Swift value types are their bridged Objective-C classes can have
different hash values. To address this, AnyHashable's responds to the
_HasCustomAnyHashableRepresentation protocol, which bridge objects of
those class types---NSString, NSNumber, etc---into their Swift
counterparts. That way, we get consistent (Swift) hashing behavior
across platforms.
However, there are cases where multiple Swift value types map to the
same Objective-C class type. In such cases, AnyHashable ends up
converting the object of class type back to some canonical type. For
example, an NS_STRING_ENUM (such as (NS)RunLoopMode) is a Swift
wrapper around a String. If an (NS)RunLoopMode is placed into an
AnyHashable, it maintains it's Swift type identity (which is correct
behavior). If it is bridged to Objective-C, it becomes an NSString; if
that NSString is placed into an AnyHashable, it produces a String. The
hash values still line up, but equality of the AnyHashable values
fails, which breaks when (for example) a dictionary with AnyHashable
keys is used from Objective-C. See SR-2648 / rdar://problem/27992351
for a case where this breaks interoperability.
To address this problem, make AnyHashable's casting and equality
sensitive to the origin of the hashed value: if the AnyHashable was
created through a _HasCustomAnyHashableRepresentation conformance,
treat comparisons/casting from it as "fuzzy":
* For equality, if one of the AnyHashable's comes from a custom
representation (e.g., it originated with an Objective-C type like
NSString) but the other did not, bridge the value of the *other*
AnyHashable to Objective-C, re-wrap it in an AnyHashable, and
compare that. This allows, e.g., an (NS)RunLoopMode created in Swift
to compare to an NSString constant with the same string value.
* For casting, if the AnyHashable we're casting from came from a
custom representation and the cast would fail, bridge to Objective-C
and then initiate the cast again. This allows an NSString to be
casted to (NS)RunLoopMode.
Fixes SR-2648 / rdar://problem/27992351.
