Merge pull request swiftlang#62871 from kavon/5.8-main-actor-conversions-combo

[5.8🍒] Resolve ambiguity error for global-actor function conversions

Author: kavon

lib/Sema/CSSimplify.cpp

@@ -3005,6 +3005,13 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
         return getTypeMatchFailure(locator);
     } else if (kind < ConstraintKind::Subtype) {
       return getTypeMatchFailure(locator);
+    } else {
+      // It is possible to convert from a function without a global actor to a
+      // similar function type that does have a global actor. But, since there
+      // is a function conversion going on here, let's increase the score to
+      // avoid ambiguity when solver can also match a global actor matching
+      // function type.
+      increaseScore(SK_FunctionConversion);
     }
   }
 

test/Concurrency/global_actor_function_types.swift

@@ -153,3 +153,10 @@ func test() {
     onSomeGlobalActor()
   }
 }
+
+// https://github.com/apple/swift/issues/61436
+let x: @MainActor () -> Void
+
+let y: () -> Void = {}
+
+x = true ? y : y // Ok