swift234

Introduce "near-miss" warnings for unsatisfied optional requirements.

It's a common mistake to mistype a declaration that is intended to
satisfy an optional requirement. In such "near misses", we want to
warn about the mistake and give the user options to either fix the
declaration or suppress the warning. Approach this problem be walking
over all of the members of each nominal type declaration or extension
therefore and looking to see if there are any members remaining that

(1) are similarly-named to an unfilfilled optional requirement of a
protocol whose conformance is attributed to that nominal type
declaration or extension,
(2) are not witnesses to another optional requirement,
(3) haven't explicitly suppressed the warning (e.g., by adding
explicit "private" or explicit "@nonobjc"), and
(4) have a useful suppression mechanism.

In addition to the suppression mechanisms described in (3), one can
suppress this warning by moving the declaration to an(other)
extension. This encourages a programming style where one breaks an
interface into extensions each implement conformance to one
protocol. Note that we encode the various cases where one cannot move
the declaration to another extension (e.g., one cannot move a
designated initializer or stored property out of a class declaration)
and suppress the warning when there's no way for the user to cope with
it.

Each warning produced by this diagnostic can have a bunch of notes on
it for various courses of action. For example:

t2.swift:7:14: warning: instance method 'doSomething(z:)' nearly
matches optional requirement 'doSomething(x:)' of protocol 'P1'
  @objc func doSomething(z: Double) { }
             ^
t2.swift:7:14: note: rename to 'doSomething(x:)' to satisfy this
requirement
  @objc func doSomething(z: Double) { }
             ^
                         x
t2.swift:7:14: note: move 'doSomething(z:)' to an extension to silence
this warning
  @objc func doSomething(z: Double) { }
             ^
t2.swift:7:14: note: make 'doSomething(z:)' private to silence this
warning
  @objc func doSomething(z: Double) { }
             ^
        private
t2.swift:2:17: note: requirement 'doSomething(x:)' declared here
  optional func doSomething(x: Int)
                ^

It's a *lot* of detail, but is intended to cover the various choices
available to the user: Fix-It to the names of the requirement (for
naming-related mistakes) or suppress via various mechanisms. Combining
notes means losing Fix-Its, while dropping notes can lead users to
non-optimal solutions.

This is more of rdar://problem/25159872.