[Bug libstdc++/70437] [6 Regression] Instantiation loop with pair and is_constructible

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70437

--- Comment #2 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Created attachment 38179
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38179&action=edit
First stab at a patch

Initial patch done, testsuite additions to follow, will submit once
compile-farm testing is done.