c++: init-list opt and lvalue initializers [PR118673]

When fn returns {extension}, the ArrayRef in the initializer_list is
constructed to point to 'extension', the variable with static storage
duration.  The optimization was copying extension's value into a temporary
array and constructing the ArrayRef to point to that temporary copy instead,
resulting in a dangling pointer.  So suppress this optimization if the
element constructor takes a reference and the initializer is a non-mergeable
lvalue.

	PR c++/118673

gcc/cp/ChangeLog:

	* call.cc (maybe_init_list_as_array): Check for lvalue
	initializers.
	* cp-tree.h (enum cp_lvalue_kind_flags): Add clk_mergeable.
	* tree.cc (lvalue_kind): Return it.
	(non_mergeable_glvalue_p): New.
	(test_lvalue_kind): Adjust.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/initlist-opt6.C: New test.

1 files changed, 26 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist-opt6.C b/gcc/testsuite/g++.dg/cpp0x/initlist-opt6.C
new file mode 100644
index 0000000..8019289
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist-opt6.C
@@ -0,0 +1,26 @@
+// PR c++/118673
+// { dg-do run { target c++11 } }
+
+#include <initializer_list>
+
+struct ArrayRef {
+  const int *Data = nullptr;
+  ArrayRef(const int &OneElt) : Data(&OneElt) {}
+};
+
+struct Vec
+{
+  ArrayRef Elts[1];
+  Vec(std::initializer_list<ArrayRef> IL)
+    : Elts{*IL.begin()}
+    {  }
+};
+
+[[gnu::noinline]] Vec fn() {
+  static const auto extension = 42;
+  return {extension};
+}
+int main() {
+  auto t = fn();
+  if (t.Elts[0].Data[0] != 42) __builtin_abort();
+}