Commits on Mar 3, 2021

1. [Concurrency] Tune overloading to to allow sync overloads in async co… …

   …ntexts.
   
   The existing overloading rules strongly prefer async functions within
   async contexts, and synchronous functions in synchronous contexts.
   However, when there are other differences in the
   signature, particularly parameters of function type that differ in
   async vs. synchronous, the overloading rule would force the use of the
   synchronous function even in cases where the synchronous function
   would be better. An example:
   
       func f(_: (Int) -> Int) { }
       func f(_: (Int) async -> Int) async { }
   
       func g(_ x: Int) -> Int { -x }
   
       func h() async {
         f(g) // currently selects async f, want to select synchronous f
       }
   
   Effect the semantics change by splitting the "sync/async mismatch"
   score in the constraint system into an "async in sync mismatch" score
   that is mostly disqualifying (because the call will always fail) and a
   less-important score for "sync used in an async context", which also
   includes conversion from a synchronous function to an asynchronous
   one. This way, only synchronous functions are still considered within
   a synchronous context, but we get more natural overloading behavior
   within an asynchronous context. The end result is intended to be
   equivalent to what one would get with reasync:
   
     func f(_: (Int) async -> Int) async { ... }
   
   Addresses rdar://74289867.

   

   DougGregor committed Mar 3, 2021
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