Clang is using C++17 standard version by default since Clang 16.
Swift’s ClangImporter should do the same, to make sure that clients who run clang and then swiftc without explicit std version see consistent behavior.
rdar://125777068
If a C++ type `Derived` inherits from `Base` privately, the public methods from `Base` should not be callable on an instance of `Derived`. However, C++ supports exposing such methods via a using declaration: `using MyPrivateBase::myPublicMethod;`.
MSVC started using this feature for `std::optional` which means Swift doesn't correctly import `var pointee: Pointee` for instantiations of `std::optional` on Windows. This prevents the automatic conformance to `CxxOptional` from being synthesized.
rdar://114282353 / resolves https://github.com/apple/swift/issues/68068
This disables TBD validation when C++ interop is enabled, unless an explicit `-validate-tbd-against-ir=` flag was passed.
rdar://83405989 / https://github.com/apple/swift/issues/56458
This adds a protocol to the C++ standard library overlay which will improve the ergonomics of `std::optional` when used from Swift code.
As of now, the overlay adds an initializer of `Swift.Optional` that takes an instance of `CxxOptional` as a parameter.
