Also test std::hash with bitsets having equal underlying vectors

This makes sure the size of the bitset is taken into account in the
calculation of the hash value.

Author: gennaroprota

test/test_std_hash.cpp

@@ -1,5 +1,6 @@
 //
 // Copyright (C) 2019 James E. King III
+// Copyright (C) 2025 Gennaro Prota
 //
 // Distributed under the Boost Software License, Version 1.0. (See
 // accompanying file LICENSE_1_0.txt or copy at
@@ -17,16 +18,23 @@ main( int, char *[] )
     const std::string                              long_string =
         "01001110101110110101011010000000000011110101101111111111";
 
+    // Some bitsets with the same size but different underlying vectors.
     const bitset_type                 zeroes( long_string.size(), 0 );
     const bitset_type                 stuff( long_string );
     const bitset_type                 one( long_string.size(), 1 );
 
+    // Some bitsets with different sizes but equal underlying vectors.
+    const bitset_type                 zeroes2( 2, 0 );
+    const bitset_type                 zeroes3( 3, 0 );
+
     std::unordered_set< bitset_type > bitsets;
     bitsets.insert( zeroes );
     bitsets.insert( stuff );
     bitsets.insert( one );
+    bitsets.insert( zeroes2 );
+    bitsets.insert( zeroes3 );
 
-    BOOST_TEST_EQ( bitsets.size(), 3 );
+    BOOST_TEST_EQ( bitsets.size(), 5 );
 
     return boost::report_errors();
 }