[Bug c++/68926] [4.9/5/6 Regression] decltype and sfinae to check for template instance availability fails to compile

[grygoriy.fuchedzhy at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68926

--- Comment #3 from Grygoriy Fuchedzhy <grygoriy.fuchedzhy at gmail dot com> ---
Created attachment 37173
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37173&action=edit
patch