libstdc++: Prevent -Wstringop-overread warning in std::deque [PR100516]

The compiler warns about the loop in deque::_M_range_initialize because
it doesn't know that the number of nodes has already been correctly
sized to match the size of the input. Use __builtin_unreachable to tell
it that the loop will never be entered if the number of elements is
smaller than a single node.

libstdc++-v3/ChangeLog:

	PR libstdc++/100516
	* include/bits/deque.tcc (_M_range_initialize<ForwardIterator>):
	Add __builtin_unreachable to loop.
	* testsuite/23_containers/deque/100516.cc: New test.

libstdc++-v3/include/bits/deque.tcc

@@ -454,6 +454,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 		 __cur_node < this->_M_impl._M_finish._M_node;
 		 ++__cur_node)
 	      {
+		if (__n < _S_buffer_size())
+		  __builtin_unreachable(); // See PR 100516
+
 		_ForwardIterator __mid = __first;
 		std::advance(__mid, _S_buffer_size());
 		std::__uninitialized_copy_a(__first, __mid, *__cur_node,

libstdc++-v3/testsuite/23_containers/deque/100516.cc

@@ -0,0 +1,14 @@
+// { dg-options "-O2 -Wstringop-overread" }
+// { dg-do compile { target c++11 } }
+
+// Bug 100516
+// Unexpected -Wstringop-overread in deque<char> initialization from empty
+// initializer_list
+
+#include <deque>
+
+void f()
+{
+    std::initializer_list<char> il{};
+    std::deque<char>{il};
+}