[Refactoring] Avoid duplicating return statements

It's possible the user has already written an
explicit return for the call to the completion
handler. In that case, avoid adding another return.

rdar://77789360

Author: hamishknight

lib/IDE/Refactoring.cpp

@@ -5736,15 +5736,23 @@ class AsyncConverter : private SourceEntityWalker {
   void addHandlerCall(const CallExpr *CE) {
     auto Exprs = TopHandler.extractResultArgs(CE);
 
+    bool AddedReturnOrThrow = true;
     if (!Exprs.isError()) {
-      OS << tok::kw_return;
+      // It's possible the user has already written an explicit return statement
+      // for the completion handler call, e.g 'return completion(args...)'. In
+      // that case, be sure not to add another return.
+      auto *parent = getWalker().Parent.getAsStmt();
+      AddedReturnOrThrow = !(parent && isa<ReturnStmt>(parent));
+      if (AddedReturnOrThrow)
+        OS << tok::kw_return;
     } else {
       OS << tok::kw_throw;
     }
 
     ArrayRef<Expr *> Args = Exprs.args();
     if (!Args.empty()) {
-      OS << " ";
+      if (AddedReturnOrThrow)
+        OS << " ";
       if (Args.size() > 1)
         OS << tok::l_paren;
       for (size_t I = 0, E = Args.size(); I < E; ++I) {

test/refactoring/ConvertAsync/convert_function.swift

@@ -247,3 +247,13 @@ func voidResultCompletion(completion: (Result<Void, Error>) -> Void) {
 // RUN: %refactor -add-async-alternative -dump-text -source-filename %s -pos=%(line+1):1 | %FileCheck -check-prefix=NON-COMPLETION-HANDLER %s
 func functionWithSomeHandler(handler: (String) -> Void) {}
 // NON-COMPLETION-HANDLER: func functionWithSomeHandler() async -> String {}
+
+// rdar://77789360 Make sure we don't print a double return statement.
+// RUN: %refactor -add-async-alternative -dump-text -source-filename %s -pos=%(line+1):1 | %FileCheck -check-prefix=RETURN-HANDLING %s
+func testReturnHandling(_ completion: (String?, Error?) -> Void) {
+  return completion("", nil)
+}
+// RETURN-HANDLING:      func testReturnHandling() async throws -> String {
+// RETURN-HANDLING-NEXT:   {{^}} return ""{{$}}
+// RETURN-HANDLING-NEXT: }
+