[Property delegates] Fix printing of memberwise initializer default arguments

lib/AST/Decl.cpp

@@ -42,6 +42,7 @@
 #include "swift/AST/TypeCheckRequests.h"
 #include "swift/AST/TypeLoc.h"
 #include "swift/AST/SwiftNameTranslation.h"
+#include "swift/Parse/Lexer.h"
 #include "clang/Lex/MacroInfo.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallPtrSet.h"
@@ -5522,6 +5523,31 @@ void ParamDecl::setDefaultArgumentInitContext(Initializer *initContext) {
   DefaultValueAndFlags.getPointer()->InitContext = initContext;
 }
 
+Expr *swift::findOriginalPropertyDelegateInitialValue(VarDecl *var,
+                                                      Expr *init) {
+  auto attr = var->getAttachedPropertyDelegate();
+  assert(attr && "No attached property delegate?");
+
+  // Direct initialization implies no original initial value.
+  if (attr->getArg())
+    return nullptr;
+
+  // Look through any expressions wrapping the initializer.
+  init = init->getSemanticsProvidingExpr();
+  auto initCall = dyn_cast<CallExpr>(init);
+  if (!initCall)
+    return nullptr;
+
+  auto initArg = cast<TupleExpr>(initCall->getArg())->getElement(0);
+  initArg = initArg->getSemanticsProvidingExpr();
+  if (auto autoclosure = dyn_cast<AutoClosureExpr>(initArg)) {
+    initArg =
+        autoclosure->getSingleExpressionBody()->getSemanticsProvidingExpr();
+  }
+
+  return initArg;
+}
+
 StringRef
 ParamDecl::getDefaultValueStringRepresentation(
   SmallVectorImpl<char> &scratch) const {
@@ -5552,6 +5578,23 @@ ParamDecl::getDefaultValueStringRepresentation(
     if (!existing.empty())
       return existing;
     auto var = getStoredProperty();
+
+    if (auto original = var->getOriginalDelegatedProperty()) {
+      if (auto attr = original->getAttachedPropertyDelegate()) {
+        if (auto arg = attr->getArg()) {
+          SourceRange fullRange(attr->getTypeLoc().getSourceRange().Start,
+                                arg->getEndLoc());
+          auto charRange = Lexer::getCharSourceRangeFromSourceRange(
+              getASTContext().SourceMgr, fullRange);
+          return getASTContext().SourceMgr.extractText(charRange);
+        }
+
+        auto init = findOriginalPropertyDelegateInitialValue(
+            original, original->getParentInitializer());
+        return extractInlinableText(getASTContext().SourceMgr, init, scratch);
+      }
+    }
+
     return extractInlinableText(getASTContext().SourceMgr,
                                 var->getParentInitializer(),
                                 scratch);