Improve error for assignment to incomplete class.

PR7681.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189510 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaOverload.cpp b/lib/Sema/SemaOverload.cpp
index 06d1f0b..a8b0bd2 100644
--- a/lib/Sema/SemaOverload.cpp
+++ b/lib/Sema/SemaOverload.cpp
@@ -10714,6 +10714,11 @@
         Diag(OpLoc,  diag::err_ovl_no_viable_oper)
              << BinaryOperator::getOpcodeStr(Opc)
              << Args[0]->getSourceRange() << Args[1]->getSourceRange();
+        if (Args[0]->getType()->isIncompleteType()) {
+          Diag(OpLoc, diag::note_assign_lhs_incomplete)
+            << Args[0]->getType()
+            << Args[0]->getSourceRange() << Args[1]->getSourceRange();
+        }
       } else {
         // This is an erroneous use of an operator which can be overloaded by
         // a non-member function. Check for non-member operators which were