From dc5320630dbf5b923d0f35000015c66f3f3c1816 Mon Sep 17 00:00:00 2001
From: Tim Pope <code@tpope.net>
Date: Tue, 19 Nov 2019 15:05:11 -0500
Subject: [PATCH] Provide explanatory error on :Gdiff ~

References https://github.com/tpope/vim-fugitive/issues/1402
---
 autoload/fugitive.vim | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/autoload/fugitive.vim b/autoload/fugitive.vim
index 2b05056..9bd089c 100644
--- a/autoload/fugitive.vim
+++ b/autoload/fugitive.vim
@@ -4633,6 +4633,8 @@ function! fugitive#Diffsplit(autodir, keepfocus, mods, arg, args) abort
       elseif arg =~# '^:\d$'
         exe s:DirCheck()
         let file = s:Relative(arg . ':')
+      elseif arg =~# '^[~^]\d*$'
+        return 'echoerr ' . string('fugitive: change ' . arg . ' to !' . arg . ' to diff against ancestor')
       else
         try
           let file = arg =~# '^:/.' ? fugitive#RevParse(arg) . s:Relative(':') : s:Expand(arg)