[PATCH] D53713: Add extension to always default-initialize nullptr_t.

[Erich Keane via Phabricator via cfe-commits]

erichkeane created this revision.
erichkeane added reviewers: rsmith, aaron.ballman.

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.


Repository:
  rC Clang

https://reviews.llvm.org/D53713

Files:
  lib/Sema/SemaInit.cpp
  test/SemaCXX/nullptr_t-init.cpp


Index: test/SemaCXX/nullptr_t-init.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/nullptr_t-init.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -ffreestanding -Wuninitialized %s
+// expected-no-diagnostics
+typedef decltype(nullptr) nullptr_t;
+
+// Ensure no 'uninitialized when used here' warnings (Wuninitialized), for 
+// nullptr_t always-initialized extension.
+nullptr_t default_init() {
+  nullptr_t a;
+  return a;
+}
Index: lib/Sema/SemaInit.cpp
===================================================================
--- lib/Sema/SemaInit.cpp
+++ lib/Sema/SemaInit.cpp
@@ -4877,6 +4877,13 @@
     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


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