Add extension to always default-initialize nullptr_t.

Core issue 1013 suggests that having an uninitialied std::nullptr_t be
UB is a bit foolish, since there is only a single valid value. This DR
reports that DR616 fixes it, which does so by making lvalue-to-rvalue
conversions from nullptr_t be equal to nullptr.

However, just implementing that results in warnings/etc in many places.
In order to fix all situations where nullptr_t would seem uninitialized,
this patch instead (as an otherwise transparent extension) default
initializes uninitialized VarDecls of nullptr_t.

Differential Revision: https://reviews.llvm.org/D53713

Change-Id: I84d72a9290054fa55341e8cbdac43c8e7f25b885
llvm-svn: 349201

1 files changed, 7 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index c2f14229d8d..57277f6e829 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -4881,6 +4881,13 @@ static void TryDefaultInitialization(Sema &S,
     return;
   }
 
+  // As an extension, and to fix Core issue 1013, zero initialize nullptr_t.
+  // Since there is only 1 valid value of nullptr_t, we can just use that.
+  if (DestType->isNullPtrType()) {
+    Sequence.AddZeroInitializationStep(Entity.getType());
+    return;
+  }
+
   //     - otherwise, no initialization is performed.
 
   //   If a program calls for the default initialization of an object of