diff --git a/lib/IRGen/GenReflection.cpp b/lib/IRGen/GenReflection.cpp
index 51ad0a295abc8..0ed853fc1d6d2 100644
--- a/lib/IRGen/GenReflection.cpp
+++ b/lib/IRGen/GenReflection.cpp
@@ -315,9 +315,8 @@ getRuntimeVersionThatSupportsDemanglingType(CanType type) {
 // where completing the metadata during demangling might cause cyclic
 // dependencies.
 static std::pair<llvm::Constant *, unsigned>
-getTypeRefByFunction(IRGenModule &IGM,
-                     CanGenericSignature sig,
-                     CanType t) {
+getTypeRefByFunction(IRGenModule &IGM, CanGenericSignature sig, CanType t,
+                     MangledTypeRefRole role) {
   IRGenMangler mangler(IGM.Context);
   std::string symbolName =
     mangler.mangleSymbolNameForMangledMetadataAccessorString(
@@ -433,7 +432,11 @@ getTypeRefByFunction(IRGenModule &IGM,
       // Form the mangled name with its relative reference.
       auto S = B.beginStruct();
       S.setPacked(true);
-      S.add(llvm::ConstantInt::get(IGM.Int8Ty, 255));
+      if (role == MangledTypeRefRole::DefaultAssociatedTypeWitness) {
+        S.add(llvm::ConstantInt::get(
+            IGM.Int8Ty,
+            ProtocolRequirementFlags::AssociatedTypeInProtocolContextByte));
+      }
       S.add(llvm::ConstantInt::get(IGM.Int8Ty, 9));
       S.addCompactFunctionReference(accessor);
 
@@ -513,7 +516,7 @@ getTypeRefImpl(IRGenModule &IGM,
     // the field will be artificially hidden to reflectors.
     if (isAlwaysNoncopyable) {
       IGM.IRGen.noteUseOfTypeMetadata(type);
-      return getTypeRefByFunction(IGM, sig, type);
+      return getTypeRefByFunction(IGM, sig, type, role);
     }
   }
   LLVM_FALLTHROUGH;
@@ -524,12 +527,12 @@ getTypeRefImpl(IRGenModule &IGM,
     // ensuring that we can always reconstruct type metadata from a mangled name
     // in-process.
     IGM.IRGen.noteUseOfTypeMetadata(type);
-    
+
     // If the minimum deployment target's runtime demangler wouldn't understand
     // this mangled name, then fall back to generating a "mangled name" with a
     // symbolic reference with a callback function.
     if (mangledNameIsUnknownToDeployTarget(IGM, type)) {
-      return getTypeRefByFunction(IGM, sig, type);
+      return getTypeRefByFunction(IGM, sig, type, role);
     }
 
     break;
diff --git a/test/IRGen/Inputs/associated_type_by_mangled_name.swift b/test/IRGen/Inputs/associated_type_by_mangled_name.swift
new file mode 100644
index 0000000000000..3e8ee23c00200
--- /dev/null
+++ b/test/IRGen/Inputs/associated_type_by_mangled_name.swift
@@ -0,0 +1,21 @@
+public protocol Proto {
+    associatedtype T: Proto
+
+    var value: T {get}
+}
+
+public func makeThing<P: Proto>(_ t: P) -> some Proto {
+    return ProtoImpl(t.value)
+}
+
+public struct ProtoImpl<V>: Proto {
+    public let genValue: V
+
+    public init(_ genValue: V) {
+        self.genValue = genValue
+    }
+
+    public var value: some Proto {
+        return self
+    }
+}
diff --git a/test/IRGen/nonisolated_nonsending_closure.swift b/test/IRGen/nonisolated_nonsending_closure.swift
new file mode 100644
index 0000000000000..32edb5efbb14d
--- /dev/null
