[Sema][ObjC] Preserve syntactic sugar when removing ARCReclaimReturnedObject cast

[ahatanak]

Summary

This is a follow-up patch to r314370.

Rather than throwing away the enclosing parentheses in maybeUndoReclaimObject, this patch walks down the expression until an ARCReclaimReturnedObject cast is found and removes just the cast, preserving the syntactic sugar expressions (parens and casts) that were visited up to that point.

rdar://problem/34705720

[rjmccall]

LGTM.

lib/Sema/SemaExprObjC.cpp

if (const ObjCObjectPointerType *ObjT =
       if (const ObjCInterfaceDecl *ObjI = ObjT->getInterfaceDecl())
         return !ObjI->isArcWeakrefUnavailable();
   }
   return true;
 }
 
 /// Look for an ObjCReclaimReturnedObject cast and destroy it.
 static Expr *maybeUndoReclaimObject(Expr *e) {
-  // For now, we just undo operands that are *immediately* reclaim
-  // expressions, which prevents the vast majority of potential
-  // problems here.  To catch them all, we'd need to rebuild arbitrary
-  // value-propagating subexpressions --- we can't reliably rebuild
-  // in-place because of expression sharing.
-  if (auto *ice = dyn_cast<ImplicitCastExpr>(e->IgnoreParens()))
-    if (ice->getCastKind() == CK_ARCReclaimReturnedObject)
+  Expr *curExpr = e, *prevExpr = nullptr;
+
+  // Walk down the expression until we hit an implicit cast of kind
+  // ARCReclaimReturnedObject or an Expr that is neither a Paren nor a Cast.
+  while (true) {
+    if (auto *pe = dyn_cast<ParenExpr>(curExpr)) {
+      prevExpr = curExpr;
+      curExpr = pe->getSubExpr();
+      continue;
+    }
+
+    if (auto *ce = dyn_cast<CastExpr>(curExpr)) {
+      if (auto *ice = dyn_cast<ImplicitCastExpr>(ce))
+        if (ice->getCastKind() == CK_ARCReclaimReturnedObject) {
+          if (!prevExpr)
             return ice->getSubExpr();
+          if (auto *pe = dyn_cast<ParenExpr>(prevExpr))
+            pe->setSubExpr(ice->getSubExpr());
+          else
+            cast<CastExpr>(prevExpr)->setSubExpr(ice->getSubExpr());
+          return e;
+        }
+
+      prevExpr = curExpr;
+      curExpr = ce->getSubExpr();
+      continue;
+    }
+
+    // Break out of the loop if curExpr is neither a Paren nor a Cast.
+    break;
+  }
 
   return e;
 }
 
 ExprResult Sema::BuildObjCBridgedCast(SourceLocation LParenLoc,
                                       ObjCBridgeCastKind Kind,
                                       SourceLocation BridgeKeywordLoc,
                                       TypeSourceInfo *TSInfo,

test/CodeGenObjC/arc-bridged-cast.m

void bridge_of_cf(int *i) {
   // CHECK-NEXT: ret void
 }
 
 // CHECK-LABEL: define %struct.__CFString* @bridge_of_paren_expr()
 CFStringRef bridge_of_paren_expr() {
   // CHECK-NOT: call i8* @objc_retainAutoreleasedReturnValue(
   // CHECK-NOT: call void @objc_release(
   CFStringRef r = (__bridge CFStringRef)(CreateNSString());
+  r = (__bridge CFStringRef)((NSString *)(CreateNSString()));
   return r;
 }