This statement:
return BOOST_ASSERT_MSG( i < N, "out of range" ), elems[i];

is a non-standard form of return.

In case we compile optimized code that lacks assert function but still want something this code fails to compile.

Separating this to a more standard form solves the issue:

BOOST_ASSERT_MSG( i < N, "out of range" );
return elems[i];