[Concurrency] Allow synchronous witnesses to satisfy 'async' requirements.

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -713,8 +713,8 @@ swift::matchWitness(
     }
 
     // If the witness is 'async', the requirement must be.
-    if (witnessFnType->getExtInfo().isAsync() !=
-          reqFnType->getExtInfo().isAsync()) {
+    if (witnessFnType->getExtInfo().isAsync() &&
+          !reqFnType->getExtInfo().isAsync()) {
       return RequirementMatch(witness, MatchKind::AsyncConflict);
     }
 

test/SILGen/async_conversion.swift

@@ -11,3 +11,13 @@ func test() {
   // CHECK: partial_apply [callee_guaranteed] [[THUNK]]([[THICK_F]]) : $@convention(thin) @async (Int, @guaranteed String, @guaranteed @callee_guaranteed (Int, @guaranteed String) -> @owned Optional<String>) -> @owned Optional<String>
   let _: (Int, String) async -> String? = f
 }
+
+protocol P {
+  func f(_: Int, _: String) async -> String?
+}
+
+struct X: P {
+  // CHECK-LABEL: sil private [transparent] [thunk] [ossa] @$s4test1XVAA1PA2aDP1fySSSgSi_SStYFTW : $@convention(witness_method: P) @async (Int, @guaranteed String, @in_guaranteed X) -> @owned Optional<String>
+  // CHECK: function_ref @$s4test1XV1fySSSgSi_SStF : $@convention(method) (Int, @guaranteed String, X) -> @owned Optional<String>
+  func f(_: Int, _: String) -> String? { nil }
+}

test/decl/func/async.swift

@@ -33,9 +33,9 @@ struct ConformsToP1: P1 { // expected-error{{type 'ConformsToP1' does not confor
 }
 
 protocol P2 {
-  func f() async // expected-note{{protocol requires function 'f()' with type '() async -> ()'; do you want to add a stub?}}
+  func f() async
 }
 
-struct ConformsToP2: P2 { // expected-error{{type 'ConformsToP2' does not conform to protocol 'P2'}}
-  func f() { }  // expected-note{{candidate is not 'async', but protocol requirement is}}
+struct ConformsToP2: P2 {
+  func f() { }  // okay
 }