+++ b/test/IRGen/nonisolated_nonsending_closure.swift
@@ -0,0 +1,55 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-build-swift-dylib(%t/%target-library-name(associated_type_by_mangled_name)) -target %target-cpu-apple-macosx15.0 -enable-library-evolution %S/Inputs/associated_type_by_mangled_name.swift -emit-module -emit-module-path %t/associated_type_by_mangled_name.swiftmodule -module-name associated_type_by_mangled_name
+// RUN: %target-codesign %t/%target-library-name(associated_type_by_mangled_name)
+
+// RUN: %target-swift-frontend -target %target-cpu-apple-macosx15.0 -I %t %s -emit-ir -o - | %FileCheck %s
+// RUN: %target-swift-frontend -target %target-cpu-apple-macosx26.0 -I %t %s -emit-ir -o - | %FileCheck %s -check-prefix CHECK-SUPPORTED
+
+// RUN: %target-build-swift -target %target-cpu-apple-macosx15.0 %s -lassociated_type_by_mangled_name -I %t -L %t -o %t/main15 %target-rpath(%t)
+// RUN: %target-codesign %t/main15
+
+// RUN: %target-run %t/main15 %t/%target-library-name(associated_type_by_mangled_name)
+
+// RUN: %target-build-swift -target %target-cpu-apple-macosx26.0 %s -lassociated_type_by_mangled_name -I %t -L %t -o %t/main26 %target-rpath(%t)
+// RUN: %target-codesign %t/main26
+
+// RUN: %target-run %t/main26 %t/%target-library-name(associated_type_by_mangled_name)
+
+// REQUIRES: executable_test
+// REQUIRES: OS=macosx
+// UNSUPPORTED: back_deployment_runtime || use_os_stdlib
+
+import associated_type_by_mangled_name
+
+protocol P: Sendable {
+    associatedtype T: Proto
+    func foo() -> T
+}
+
+struct PImpl: P {
+  func foo() -> some Proto {
+    return ProtoImpl(2)
+  }
+}
+
+// CHECK: @"get_type_metadata 30nonisolated_nonsending_closure1PRzl31associated_type_by_mangled_name9ProtoImplVyySiYaKYCcG.3" = linkonce_odr hidden constant <{ i8, i32, i8 }> <{ i8 9, i32 trunc (i64 sub (i64 ptrtoint (ptr @"get_type_metadata 30nonisolated_nonsending_closure1PRzl31associated_type_by_mangled_name9ProtoImplVyySiYaKYCcG" to i64), i64 ptrtoint (ptr getelementptr inbounds (<{ i8, i32, i8 }>, ptr @"get_type_metadata 30nonisolated_nonsending_closure1PRzl31associated_type_by_mangled_name9ProtoImplVyySiYaKYCcG.3", i32 0, i32 1) to i64)) to i32), i8 0 }>
+// CHECK: define linkonce_odr hidden ptr @"$s31associated_type_by_mangled_name9ProtoImplVyySiYaKYCcGACyxGAA0F0AAWl"() #1 {
+// CHECK:   call swiftcc %swift.metadata_response @"$s31associated_type_by_mangled_name9ProtoImplVyySiYaKYCcGMa"(i64 255)
+// CHECK: }
+
+// CHECK-SUPPORTED-NOT: @"get_type_metadata 30nonisolated_nonsending_closure1PRzl31associated_type_by_mangled_name9ProtoImplVyySiYaKYCcG.3"
+// CHECK-SUPPORTED: define linkonce_odr hidden ptr @"$s31associated_type_by_mangled_name9ProtoImplVyySiYaKYCcGACyxGAA0F0AAWl"() #1 {
+// CHECK-SUPPORTED:   call ptr @__swift_instantiateConcreteTypeFromMangledNameAbstract(ptr @"$s31associated_type_by_mangled_name9ProtoImplVyySiYaKYCcGMD")
+// CHECK-SUPPORTED: }
+struct MyStruct<T: P>: Proto {
+  typealias Closure = nonisolated(nonsending) (Int) async throws -> Void
+
+  let x: T
+
+  public var value: some Proto {
+    return ProtoImpl<Closure> { _ in }
+  }
+}
+
+print(makeThing(MyStruct(x: PImpl())))