vector<bool> _M_start and 0 offset

2018-10-02  Marc Glisse  <marc.glisse@inria.fr>

	PR libstdc++/87258
	* include/bits/stl_bvector.h (vector::begin(), vector::cbegin()):
	Rebuild _M_start with an explicit 0 offset.

From-SVN: r264791

libstdc++-v3/ChangeLog

+2018-10-02  Marc Glisse  <marc.glisse@inria.fr>
+
+	PR libstdc++/87258
+	* include/bits/stl_bvector.h (vector::begin(), vector::cbegin()):
+	Rebuild _M_start with an explicit 0 offset.
+
 2018-10-02  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/std/variant (__gen_vtable_impl::__visit_invoke): Call __get

libstdc++-v3/include/bits/stl_bvector.h

@@ -809,11 +809,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
       iterator
       begin() _GLIBCXX_NOEXCEPT
-      { return this->_M_impl._M_start; }
+      { return iterator(this->_M_impl._M_start._M_p, 0); }
 
       const_iterator
       begin() const _GLIBCXX_NOEXCEPT
-      { return this->_M_impl._M_start; }
+      { return const_iterator(this->_M_impl._M_start._M_p, 0); }
 
       iterator
       end() _GLIBCXX_NOEXCEPT
@@ -842,7 +842,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 #if __cplusplus >= 201103L
       const_iterator
       cbegin() const noexcept
-      { return this->_M_impl._M_start; }
+      { return const_iterator(this->_M_impl._M_start._M_p, 0); }
 
       const_iterator
       cend() const noexcept