commit https://github.com/vim/vim/commit/24c77a1e3a2ad510582116229462b482d69b4b8e Author: Bram Moolenaar <Bram@vim.org> Date: Thu Mar 24 21:23:06 2016 +0100 patch 7.4.1644 Problem: Using string() on a partial that exists in the dictionary it binds results in an error. (Nikolai Pavlov) Solution: Make string() not fail on a recursively nested structure. (Ken Takta)

--- a/src/testdir/test_partial.vim
+++ b/src/testdir/test_partial.vim
@@ -180,3 +180,16 @@ func Test_func_unref()
   unlet obj
   call assert_false(exists('*{' . funcnumber . '}'))
 endfunc
+
+func Test_tostring()
+  let d = {}
+  let d.d = d
+  function d.test3()
+    echo 42
+  endfunction
+  try
+    call string(d.test3)
+  catch
+    call assert_true(v:false, v:exception)
+  endtry
+endfunc