diff --git a/lib/AST/ASTPrinter.cpp b/lib/AST/ASTPrinter.cpp
index 349f18e6fa01f..98b51a127859f 100644
--- a/lib/AST/ASTPrinter.cpp
+++ b/lib/AST/ASTPrinter.cpp
@@ -6619,7 +6619,18 @@ class TypePrinter : public TypeVisitor<TypePrinter, void, NonRecursivePrintOptio
 
     auto isolation = info.getIsolation();
     switch (isolation.getKind()) {
-    case FunctionTypeIsolation::Kind::NonIsolated:
+    case FunctionTypeIsolation::Kind::NonIsolated: {
+      // When `NonisolatedNonsendingByDefault` is enabled and async
+      // function type is nonisolated it could only mean that it is
+      // either explicitly `@concurrent` or inferred to be one and
+      // it should be printed accordingly.
+      if (ctx.LangOpts.hasFeature(Feature::NonisolatedNonsendingByDefault) &&
+          fnType->isAsync()) {
+        Printer << "@concurrent ";
+      }
+      break;
+    }
+
     case FunctionTypeIsolation::Kind::Parameter:
       break;
 
diff --git a/test/Concurrency/attr_execution/nonisolated_nonsending_by_default.swift b/test/Concurrency/attr_execution/nonisolated_nonsending_by_default.swift
index 6c240ec88f364..19ff4eb2587e5 100644
--- a/test/Concurrency/attr_execution/nonisolated_nonsending_by_default.swift
+++ b/test/Concurrency/attr_execution/nonisolated_nonsending_by_default.swift
@@ -5,6 +5,19 @@
 func testCasts() {
   let defaultedType = (() async -> ()).self
   _ = defaultedType as (@concurrent () async -> ()).Type
-  // expected-error@-1 {{cannot convert value of type '(nonisolated(nonsending) () async -> ()).Type' to type '(() async -> ()).Type' in coercion}}
+  // expected-error@-1 {{cannot convert value of type '(nonisolated(nonsending) () async -> ()).Type' to type '(@concurrent () async -> ()).Type' in coercion}}
   _ = defaultedType as (nonisolated(nonsending) () async -> ()).Type // Ok
 }
+
+protocol TestWitnessFixIts {
+  func test(_: @concurrent () async -> Void)
+  // expected-note@-1 {{protocol requires function 'test' with type '(@concurrent () async -> Void) -> ()'}}
+}
+
+do {
+  struct Test: TestWitnessFixIts { // expected-error {{type 'Test' does not conform to protocol 'TestWitnessFixIts'}}
+    // expected-note@-1 {{add stubs for conformance}} {{35-35=\n      func test(_: @concurrent () async -> Void) {\n          <#code#>\n      \}\n}}
+    func test(_: () async -> Void) {}
+    // expected-note@-1 {{candidate has non-matching type '(nonisolated(nonsending) () async -> Void) -> ()'}}
+  }
+}
diff --git a/test/Concurrency/transfernonsendable.swift b/test/Concurrency/transfernonsendable.swift
index 345627a67beb6..c8103a80348c0 100644
--- a/test/Concurrency/transfernonsendable.swift
+++ b/test/Concurrency/transfernonsendable.swift
@@ -1878,8 +1878,11 @@ func mutableLocalCaptureDataRace() async {
   x = 0
   _ = x
 
-  Task.detached { x = 1 } // expected-warning {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-  // expected-note @-1 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+  Task.detached { x = 1 }
+  // expected-ni-warning @-1 {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+  // expected-ni-note @-2 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+  // expected-ni-ns-warning @-3 {{sending value of non-Sendable type '@concurrent () async -> ()' risks causing data races}}
+  // expected-ni-ns-note @-4 {{Passing value of non-Sendable type '@concurrent () async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
 
   x = 2 // expected-note {{access can happen concurrently}}
 }
@@ -1888,8 +1891,11 @@ func mutableLocalCaptureDataRace2() async {
   var x = 0
   x = 0
 
-  Task.detached { x = 1 } // expected-warning {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-  // expected-note @-1 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+  Task.detached { x = 1 }
+  // expected-ni-warning @-1 {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+  // expected-ni-note @-2 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+  // expected-ni-ns-warning @-3 {{sending value of non-Sendable type '@concurrent () async -> ()' risks causing data races}}
+  // expected-ni-ns-note @-4 {{Passing value of non-Sendable type '@concurrent () async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
 
   print(x) // expected-note {{access can happen concurrently}}
 }
diff --git a/test/Concurrency/transfernonsendable_closureliterals_isolationinference.swift b/test/Concurrency/transfernonsendable_closureliterals_isolationinference.swift
index fb3669656dfd4..a4153bb3d3172 100644
--- a/test/Concurrency/transfernonsendable_closureliterals_isolationinference.swift
+++ b/test/Concurrency/transfernonsendable_closureliterals_isolationinference.swift
@@ -170,8 +170,11 @@ func test_CallerSyncNormal_CalleeAsyncMainActorIsolated() async {
 
