Marcel Hlopko 4f5c75a236 [cxx-interop] Instantiate C++ class templates from Swift (#33284) ...

This PR makes it possible to instantiate C++ class templates from Swift. Given a C++ header:

```c++
// C++ module `ClassTemplates`
template<class T>
struct MagicWrapper {
  T t;
};

struct MagicNumber {};
```

it is now possible to write in Swift:

```swift
import ClassTemplates

func x() -> MagicWrapper<MagicNumber> {
  return MagicWrapper<MagicNumber>()
}
```

This is achieved by importing C++ class templates as generic structs, and then when Swift type checker calls `applyGenericArguments` we detect when the generic struct is backed by the C++ class template and call Clang to instantiate the template. In order to make it possible to put class instantiations such as `MagicWrapper<MagicNumber>` into Swift signatures, we have created a new field in `StructDecl` named `TemplateInstantiationType` where the typechecker stores the `BoundGenericType` which we serialize. Deserializer then notices that the `BoundGenericType` is actually a C++ class template and performs the instantiation logic.

Depends on https://github.com/apple/swift/pull/33420.
Progress towards https://bugs.swift.org/browse/SR-13261.
Fixes https://bugs.swift.org/browse/SR-13775.

Co-authored-by: Dmitri Gribenko <gribozavr@gmail.com>
Co-authored-by: Rosica Dejanovska <rosica@google.com>

2021-01-27 13:01:20 +01:00

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