If a non-member operator is declared in a C++ namespace, we previously imported it as a static member of the enum that represents the C++ namespace.
This is not always correct under Swift rules for operators. In pure Swift, this code is valid:
```
public protocol UnsafeCxxRandomAccessIterator {
static func +=(lhs: inout Self, rhs: Int)
}
enum std {
public struct A : UnsafeCxxRandomAccessIterator {
public static func += (lhs: inout A, rhs: Int) {
}
}
}
```
but this is not valid:
```
public protocol UnsafeCxxRandomAccessIterator {
static func +=(lhs: inout Self, rhs: Int)
}
enum std {
public struct A : UnsafeCxxRandomAccessIterator {}
public static func += (lhs: inout A, rhs: Int) {}
}
// error: Member operator '+=' must have at least one argument of type 'std'
```
This caused assertion failures in SILGen when conforming C++ iterator types to `UnsafeCxxRandomAccessIterator`.
This does not include subscript operators.
Before this is re-enabled operators need to be re-implemented. Right now they are the source of a lot of bugs. They cause frequent crashes and mis compiles. Also, templated operators insert a lot of names into global lookup which causes problems.
They also don't work on Windows.
Previously a namespace declaration was imported along with all of its redeclarations, and their members were added to a single Swift extension. This was problematic when a single namespace is declared in multiple modules – the extension belonged to only one of them.
For an example of this, try printing a module interface for `std.string`/`std.iosfwd` – it will be empty, even though the declarations from those modules are actually imported into Swift correctly.
This change makes sure that when we're importing different redeclarations of the same namespace, we're adding them as separate extensions to appropriate modules.
C++ namespaces are module-independent, but enums are owned by their module's in Swift. So, to prevent declaring two enums with the same name, this patch implements a new approach to namespaces: enums with extensions.
Here's an example:
```
// Module A
namespace N { void test1(); }
// Module B
namespace N { void test2(); }
// __ObjC module
enum N { }
// Swift module A
extension N { func test1() }
// Swift module B
extension N { func test1() }
```
Thanks to @gribozavr for the great idea.
