patch 8.2.2445: Vim9: no proper error for lambda missing return type

Problem:    Vim9: no proper error for lambda missing return type.
Solution:   Check for this error. (closes #7758)

Author: brammool

src/errors.h

@@ -349,3 +349,5 @@ EXTERN char e_cannot_define_autocommands_for_all_events[]
 	INIT(= N_("E1155: Cannot define autocommands for ALL events"));
 EXTERN char e_cannot_change_arglist_recursively[]
 	INIT(= N_("E1156: Cannot change the argument list recursively"));
+EXTERN char e_missing_return_type[]
+	INIT(= N_("E1157: Missing return type"));

src/testdir/test_vim9_func.vim

@@ -687,6 +687,13 @@ def Test_call_lambda_args()
   CheckDefAndScriptFailure(lines, 'E1012:')
 enddef
 
+def Test_lambda_return_type()
+  var lines =<< trim END
+    var Ref = (): => 123
+  END
+  CheckDefAndScriptFailure(lines, 'E1157:', 1)
+enddef
+
 def Test_lambda_uses_assigned_var()
   CheckDefSuccess([
         'var x: any = "aaa"'

src/userfunc.c

@@ -491,6 +491,11 @@ skip_arrow(
 	    s = skipwhite(s + 1);
 	    *ret_type = s;
 	    s = skip_type(s, TRUE);
+	    if (s == *ret_type)
+	    {
+		emsg(_(e_missing_return_type));
+		return NULL;
+	    }
 	}
 	bef = s;
 	s = skipwhite(s);
@@ -543,6 +548,7 @@ get_lambda_tv(
     char_u	*tofree2 = NULL;
     int		equal_arrow = **arg == '(';
     int		white_error = FALSE;
+    int		called_emsg_start = called_emsg;
 
     if (equal_arrow && !in_vim9script())
 	return NOTDONE;
@@ -560,7 +566,7 @@ get_lambda_tv(
     {
 	if (types_optional)
 	    ga_clear_strings(&argtypes);
-	return NOTDONE;
+	return called_emsg == called_emsg_start ? NOTDONE : FAIL;
     }
 
     // Parse the arguments for real.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2445,
 /**/
     2444,
 /**/