diff --git a/lib/IRGen/TBDGen.cpp b/lib/IRGen/TBDGen.cpp
index 75f55823bd376..d4f1efbe0956c 100644
--- a/lib/IRGen/TBDGen.cpp
+++ b/lib/IRGen/TBDGen.cpp
@@ -472,9 +472,16 @@ void TBDGenVisitor::addProtocolWitnessThunk(RootProtocolConformance *C,
                                             ValueDecl *requirementDecl) {
   Mangle::ASTMangler Mangler;
 
+  std::string decorated = Mangler.mangleWitnessThunk(C, requirementDecl);
   // FIXME: We should have a SILDeclRef SymbolSource for this.
-  addSymbol(Mangler.mangleWitnessThunk(C, requirementDecl),
-            SymbolSource::forUnknown());
+  addSymbol(decorated, SymbolSource::forUnknown());
+
+  if (requirementDecl->isProtocolRequirement()) {
+    ValueDecl *PWT = C->getWitness(requirementDecl).getDecl();
+    if (const auto *AFD = dyn_cast<AbstractFunctionDecl>(PWT))
+      if (AFD->hasAsync())
+        addSymbol(decorated + "Tu", SymbolSource::forUnknown());
+  }
 }
 
 void TBDGenVisitor::addFirstFileSymbols() {
diff --git a/test/IRGen/serialised-pwt-afp.swift b/test/IRGen/serialised-pwt-afp.swift
new file mode 100644
index 0000000000000..155dd92529d2a
--- /dev/null
+++ b/test/IRGen/serialised-pwt-afp.swift
@@ -0,0 +1,23 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -disable-availability-checking -emit-module -emit-module-path %t/P.swiftmodule -parse-as-library -module-name P -DP %s
+// RUN: %target-swift-frontend -disable-availability-checking -I%t -parse-as-library -module-name Q -c %s -o /dev/null -validate-tbd-against-ir=missing
+
+// REQUIRES: concurrency
+
+#if P
+public protocol P {
+  func f() async
+}
+#else
+import P
+
+protocol Q: P { }
+
+extension Q {
+  public func f() async { }
+}
+
+public struct S: Q {
+  public init() { }
+}
+#endif