tree 8f45e255b94a67510843c7a57f979279ab460fd7
parent a50f5035629b7621e92acef968403f71b7d48553
author Eric Fiselier <eric@efcs.ca> 1552344921 +0000
committer Eric Fiselier <eric@efcs.ca> 1552344921 +0000

Allow optional to tolerate being used with a nested class.

When Clang tries to complete a type containing `std::optional` it
considers the `in_place_t` constructor with no arguments which checks
if the value type is default constructible. If the value type is a
nested class type, then this check occurs too early and poisons the
is_default_constructible trait.

This patch makes optional deduce `in_place_t` so we can prevent
this early SFINAE evaluation. Technically this could break people
doing weird things with the in_place_t tag, but that seems less
important than making the nested class case work.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@355877 91177308-0d34-0410-b5e6-96231b3b80d8