patch 8.1.1816: cannot use a user defined function as a method

Problem:    Cannot use a user defined function as a method.
Solution:   Pass the base as the first argument to the user defined function
            after "->". (partly by FUJIWARA Takuya)
diff --git a/src/testdir/sautest/autoload/foo.vim b/src/testdir/sautest/autoload/foo.vim
index d7dcd5c..298e727 100644
--- a/src/testdir/sautest/autoload/foo.vim
+++ b/src/testdir/sautest/autoload/foo.vim
@@ -5,3 +5,7 @@
 func foo#bar.echo()
   let g:called_foo_bar_echo += 1
 endfunc
+
+func foo#addFoo(head)
+  return a:head .. 'foo'
+endfunc
diff --git a/src/testdir/test_autoload.vim b/src/testdir/test_autoload.vim
index 7396c22..b8c4fa2 100644
--- a/src/testdir/test_autoload.vim
+++ b/src/testdir/test_autoload.vim
@@ -8,6 +8,8 @@
   call g:foo#bar.echo()
   call assert_equal(1, g:loaded_foo_vim)
   call assert_equal(1, g:called_foo_bar_echo)
+
+  eval 'bar'->g:foo#addFoo()->assert_equal('barfoo')
 endfunc
 
 func Test_source_autoload()
diff --git a/src/testdir/test_user_func.vim b/src/testdir/test_user_func.vim
index 666c06c..4b4a276 100644
--- a/src/testdir/test_user_func.vim
+++ b/src/testdir/test_user_func.vim
@@ -47,7 +47,7 @@
 endfunc
 
 func Test_user_func()
-  let g:FuncRef=function("FuncWithRef")
+  let g:FuncRef = function("FuncWithRef")
   let g:counter = 0
   inoremap <expr> ( ListItem()
   inoremap <expr> [ ListReset()
@@ -62,6 +62,14 @@
   call assert_equal(9, g:retval)
   call assert_equal(333, g:FuncRef(333))
 
+  let g:retval = "nop"
+  call assert_equal('xxx4asdf', "xxx"->Table(4, "asdf"))
+  call assert_equal('fail', 45->Compute(0, "retval"))
+  call assert_equal('nop', g:retval)
+  call assert_equal('ok', 45->Compute(5, "retval"))
+  call assert_equal(9, g:retval)
+  " call assert_equal(333, 333->g:FuncRef())
+
   enew
 
   normal oXX+-XX
@@ -144,3 +152,11 @@
 	\ .. "   endfunction",
 	\ execute('func Args2'))
 endfunc
+
+func s:addFoo(lead)
+  return a:lead .. 'foo'
+endfunc
+
+func Test_user_method()
+  eval 'bar'->s:addFoo()->assert_equal('barfoo')
+endfunc