Stefan's comment "How does it interact with electric-pair-mode?" in 
<http://bugs.gnu.org/20545#11> prompted me to go back to the patch proposed for 
Bug#20385 and to get it to work with electric-pair-mode as well.  The revised 
patch is attached.  Most of the new revisions are to 
lisp/progmodes/elisp-mode.el.  They also add support for C-M-i completion in the 
presence of curved quotes, and tweak some wording in the documentation.

I haven't installed this as I am still mulling over similar revisions to the 
patch proposed for Bug#20545 (this is to make it easier to type curved quotes in 
doc strings).