Boost.Exception regression in 1.75.0 beta 1

From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2020-11-13 11:17:01

If you diagnose the test failures (which are present across all
compilers) in
https://www.boost.org/development/tests/develop/developer/outcome.html
which report failures of the form:

```
unknown location(0): fatal error: in "works_outcome": Throw location
unknown (consider using BOOST_THROW_EXCEPTION)
Dynamic exception type: struct boost::wrapexcept<class boost::exception_ptr>

libs\outcome\test\tests\core-outcome.cpp(173): last checkpoint

*** 1 failure is detected in the test module "Outcome"
```

The cause is test code such as:

```
    boost::system::error_code ec(5, boost::system::system_category());
    auto e = boost::copy_exception(boost::system::system_error(ec));  //
NOLINT
    outcome<int> m(e);
    BOOST_CHECK_THROW(m.value(), boost::system::system_error);
```

Outcome calls boost::rethrow_exception() to rethrow the exception_ptr.
boost::rethrow_exception() is not rethrowing the
boost::system::system_error held within the boost::exception_ptr. It is
throwing instead boost::wrapexcept<class boost::exception_ptr>. This
causes BOOST_CHECK_THROW() to not correctly trap the type of exception
thrown, and thus the test fails.

boost::wrapexcept<E> inherits from E. It seems to me therefore that
catching a const E& ought to catch throws of boost::wrapexcept<E>, and
therefore this code is correct. I double checked this at
https://wandbox.org/permlink/n9Wjb5cPpfHEFalp, and indeed it works as
expected.

However, obviously enough this is failing, and it was not failing in
Boost 1.74, so this is a regression. I am stumped as to the exact cause.

This affects all platforms, POSIX, Windows, clang, GCC and MSVC.

Niall