Merge pull request swiftlang#36828 from slavapestov/reasync-ranking-fix

Sema: Don't score a solution higher if there's a sync-vs-async mismatch with a 'reasync' function

Author: slavapestov

lib/Sema/CSRanking.cpp

@@ -627,8 +627,8 @@ bool CompareDeclSpecializationRequest::evaluate(
     // If they both have trailing closures, compare those separately.
     bool compareTrailingClosureParamsSeparately = false;
     if (numParams1 > 0 && numParams2 > 0 &&
-        params1.back().getOldType()->is<AnyFunctionType>() &&
-        params2.back().getOldType()->is<AnyFunctionType>()) {
+        params1.back().getParameterType()->is<AnyFunctionType>() &&
+        params2.back().getParameterType()->is<AnyFunctionType>()) {
       compareTrailingClosureParamsSeparately = true;
     }
 
@@ -1132,8 +1132,8 @@ SolutionCompareResult ConstraintSystem::compareSolutions(
         auto params = fnTy->getParams();
         assert(params.size() == 2);
 
-        auto param1 = params[0].getOldType();
-        auto param2 = params[1].getOldType()->castTo<AnyFunctionType>();
+        auto param1 = params[0].getParameterType();
+        auto param2 = params[1].getParameterType()->castTo<AnyFunctionType>();
 
         assert(param1->getOptionalObjectType());
         assert(params[1].isAutoClosure());

lib/Sema/ConstraintSystem.cpp

@@ -2890,7 +2890,8 @@ void ConstraintSystem::resolveOverload(ConstraintLocator *locator,
     // If we're choosing an asynchronous declaration within a synchronous
     // context, or vice-versa, increase the async/async mismatch score.
     if (auto func = dyn_cast<AbstractFunctionDecl>(decl)) {
-      if (func->isAsyncContext() != isAsynchronousContext(useDC)) {
+      if (!func->hasPolymorphicEffect(EffectKind::Async) &&
+          func->isAsyncContext() != isAsynchronousContext(useDC)) {
         increaseScore(
             func->isAsyncContext() ? SK_AsyncInSyncMismatch : SK_SyncInAsync);
       }

test/Concurrency/reasync_ranking.swift

@@ -0,0 +1,30 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency
+
+// We don't want 'reasync' overloads to have a higher score in the
+// case of a sync vs reasync mismatch the way 'async' overloads do,
+// since this would change solver performance characteristics when
+// using the reasync '&&', '||' and '??' operators.
+
+func asyncOverload(_: () async -> (), _: Int) async {}
+func asyncOverload(_: () -> (), _: String) {}
+
+func referencesAsyncOverload() {
+  _ = asyncOverload // we prefer the sync overload
+}
+
+func referencesAsyncOverloadAsync() async {
+  _ = asyncOverload // we prefer the async overload
+}
+
+func reasyncOverload(_: () async -> (), _: Int) reasync {} // expected-note {{found this candidate}}
+func reasyncOverload(_: () -> (), _: String) {} // expected-note {{found this candidate}}
+
+func referencesReasyncOverload() {
+  _ = reasyncOverload // expected-error {{ambiguous use of 'reasyncOverload'}}
+}
+
+func referencesReasyncOverloadAsync() async {
+  // we prefer the async overload because the sync overload scores higher
+  // due to a sync-vs-async mismatch.
+  _ = reasyncOverload
+}