For reasons lost to history and not interesting enough to uncover, the
“override” computation performed a check of the overriding declaration’s
Objective-C selector against the potentially-overridden declaration’s
Objective-C selector. This introduced a cycle dependency between
the “is exposed to Objective-C” computation and the “overridden declarations”
computation, because @objc is inherited from an overridden declaration.
We’ll detect the same errors via other, existing diagnostics, so prune
out the Objective-C selector checks from the override path.
The Swift class model does not support overriding declarations where either
the overridden declaration or the overriding declaration are in an extension.
However, the Objective-C class model does, so marking the declaration as
@objc (when possible) will work around the limitation.
Customize the "cannot override declaration in extension" diagnostic to
suggest adding @objc to the overridden declaration in cases where
@objc is permitted. Fixes SR-6512 / rdar://problem/35787914.
Migrate the check for whether a given type is representable in
Objective-C, which is currently used to verify when @objc can be
inferred or verify that an explicitly-written @objc is well-formed,
from Sema into a set of queries on the Type within the AST library, so
it can be used in other parts of the compiler.
As part of this refactoring, clean up and improve a number of aspects
of this code:
* Unify the "trivially representable" and "representable" code paths
into a single code path that covers these cases. Clarify the
different levels of "representable" we have in both the code and
in comments.
* Distinguish between representation in C vs. representation in
Objective-C. While we aren't using this now, I'm anticipating it
being useful to allow exporting C interfaces via @_cdecl (or
similar).
* Eliminate the special cases for bridging String/Array/Dictionary/Set
with their Foundation counterparts; we now consult
_ObjectiveCBridgeable conformances exclusively to get this
information.
* Cache foreign-representation information on the ASTContext in a
manner that will let us more easily get the right answer across
different contexts while providing more sharing than the TypeChecker
version.
Annoyingly, this only seemed to fix a small class of error where we
were permitting Unsafe(Mutable)Pointer<T> to be representable in
Objective-C when T was representable but not trivially representable,
e.g., T=String or T=AnyObject.Type.
