. cmd .. ' expr'], 'E1189:')
+  endfor
+enddef
+
 def DoFilterThis(a: string): list<string>
   # closure nested inside another closure using argument
   var Filter = (l) => filter(l, (_, v) => stridx(v, a) == 0)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2926,
 /**/
     2925,
 /**/

src/vim9compile.c

@@ -9222,6 +9222,27 @@ compile_def_function(
 	{
 	    char_u *start = ea.cmd;
 
+	    switch (ea.cmdidx)
+	    {
+		case CMD_if:
+		case CMD_elseif:
+		case CMD_else:
+		case CMD_endif:
+		case CMD_for:
+		case CMD_endfor:
+		case CMD_continue:
+		case CMD_break:
+		case CMD_while:
+		case CMD_endwhile:
+		case CMD_try:
+		case CMD_catch:
+		case CMD_finally:
+		case CMD_endtry:
+			semsg(_(e_cannot_use_legacy_with_command_str), ea.cmd);
+			goto erret;
+		default: break;
+	    }
+
 	    // ":legacy return expr" needs to be handled differently.
 	    if (checkforcmd(&start, "return", 4))
 		ea.cmdidx = CMD_return;