     // CHECK-LABEL: // closure #1 in test_CallerSyncNormal_CalleeAsyncMainActorIsolated()
     // CHECK-NEXT: // Isolation: global_actor. type: CustomActor
-    await normalGlobalActorAcceptsAsyncClosure { } // expected-error {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-    // expected-note @-1 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '() async -> ()' to main actor-isolated global function 'normalGlobalActorAcceptsAsyncClosure' risks causing races in between global actor 'CustomActor'-isolated and main actor-isolated uses}}
+    await normalGlobalActorAcceptsAsyncClosure { }
+    // expected-ni-error @-1 {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+    // expected-ni-note @-2 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '() async -> ()' to main actor-isolated global function 'normalGlobalActorAcceptsAsyncClosure' risks causing races in between global actor 'CustomActor'-isolated and main actor-isolated uses}}
+    // expected-ni-ns-error @-3 {{sending value of non-Sendable type '@concurrent () async -> ()' risks causing data races}}
+    // expected-ni-ns-note @-4 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '@concurrent () async -> ()' to main actor-isolated global function 'normalGlobalActorAcceptsAsyncClosure' risks causing races in between global actor 'CustomActor'-isolated and main actor-isolated uses}}
 
     // CHECK-LABEL: // closure #2 in test_CallerSyncNormal_CalleeAsyncMainActorIsolated()
     // CHECK-NEXT: // Isolation: nonisolated
@@ -286,8 +289,11 @@ func test_CallerAsyncNormal_CalleeAsyncMainActorIsolated() async {
 
     // CHECK-LABEL: // closure #1 in test_CallerAsyncNormal_CalleeAsyncMainActorIsolated()
     // CHECK-NEXT: // Isolation: global_actor. type: CustomActor
-    await asyncNormalGlobalActorAcceptsAsyncClosure { } // expected-error {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-    // expected-note @-1 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '() async -> ()' to main actor-isolated global function 'asyncNormalGlobalActorAcceptsAsyncClosure' risks causing races in between global actor 'CustomActor'-isolated and main actor-isolated uses}}
+    await asyncNormalGlobalActorAcceptsAsyncClosure { }
+    // expected-ni-error @-1 {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+    // expected-ni-note @-2 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '() async -> ()' to main actor-isolated global function 'asyncNormalGlobalActorAcceptsAsyncClosure' risks causing races in between global actor 'CustomActor'-isolated and main actor-isolated uses}}
+    // expected-ni-ns-error @-3 {{sending value of non-Sendable type '@concurrent () async -> ()' risks causing data races}}
+    // expected-ni-ns-note @-4 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '@concurrent () async -> ()' to main actor-isolated global function 'asyncNormalGlobalActorAcceptsAsyncClosure' risks causing races in between global actor 'CustomActor'-isolated and main actor-isolated uses}}
 
     // CHECK-LABEL: // closure #2 in test_CallerAsyncNormal_CalleeAsyncMainActorIsolated()
     // CHECK-NEXT: // Isolation: nonisolated
@@ -397,8 +403,11 @@ extension MyActor {
 
         // CHECK-LABEL: // closure #1 in MyActor.test_CallerSyncNormal_CalleeAsyncMainActorIsolated()
         // CHECK-NEXT: // Isolation: actor_instance. name: 'self'
-        await normalGlobalActorAcceptsAsyncClosure { print(self) } // expected-error {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-        // expected-note @-1 {{sending 'self'-isolated value of non-Sendable type '() async -> ()' to main actor-isolated global function 'normalGlobalActorAcceptsAsyncClosure' risks causing races in between 'self'-isolated and main actor-isolated uses}}
+        await normalGlobalActorAcceptsAsyncClosure { print(self) }
+        // expected-ni-error @-1 {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+        // expected-ni-note @-2 {{sending 'self'-isolated value of non-Sendable type '() async -> ()' to main actor-isolated global function 'normalGlobalActorAcceptsAsyncClosure' risks causing races in between 'self'-isolated and main actor-isolated uses}}
+        // expected-ni-ns-error @-3 {{sending value of non-Sendable type '@concurrent () async -> ()' risks causing data races}}
+        // expected-ni-ns-note @-4 {{sending 'self'-isolated value of non-Sendable type '@concurrent () async -> ()' to main actor-isolated global function 'normalGlobalActorAcceptsAsyncClosure' risks causing races in between 'self'-isolated and main actor-isolated uses}}
 
         // CHECK-LABEL: // closure #2 in MyActor.test_CallerSyncNormal_CalleeAsyncMainActorIsolated()
         // CHECK-NEXT: // Isolation: nonisolated
@@ -513,8 +522,11 @@ extension MyActor {
 
