PR libstdc++/86676 Do not assume stack buffer is aligned

	PR libstdc++/86676
	* testsuite/20_util/monotonic_buffer_resource/release.cc: Allow for
	buffer being misaligned and so returned pointer not being at start.

From-SVN: r262980

libstdc++-v3/ChangeLog

 2018-07-25  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/86676
+	* testsuite/20_util/monotonic_buffer_resource/release.cc: Allow for
+	buffer being misaligned and so returned pointer not being at start.
+
 	* include/experimental/memory_resource: Include <cstddef> header.
 
 	* acinclude.m4 (glibcxx_SUBDIRS): Add src/c++17.

libstdc++-v3/testsuite/20_util/monotonic_buffer_resource/release.cc

@@ -115,10 +115,12 @@ test04()
   unsigned char buffer[1024];
   std::pmr::monotonic_buffer_resource mbr(buffer, sizeof(buffer), &r);
   void* p = mbr.allocate(800, 16);
-  VERIFY( p == buffer );
+  VERIFY( p >= buffer && p < (buffer + 16) );
+  void* const p_in_buffer = p;
   VERIFY( r.allocate_calls == 0 );
   p = mbr.allocate(300, 1);
   VERIFY( p != buffer );
+  VERIFY( p != buffer );
   VERIFY( r.allocate_calls == 1 );
   mbr.release();
   VERIFY( r.deallocate_calls == 1 );
@@ -126,7 +128,7 @@ test04()
   VERIFY( r.number_of_active_allocations() == 0 );
   // initial buffer should be used again now:
   p = mbr.allocate(1000);
-  VERIFY( p == buffer );
+  VERIFY( p == p_in_buffer );
   VERIFY( r.allocate_calls == 1 );
 }