[Async Refactoring] Add @discardableResult for defaulted completion

lib/IDE/Refactoring.cpp

@@ -4413,6 +4413,14 @@ struct AsyncHandlerParamDesc : public AsyncHandlerDesc {
     OS << tok::r_paren;
   }
 
+  /// Retrieves the parameter decl for the completion handler parameter, or
+  /// \c nullptr if no valid completion parameter is present.
+  const ParamDecl *getHandlerParam() const {
+    if (!isValid())
+      return nullptr;
+    return Func->getParameters()->get(Index);
+  }
+
   bool operator==(const AsyncHandlerParamDesc &Other) const {
     return Handler == Other.Handler && Type == Other.Type &&
            HasError == Other.HasError && Index == Other.Index;
@@ -6313,6 +6321,13 @@ class AsyncConverter : private SourceEntityWalker {
 
   void addFuncDecl(const FuncDecl *FD) {
     auto *Params = FD->getParameters();
+    auto *HandlerParam = TopHandler.getHandlerParam();
+
+    // If the completion handler parameter has a default argument, the async
+    // version is effectively @discardableResult, as not all the callers care
+    // about receiving the completion call.
+    if (HandlerParam && HandlerParam->isDefaultArgument())
+      OS << tok::at_sign << "discardableResult" << "\n";
 
     // First chunk: start -> the parameter to remove (if any)
     SourceLoc LeftEndLoc = Params->getLParenLoc().getAdvancedLoc(1);

test/refactoring/ConvertAsync/convert_function.swift

@@ -1,3 +1,5 @@
+// RUN: %empty-directory(%t)
+
 enum CustomError : Error {
   case Bad
 }
@@ -309,3 +311,21 @@ func rdar78693050(_ completion: () -> Void) {
 // RDAR78693050-NEXT:   }
 // RDAR78693050-NEXT:   return
 // RDAR78693050-NEXT: }
+
+// RUN: %refactor-check-compiles -convert-to-async -dump-text -source-filename %s -pos=%(line+1):1 | %FileCheck -check-prefix=DISCARDABLE-RESULT %s
+func withDefaultedCompletion(arg: String, completion: @escaping (String) -> Void = {_ in}) {
+  completion(arg)
+}
+
+// DISCARDABLE-RESULT:      @discardableResult
+// DISCARDABLE-RESULT-NEXT: func withDefaultedCompletion(arg: String) async -> String {
+// DISCARDABLE-RESULT-NEXT:   return arg
+// DISCARDABLE-RESULT-NEXT: }
+
+// RUN: %refactor-check-compiles -convert-to-async -dump-text -source-filename %s -pos=%(line+1):1 | %FileCheck -check-prefix=DEFAULT-ARG %s
+func withDefaultArg(x: String = "") {
+}
+
+// DEFAULT-ARG:      convert_function.swift [[# @LINE-3]]:1 -> [[# @LINE-2]]:2
+// DEFAULT-ARG-NOT:  @discardableResult
+// DEFAULT-ARG-NEXT: {{^}}func withDefaultArg(x: String = "") async