re PR c++/52988 (std::async not executed on function returning nullptr_t)

	PR c++/52988
	* typeck.c (decay_conversion): Don't discard side-effects from
	expressions of nullptr_t.

From-SVN: r188955

gcc/cp/ChangeLog

+2012-06-25  Jason Merrill  <jason@redhat.com>
+
+	PR c++/52988
+	* typeck.c (decay_conversion): Don't discard side-effects from
+	expressions of nullptr_t.
+
 2012-06-25  Florian Weimer  <fweimer@redhat.com>
 
 	* init.c (build_new_1): Warn about (T[N]) for variable N, and

gcc/cp/typeck.c

@@ -1843,7 +1843,7 @@ decay_conversion (tree exp, tsubst_flags_t complain)
   if (error_operand_p (exp))
     return error_mark_node;
 
-  if (NULLPTR_TYPE_P (type))
+  if (NULLPTR_TYPE_P (type) && !TREE_SIDE_EFFECTS (exp))
     return nullptr_node;
 
   /* build_c_cast puts on a NOP_EXPR to make the result not an lvalue.

gcc/testsuite/ChangeLog

+2012-06-25  Jason Merrill  <jason@redhat.com>
+
+	PR c++/52988
+	* g++.dg/cpp0x/nullptr28.C: New.
+
 2012-06-25  Christophe Lyon  <christophe.lyon@st.com>
 
 	* gcc.target/arm/neon-vld1_dupQ.c: New.

gcc/testsuite/g++.dg/cpp0x/nullptr28.C

+// { dg-do run { target c++11 } }
+
+typedef decltype(nullptr) nullptr_t;
+
+int i;
+nullptr_t n;
+const nullptr_t& f() { ++i; return n; }
+
+nullptr_t g() { return f(); }
+
+int main()
+{
+  g();
+  if (i != 1)
+    __builtin_abort ();
+}