DougGregor

First steps for Swift/C++ Interoperability

Just a quick brain dump of "first steps" to move Swift/C++ interoperability forward. While there are many big and interesting design questions for a good interoperability story, there are also a large number of implementation tasks that can proceed independently of those design discussions (and often in parallel). This list focuses on those implementation tasks:

1. (DONE) Add a flag -enable-cxx-interop to the frontend, and have it enable (Objective-)C++ mode in the Clang importer.
2. Add a lit configuration flag to enable -enable-cxx-interop for the full Swift testsuite and fix all of the issues that prevent (Objective-)C code from being imported correctly when it's compiled as C++. The testsuite will likely need a lot of extern "C" annotations throughout it's headers---those fixes can be committed to the testsuite behind appropriate #ifdefs.
3. Import C++ namespaces as Swift enums that have no cases (so-called "uninhabited enums") so the C++ lexical struct

DougGregor

import Darwin

@dynamicMemberLookup
struct Environment {
  subscript(dynamicMember name: String) -> String? {
    get {
      guard let value = getenv(name) else { return nil }

      return String(validatingUTF8: value)
    }

DougGregor

protocol Fooable {
  associatedtype Foo
}

protocol Barrable {
  associatedtype Bar: Fooable
}

struct X {}
struct Y: Fooable {

DougGregor

protocol P { }

protocol Q {
	associatedtype A: P
}

protocol R: Q {
	associatedtype A: P
}

DougGregor

  protocol P {
      associatedtype PType
  }

  public protocol Q {
      associatedtype QType: R
  }

  public protocol R {
      associatedtype RType

DougGregor

protocol P { }

protocol Q {
  associatedtype Assoc
  func assoc() -> Assoc
}

protocol R : Q {
  associatedtype Assoc: P
}

DougGregor

Implementing Recursive Protocol Constraints

Recursive protocol constraints are a small-looking feature (really, eliminating a limitation that feels arbitrary to users) that can also greatly improve the standard library. However, their implementation will require a number of changes throughout the Swift compiler. This document attempts to catalogue the kinds of changes required in the compiler to implement this feature, to provide some scoping and direction for the work.

ArchetypeBuilder

The ArchetypeBuilder is responsible for processing the requirements in a protocol, generic function, generic type, or extension of a generic type to determine whether they are consistent. It also plays a central role in producing and canonicalizing generic signatures, as well as producing archetypes for a generic signature in a given generic environment. It is where recursive protocol constraints are detected and diagnosed, and the primary place where we will need to deal with recursion.

Lazily expand nested t