Non standard return fails to compile

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];

Search the Boost.Array source for the exact return statement containing BOOST_ASSERT_MSG and elems[i], then inspect its surrounding function. Separate the assertion from the return as described and build the optimized configuration to confirm it compiles; run the relevant Boost.Array tests to verify behavior remains unchanged.

Written by the indexing model from the issue text.