patch 9.2.1217: Vim9: varargs type of generic function not in function type

Problem:  Vim9: when a generic function with variable arguments is used
          with type arguments, the type variable in the variable arguments
          type is not replaced in the function type, e.g. it is
          "func(...list<any>)" instead of "func(...list<number>)".
Solution: Also replace the type variable in the function type
          (Yegappan Lakshmanan).

fixes:  #21568
closes: #21507

Assisted-by: Claude
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_vim9_generics.vim

@@ -3988,6 +3988,29 @@ def Test_generic_type_var_name_prefix()
   v9.CheckSourceFailure(lines, 'E1010: Type not recognized: T', 2)
 enddef
 
+" Test for the function type of a generic function with variable arguments
+def Test_generic_func_varargs_type()
+  var lines =<< trim END
+    vim9script
+    def Fn<T>(...l: list<T>): list<T>
+      return l
+    enddef
+    assert_equal('func(...list<number>): list<number>', typename(Fn<number>))
+    var F: func(...list<number>): list<number> = Fn<number>
+    assert_equal([1, 2], F(1, 2))
+  END
+  v9.CheckSourceSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+    def Fn<T>(...l: list<T>): list<T>
+      return l
+    enddef
+    var F: func(...list<string>): list<number> = Fn<number>
+  END
+  v9.CheckSourceFailure(lines, 'E1012: Type mismatch; expected func(...list<string>): list<number> but got func(...list<number>): list<number>', 5)
+enddef
+
 " Test for using a generic method to initialize an object member variable
 def Test_generic_method_in_object_member_init_expr()
   var lines =<< trim END

src/version.c

@@ -743,6 +743,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1217,
 /**/
     1216,
 /**/

src/vim9generics.c

@@ -918,10 +918,17 @@ generic_func_copy_types(ufunc_T *fp, ufunc_T *new_fp)
 					&& ft->tt_argcount > i
 						    ? &ft->tt_args[i] : NULL);
 
-    // Update the vararg type if it uses generic types
+    // Update the vararg type if it uses generic types.  It is the last
+    // argument of the function type.
     if (fp->uf_va_type != NULL)
+    {
+	int	va_idx = fp->uf_args.ga_len;
+
 	update_generic_type(fp, new_fp, fp->uf_va_type, &new_fp->uf_va_type,
-			    NULL);
+			    ft != NULL && ft->tt_args != NULL
+					&& ft->tt_argcount > va_idx
+						? &ft->tt_args[va_idx] : NULL);
+    }
 
     // Update the return type if it is a generic type
     if (fp->uf_ret_type != NULL)