1 files changed, 35 insertions, 0 deletions
diff --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index 3e06a6477d6..d28a24453e2 100644
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/clang/lib/Sema/SemaCast.cpp
@@ -389,6 +389,33 @@ static void diagnoseBadCast(Sema &S, unsigned msg, CastType castType,
 
   S.Diag(opRange.getBegin(), msg) << castType
     << src->getType() << destType << opRange << src->getSourceRange();
+
+  // Detect if both types are (ptr to) class, and note any incompleteness.
+  int DifferentPtrness = 0;
+  QualType From = destType;
+  if (auto Ptr = From->getAs<PointerType>()) {
+    From = Ptr->getPointeeType();
+    DifferentPtrness++;
+  }
+  QualType To = src->getType();
+  if (auto Ptr = To->getAs<PointerType>()) {
+    To = Ptr->getPointeeType();
+    DifferentPtrness--;
+  }
+  if (!DifferentPtrness) {
+    auto RecFrom = From->getAs<RecordType>();
+    auto RecTo = To->getAs<RecordType>();
+    if (RecFrom && RecTo) {
+      auto DeclFrom = RecFrom->getAsCXXRecordDecl();
+      if (!DeclFrom->isCompleteDefinition())
+        S.Diag(DeclFrom->getLocation(), diag::note_type_incomplete)
+          << DeclFrom->getDeclName();
+      auto DeclTo = RecTo->getAsCXXRecordDecl();
+      if (!DeclTo->isCompleteDefinition())
+        S.Diag(DeclTo->getLocation(), diag::note_type_incomplete)
+          << DeclTo->getDeclName();
+    }
+  }
 }
 
 /// UnwrapDissimilarPointerTypes - Like Sema::UnwrapSimilarPointerTypes,
@@ -1079,6 +1106,14 @@ static TryCastResult TryStaticCast(Sema &Self, ExprResult &SrcExpr,
   if (!CStyle &&
       Self.CheckTollFreeBridgeStaticCast(DestType, SrcExpr.get(), Kind))
     return TC_Success;
+
+  // See if it looks like the user is trying to convert between
+  // related record types, and select a better diagnostic if so.
+  if (auto SrcPointer = SrcType->getAs<PointerType>())
+    if (auto DestPointer = DestType->getAs<PointerType>())
+      if (SrcPointer->getPointeeType()->getAs<RecordType>() &&
+          DestPointer->getPointeeType()->getAs<RecordType>())
+       msg = diag::err_bad_cxx_cast_unrelated_class;
   
   // We tried everything. Everything! Nothing works! :-(
   return TC_NotApplicable;