[Concurrency] Fix @Sendable closures not inferring nonisolated(nonsending)

If there are no explicit concurrency attributes, isolated parameters,
or captures associated with the closure it should infer `nonisolated(nonsending)`
for the parent conversion injected by the solver (this conversion is injected
because the solver cannot check captures to elide it).

The change pushes `isIsolationInferenceBoundaryClosure` check down
with added benefit of getting preconcurrency context from the parent.

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -4988,16 +4988,6 @@ ActorIsolation ActorIsolationChecker::determineClosureIsolation(
         return ActorIsolation::forActorInstanceCapture(param);
     }
 
-    // If we have a closure that acts as an isolation inference boundary, then
-    // we return that it is non-isolated.
-    //
-    // NOTE: Since we already checked for global actor isolated things, we
-    // know that all Sendable closures must be nonisolated. That is why it is
-    // safe to rely on this path to handle Sendable closures.
-    if (isIsolationInferenceBoundaryClosure(closure,
-                                            /*canInheritActorContext=*/true))
-      return ActorIsolation::forNonisolated(/*unsafe=*/false);
-
     // A non-Sendable closure gets its isolation from its context.
     auto parentIsolation = getActorIsolationOfContext(
         closure->getParent(), getClosureActorIsolation);
@@ -5029,6 +5019,16 @@ ActorIsolation ActorIsolationChecker::determineClosureIsolation(
       }
     }
 
+    // If we have a closure that acts as an isolation inference boundary, then
+    // we return that it is non-isolated.
+    //
+    // NOTE: Since we already checked for global actor isolated things, we
+    // know that all Sendable closures must be nonisolated. That is why it is
+    // safe to rely on this path to handle Sendable closures.
+    if (isIsolationInferenceBoundaryClosure(closure,
+                                            /*canInheritActorContext=*/true))
+      return ActorIsolation::forNonisolated(/*unsafe=*/false);
+
     return normalIsolation;
   }();
 

test/Concurrency/actor_inout_isolation.swift

@@ -220,7 +220,7 @@ if #available(SwiftStdlib 5.1, *) {
   let _ = Task.detached { await { (_ foo: inout Int) async in foo += 1 }(&number) }
   // expected-error @-1 {{actor-isolated var 'number' cannot be passed 'inout' to 'async' function call}}
   // expected-minimal-error @-2 {{global actor 'MyGlobalActor'-isolated var 'number' can not be used 'inout' from a nonisolated context}}
-  // expected-complete-error @-3 {{main actor-isolated var 'number' can not be used 'inout' from a nonisolated context}}
+  // expected-complete-warning @-3 {{main actor-isolated var 'number' can not be used 'inout' from a nonisolated context}}
 }
 
 // attempt to pass global state owned by the global actor to another async function

test/Concurrency/attr_execution/conversions_silgen.swift

@@ -613,3 +613,11 @@ func testConvertToThrowing(isolation: isolated (any Actor)? = #isolation) async
     observe()
   }
 }
+
+func testSendableClosureNonisolatedNonSendingInference() {
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen49testSendableClosureNonisolatedNonSendingInferenceyyFySiYaYbYCcfU_ : $@convention(thin) @Sendable @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, Int) -> ()
+  // CHECK: bb0([[EXECUTOR:%.*]] : @guaranteed $Optional<any Actor>, %1 : $Int):
+  // CHECK: hop_to_executor [[EXECUTOR]]
+  // CHECK: // end sil function '$s21attr_execution_silgen49testSendableClosureNonisolatedNonSendingInferenceyyFySiYaYbYCcfU_'
+  let _: nonisolated(nonsending) @Sendable (Int) async -> Void = { _ in }
+}

test/Concurrency/sendable_checking_captures_swift5.swift

@@ -92,5 +92,5 @@ do {
 
   let c: Class
   test(c)
-  // expected-complete-warning@-1:8 {{implicit capture of 'c' requires that 'Class' conforms to 'Sendable'; this is an error in the Swift 6 language mode}}
+  // expected-complete-warning@-1:8 {{implicit capture of 'c' requires that 'Class' conforms to 'Sendable'}}
 }