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
When importing a C++ struct, if its owning module requires cplusplus, Swift tried to auto-conform it to certain protocols from the Cxx module. This triggers name lookup in the clang struct, specifically for `__beginUnsafe()` and `__endUnsafe` methods, which imports all of the base structs including their methods.
This moves the import of base structs out of the name lookup request, preventing cycles.
rdar://116426238
Even if a virtual method is not used directly from Swift, it might get emitted into the vtable, and in that case the IR for it should be emitted to avoid linker errors.
Fixes https://github.com/apple/swift/issues/61730.
This commit adds very basic support for importing and calling base class methods, getting and setting base class fields, and using types inside of base classes.
