Fallout from when I started checking for abstract classes as function 
parameter types; if we see an incomplete type when we check for 
abstractness, we save it to a list and check it again later when it's 
complete.  But we shouldn't do that in SFINAE context.  This change is 
already on the trunk, as part of a larger patch.

Tested x86_64-pc-linux-gnu, applying to 4.8.