Fix compilation errors for C++20 (#1329)
Summary: Compiling with gcc 10.0.1 and -std=gnu++2a, we have the following errors: ``` folly/FBString.h:1013:33: error: no type named 'reference' in 'class std::allocator<char>' 1013 | typedef typename A::reference reference; | ^~~~~~~~~ folly/FBString.h:1014:39: error: no type named 'const_reference' in 'class std::allocator<char>' 1014 | typedef typename A::const_reference const_reference; ``` This is due to the fact many members in `std::allocator` have been remove in C++20: https://en.cppreference.com/w/cpp/memory/allocator We also have this one due to the new way C++ resolves the comparison operators: ``` folly/dynamic-inl.h:854:20: error: ambiguous overload for 'operator!=' (operand types are 'folly::dynamic::const_item_iterator' and 'folly::dynamic::const_item_iterator') 854 | return find(key) != items().end() ? 1u : 0u; | ~~~~~~~~~ ^~ ~~~~~~~~~~~~~ | | | | | folly::dynamic::const_item_iterator | folly::dynamic::const_item_iterator In file included from folly/dynamic-inl.h:25, from folly/dynamic.h:796, from folly/dynamic.cpp:17: folly/detail/Iterators.h:80:8: note: candidate: 'bool folly::detail::IteratorFacade<D, V, Tag>::operator==(const D&) const [with D = folly::dynamic::const_item_iterator; V = const std::pair<const folly::dynamic, folly::dynamic>; Tag = std::f orward_iterator_tag]' (reversed) 80 | bool operator==(D const& rhs) const { | ^~~~~~~~ folly/detail/Iterators.h:98:3: note: candidate: 'typename std::enable_if<std::is_convertible<D, D2>::value, bool>::type folly::detail::IteratorFacade<D, V, Tag>::operator==(const D2&) const [with D2 = folly::dynamic::const_item_iterator; D = folly::dynamic::const_item_iterator; V = const std::pair<const folly::dynamic, folly::dynamic>; Tag = std::forward_iterator_tag; typename std::enable_if<std::is_convertible<D, D2>::value, bool>::type = bool]' (reversed) 98 | operator==(D2 const& rhs) const { | ^~~~~~~~ folly/detail/Iterators.h:84:8: note: candidate: 'bool folly::detail::IteratorFacade<D, V, Tag>::operator!=(const D&) const [with D = folly::dynamic::const_item_iterator; V = const std::pair<const folly::dynamic, folly::dynamic>; Tag = std::f orward_iterator_tag]' 84 | bool operator!=(D const& rhs) const { | ^~~~~~~~ folly/detail/Iterators.h:103:8: note: candidate: 'bool folly::detail::IteratorFacade<D, V, Tag>::operator!=(const D2&) const [with D2 = folly::dynamic::const_item_iterator; D = folly::dynamic::const_item_iterator; V = const std::pair<const f olly::dynamic, folly::dynamic>; Tag = std::forward_iterator_tag]' 103 | bool operator!=(D2 const& rhs) const { | ^~~~~~~~ ``` Pull Request resolved: https://github.com/facebook/folly/pull/1329 Reviewed By: ot, igorsugak Differential Revision: D20443631 Pulled By: yfeldblum fbshipit-source-id: 03e8210a64a3eeaefb6f14afaddf45be882e3ba6
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