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2ab98fbcd4
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