From e1a92d8bf8f07abc89350a956819b78df05bc4fe Mon Sep 17 00:00:00 2001 From: Billy Robert O'Neal III Date: Mon, 25 May 2020 20:47:58 -0700 Subject: [PATCH] Disable trivially_constructible test for atomic on MSVC++. MSVC++ implements P0883 unconditionally, which changes the rules for std::atomic. It removes atomic's trivial constructor, and makes the default constructor value initialize the T. Note that Mongo was not following the C++11 rules, because it used the atomic before calling atomic_init first. MSVC++ never implemented the C++11 rules and previously default initialized the T. All versions of MSVC++ will provide constant initialization of the guarded value "current_instance". In old versions, atomic didn't implement P0883 due to bugs in the constexpr evaluator; in current versions the constexpr evaluator was fixed and atomic value initializes unconditionally. Therefore, this PR disables the check whenever MSVC++'s standard library is detected. See https://github.com/microsoft/STL/issues/661 for further discussion. --- src/mongocxx/instance.cpp | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/src/mongocxx/instance.cpp b/src/mongocxx/instance.cpp index e67d41ec7..6eb456c65 100644 --- a/src/mongocxx/instance.cpp +++ b/src/mongocxx/instance.cpp @@ -75,7 +75,8 @@ typename std::aligned_storage::type sentine std::atomic current_instance{nullptr}; static_assert(std::is_standard_layout::value, "Must be standard layout"); -#if (!defined(__GNUC__) || (defined(__clang__) && !defined(__GLIBCXX__))) || (__GNUC__ >= 5) +#if (!defined(_MSVC_STL_VERSION)) \ + && ((!defined(__GNUC__) || (defined(__clang__) && !defined(__GLIBCXX__))) || (__GNUC__ >= 5)) static_assert(std::is_trivially_constructible::value, "Must be trivially constructible"); #endif