[CSApply] Propagate 'async' from contextual types to closures, unless the closure

is an argument to a 'reasync' function.

Author: hborla

lib/Sema/CSApply.cpp

@@ -25,6 +25,7 @@
 #include "swift/AST/ASTVisitor.h"
 #include "swift/AST/ASTWalker.h"
 #include "swift/AST/ClangModuleLoader.h"
+#include "swift/AST/Effects.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/GenericSignature.h"
@@ -7052,6 +7053,38 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
       }
     }
 
+    /// Whether the given effect should be propagated to a closure expression.
+    auto shouldApplyEffect = [&](EffectKind kind) -> bool {
+      auto last = locator.last();
+      if (!(last && last->is<LocatorPathElt::ApplyArgToParam>()))
+        return true;
+
+      // The effect should not be applied if the closure is an argument
+      // to a function where that effect is polymorphic.
+      if (auto *call = getAsExpr<ApplyExpr>(locator.getAnchor())) {
+        if (auto *declRef = dyn_cast<DeclRefExpr>(call->getFn())) {
+          if (auto *fn = dyn_cast<AbstractFunctionDecl>(declRef->getDecl()))
+            return !fn->hasPolymorphicEffect(kind);
+        }
+      }
+
+      return true;
+    };
+
+    // If we have a ClosureExpr, we can safely propagate 'async' to the closure.
+    fromEI = fromFunc->getExtInfo();
+    if (toEI.isAsync() && !fromEI.isAsync() && shouldApplyEffect(EffectKind::Async)) {
+      auto newFromFuncType = fromFunc->withExtInfo(fromEI.withAsync());
+      if (applyTypeToClosureExpr(cs, expr, newFromFuncType)) {
+        fromFunc = newFromFuncType->castTo<FunctionType>();
+
+        // Propagating 'async' might have satisfied the entire conversion.
+        // If so, we're done, otherwise keep converting.
+        if (fromFunc->isEqual(toType))
+          return expr;
+      }
+    }
+
     // If we have a ClosureExpr, then we can safely propagate the 'no escape'
     // bit to the closure without invalidating prior analysis.
     fromEI = fromFunc->getExtInfo();

test/Concurrency/actor_isolation.swift

@@ -71,7 +71,7 @@ actor MyActor: MySuperActor { // expected-error{{actor types do not support inhe
   class func synchronousClass() { }
   static func synchronousStatic() { }
 
-  func synchronous() -> String { text.first ?? "nothing" } // expected-note 20{{calls to instance method 'synchronous()' from outside of its actor context are implicitly asynchronous}}
+  func synchronous() -> String { text.first ?? "nothing" } // expected-note 19{{calls to instance method 'synchronous()' from outside of its actor context are implicitly asynchronous}}
   func asynchronous() async -> String {
     super.superState += 4
     return synchronous()
@@ -807,7 +807,8 @@ func acceptAsyncSendableClosureInheriting<T>(@_inheritActorContext _: @Sendable
 extension MyActor {
   func testSendableAndInheriting() {
     acceptAsyncSendableClosure {
-      synchronous() // expected-error{{actor-isolated instance method 'synchronous()' cannot be referenced from a concurrent closure}}
+      synchronous() // expected-error{{expression is 'async' but is not marked with 'await'}}
+      // expected-note@-1 {{call is 'async'}}
     }
 
     acceptAsyncSendableClosure {

test/Concurrency/global_actor_inference.swift

@@ -509,7 +509,7 @@ func acceptClosure<T>(_: () -> T) { }
 // ----------------------------------------------------------------------
 func takesUnsafeMainActor(@_unsafeMainActor fn: () -> Void) { }
 
-@MainActor func onlyOnMainActor() { } // expected-note{{calls to global function 'onlyOnMainActor()' from outside of its actor context are implicitly asynchronous}}
+@MainActor func onlyOnMainActor() { }
 
 func useUnsafeMainActor() {
   takesUnsafeMainActor {
@@ -525,7 +525,8 @@ func acceptAsyncSendableClosureInheriting<T>(@_inheritActorContext _: @Sendable
 
 @MainActor func testCallFromMainActor() {
   acceptAsyncSendableClosure {
-    onlyOnMainActor() // expected-error{{call to main actor-isolated global function 'onlyOnMainActor()' in a synchronous nonisolated context}}
+    onlyOnMainActor() // expected-error{{expression is 'async' but is not marked with 'await'}}
+    // expected-note@-1 {{call is 'async'}}
   }
 
   acceptAsyncSendableClosure {