         // CHECK-LABEL: // closure #1 in MyActor.test_CallerAsyncNormal_CalleeAsyncMainActorIsolated()
         // CHECK-NEXT: // Isolation: actor_instance. name: 'self'
-        await asyncNormalGlobalActorAcceptsAsyncClosure { print(self) } // expected-error {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-        // expected-note @-1 {{sending 'self'-isolated value of non-Sendable type '() async -> ()' to main actor-isolated global function 'asyncNormalGlobalActorAcceptsAsyncClosure' risks causing races in between 'self'-isolated and main actor-isolated uses}}
+        await asyncNormalGlobalActorAcceptsAsyncClosure { print(self) }
+        // expected-ni-error @-1 {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+        // expected-ni-note @-2 {{sending 'self'-isolated value of non-Sendable type '() async -> ()' to main actor-isolated global function 'asyncNormalGlobalActorAcceptsAsyncClosure' risks causing races in between 'self'-isolated and main actor-isolated uses}}
+        // expected-ni-ns-error @-3 {{sending value of non-Sendable type '@concurrent () async -> ()' risks causing data races}}
+        // expected-ni-ns-note @-4 {{sending 'self'-isolated value of non-Sendable type '@concurrent () async -> ()' to main actor-isolated global function 'asyncNormalGlobalActorAcceptsAsyncClosure' risks causing races in between 'self'-isolated and main actor-isolated uses}}
 
         // CHECK-LABEL: // closure #2 in MyActor.test_CallerAsyncNormal_CalleeAsyncMainActorIsolated()
         // CHECK-NEXT: // Isolation: nonisolated
diff --git a/test/Concurrency/transfernonsendable_nonisolatedunsafe.swift b/test/Concurrency/transfernonsendable_nonisolatedunsafe.swift
index fa1aa966c21b9..41899a36e1745 100644
--- a/test/Concurrency/transfernonsendable_nonisolatedunsafe.swift
+++ b/test/Concurrency/transfernonsendable_nonisolatedunsafe.swift
@@ -1,5 +1,5 @@
-// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
-// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix ni- %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix ni-ns- %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // READ THIS: This test is intended to centralize all tests that use
 // nonisolated(unsafe).
@@ -1009,8 +1009,12 @@ func closureTests() async {
 
   func testWithTaskDetached() async {
     let x1 = NonSendableKlass()
-    Task.detached { _ = x1 } // expected-warning {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-    // expected-note @-1 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+    Task.detached { _ = x1 }
+    // expected-ni-warning @-1 {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+    // expected-ni-note @-2 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+    // expected-ni-ns-warning @-3 {{sending value of non-Sendable type '@concurrent () async -> ()' risks causing data races}}
+    // expected-ni-ns-note @-4 {{Passing value of non-Sendable type '@concurrent () async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+
     Task.detached { _ = x1 } // expected-note {{access can happen concurrently}}
 
     nonisolated(unsafe) let x2 = NonSendableKlass()
@@ -1024,8 +1028,12 @@ func closureTests() async {
 
     nonisolated(unsafe) let x4a = NonSendableKlass()
     let x4b = NonSendableKlass()
-    Task.detached { _ = x4a; _ = x4b } // expected-warning {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-    // expected-note @-1 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+    Task.detached { _ = x4a; _ = x4b }
+    // expected-ni-warning @-1 {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+    // expected-ni-note @-2 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+    // expected-ni-ns-warning @-3 {{sending value of non-Sendable type '@concurrent () async -> ()' risks causing data races}}
+    // expected-ni-ns-note @-4 {{Passing value of non-Sendable type '@concurrent () async -> ()' as a 'sending' argument to static method 'detached(name:priority:operation:)' risks causing races in between local and caller code}}
+
     Task.detached { _ = x4a; _ = x4b } // expected-note {{access can happen concurrently}}
   }
 
diff --git a/test/Concurrency/transfernonsendable_typed_errors.swift b/test/Concurrency/transfernonsendable_typed_errors.swift
index 0772f3a855498..bd27f477440f8 100644
--- a/test/Concurrency/transfernonsendable_typed_errors.swift
+++ b/test/Concurrency/transfernonsendable_typed_errors.swift
@@ -1,5 +1,5 @@
-// RUN: %target-swift-frontend -swift-version 6 -Xllvm -sil-regionbasedisolation-force-use-of-typed-errors -emit-sil -o /dev/null %s -verify -target %target-swift-5.1-abi-triple
-// RUN: %target-swift-frontend -swift-version 6 -Xllvm -sil-regionbasedisolation-force-use-of-typed-errors -emit-sil -o /dev/null %s -verify -target %target-swift-5.1-abi-triple -enable-upcoming-feature NonisolatedNonsendingByDefault
+// RUN: %target-swift-frontend -swift-version 6 -Xllvm -sil-regionbasedisolation-force-use-of-typed-errors -emit-sil -o /dev/null %s -verify -verify-additional-prefix ni- -target %target-swift-5.1-abi-triple
+// RUN: %target-swift-frontend -swift-version 6 -Xllvm -sil-regionbasedisolation-force-use-of-typed-errors -emit-sil -o /dev/null %s -verify -verify-additional-prefix ni-ns- -target %target-swift-5.1-abi-triple -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: asserts