Unfortunately, importing them as is results in ambiguous call sites.
E.g., std::vector::push_back has overloads for lvalue reference and
rvalue reference and we have no way to distinguish them at the call site
in Swift. To overcome this issue, functions with rvalue reference
parameters are imported with 'consuming:' argument labels.
Note that, in general, move only types and consuming is not properly
supported in Swift yet. We do not invoke the dtor for the moved-from
objects. This is a preexisting problem that can be observed with move
only types before this PR, so the fix will be done in a separate PR.
Fortunately, for most types, the moved-from objects do not require
additional cleanups.
rdar://125816354
While private and protected fields coming from C++ cannot be accessed from Swift, they can affect Swift typechecking.
For instance, the Swift typechecker mechanism that adds implicit `Sendable` conformances works by iterating over all of the struct's fields and checking whether all of them are `Sendable`. This logic was broken for C++ types with private fields, since they were never accounted for. This resulted in erroneous implicit `Sendable` confromances being added.
Same applies for `BitwiseCopyable`.
In addition to this, ClangImporter used to mistakenly mark all C++ structs that have private fields as types with unreferenceable storage, which hampered optimizations.
As a side effect of this change, we now also provide a better diagnostic when someone tries to access a private C++ field from Swift.
rdar://134430857
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
