When comparing a requirement to a witness for @objc protocols, strip
optionality out of the types and keep track of the optionality
differences separately. When we have actually matched a witness,
diagnose any unsafe (via an error) or ill-advised (via a warning)
differences, providing Fix-Its to update the optionality.
This change addresses a usable problem introduced by the fix to
rdar://problem/18383574: witnesses for optional requirements of @objc
protocols could previously have completely wrong optionality, and we
would "allow" it by not actually matching the witness to the
requirement. Then it would happen to work at runtime because it's
@objc. Now, we match those witnesses and diagnose issues, with Fix-Its
to clean up the user's code.
Addresses rdar://problem/19656106.
Swift SVN r24939
