The functions std::atomic<T*>::fetch_(add|sub) and std::atomic_fetch_(add|sub) no longer accept a function pointer. While this is technically an API break, the invalid syntax isn’t supported by libstdc++ and MSVC STL. See https://godbolt.org/z/49fvzz98d.

The call of the functions std::atomic_(add|sub)(std::atomic<T*>*, ...) with the explicit template argument T are now ill-formed. While this is technically an API break, the invalid syntax isn’t supported by libstdc++ and MSVC STL. See https://godbolt.org/z/v9959re3v.

Due to this change it’s now possible to call these functions with the explicit template argument T*. This allows using the same syntax on the major Standard library implementations. See https://godbolt.org/z/oEfzPhTTb.

Calls to these functions where the template argument was deduced by the compiler are unaffected by this change.

The functions std::allocator<T>::allocate and std::experimental::pmr::polymorphic_allocator<T>::allocate now throw an exception of type std::bad_array_new_length when the requested size exceeds the maximum supported size, as required by the C++ standard. Previously the type std::length_error was used.

Removed the nonstandard methods std::chrono::file_clock::to_time_t and std::chrono::file_clock::from_time_t; neither libstdc++ nor MSVC STL had such methods. Instead, in C++20, you can use std::chrono::file_clock::from_sys and std::chrono::file_clock::to_sys, which are specified in the Standard. If you are not using C++20, you should move to it.

The declarations of functions declare_reachable, undeclare_reachable, declare_no_pointers, undeclare_no_pointers, and get_pointer_safety have been removed not only from C++2b but from all modes. Their symbols are still provided by the dynamic library for the benefit of existing compiled code. All of these functions have always behaved as no-ops.

std::filesystem::path::iterator, which (in our implementation) stashes a path value inside itself similar to istream_iterator, now sets its reference type to path and its iterator_category to input_iterator_tag, so that it is a conforming input iterator in C++17 and a conforming std::bidirectional_iterator in C++20. Before this release, it had set its reference type to const path& and its iterator_category to bidirectional_iterator_tag, making it a non-conforming bidirectional iterator. After this change, for loops of the form for (auto& c : path) must be rewritten as either for (auto&& c : path) or for (const auto& c : path). std::reverse_iterator<path::iterator> is no longer rejected.

Removed the nonstandard default constructor from std::chrono::month_weekday. You must now explicitly initialize with a chrono::month and chrono::weekday_indexed instead of “meh, whenever”.

C++20 requires that std::basic_string::reserve(n) never reduce the capacity of the string. (For that, use shrink_to_fit().) Prior to this release, libc++’s std::basic_string::reserve(n) could reduce capacity in C++17 and before, but not in C++20 and later. This caused ODR violations when mixing code compiled under different Standard modes. After this change, libc++’s std::basic_string::reserve(n) never reduces capacity, even in C++17 and before. C++20 deprecates the zero-argument overload of std::basic_string::reserve(), but specifically permits it to reduce capacity. To avoid breaking existing code assuming that std::basic_string::reserve() will shrink, libc++ maintains the behavior to shrink, even though that makes std::basic_string::reserve() not a synonym for std::basic_string::reserve(0) in any Standard mode anymore.

Building the libc++ shared or static library requires a C++ 20 capable compiler. Consider using a Bootstrapping build to build libc++ with a fresh Clang if you can’t use the system compiler to build libc++ anymore.

Historically, there has been numerous ways of building libc++ and libc++abi. This has culminated in over 5 different ways to build the runtimes, which made it impossible to maintain with a good level of support. Starting with this release, the runtimes support exactly two ways of being built, which should cater to all use-cases. Furthermore, these builds are as lightweight as possible and will work consistently even when targeting embedded platforms, which used not to be the case. Please see the documentation on building libc++ to see those two ways of building and migrate over to the appropriate build instructions as soon as possible.

All other ways to build are deprecated and will not be supported in the next release. We understand that making these changes can be daunting. For that reason, here’s a summary of how to migrate from the two most common ways to build:

• If you were rooting your CMake invocation at <monorepo>/llvm and passing -DLLVM_ENABLE_PROJECTS=<...> (which was the previously advertised way to build the runtimes), please simply root your CMake invocation at <monorepo>/runtimes and pass -DLLVM_ENABLE_RUNTIMES=<...>.

• If you were doing two CMake invocations, one rooted at <monorepo>/libcxx and one rooted at <monorepo>/libcxxabi (this used to be called a “Standalone build”), please move them to a single invocation like so:

  $ cmake -S <monorepo>/libcxx -B libcxx-build <LIBCXX-OPTIONS>
  $ cmake -S <monorepo>/libcxxabi -B libcxxabi-build <LIBCXXABI-OPTIONS>
  

  should become

  $ cmake -S <monorepo>/runtimes -B build -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" <LIBCXX-OPTIONS> <LIBCXXABI-OPTIONS>
  

Support for building the runtimes using the GCC 32 bit multilib flag (-m32) has been removed. Support for this had been flaky for a while, and we didn’t know of anyone depending on this. Instead, please perform a normal cross-compilation of the runtimes using the appropriate target, such as passing the following to your bootstrapping build:

-DLLVM_RUNTIME_TARGETS=i386-unknown-linux

Libc++, libc++abi and libunwind will not be built with -fPIC by default anymore. If you want to build those runtimes with position independent code, please specify -DCMAKE_POSITION_INDEPENDENT_CODE=ON explicitly when configuring the build, or -DRUNTIMES_<target-name>_CMAKE_POSITION_INDEPENDENT_CODE=ON if using the bootstrapping build.