exception() requires an rvalue-reference for its exception parameter.

[vector-of-bool]

The exception-aware overload of exception() look as follows (borrowing constraint syntax for conciseness):

template <typename Ex>
  requires std::is_base_of<std::exception, Ex>
exception(Ex&& ex) {
  // Throws a class derived from 'Ex' ...
}

This causes unexpected behavior if the exception object is an lvalue:

std::runtime_error exc{"Ouch"};
throw leaf::exception(exc);

Because is_base_of<std::exception, std::runtime_error&> is false, the other overload of exception() is selected, and a bare std::exception object will be returned and then thrown, instead of throwing the std::runtime_error exc. The runtime_error will instead be loaded as an exception value, which can sometimes be handled:

leaf::try_handle_some(
  [] {
    std::runtime_error exc{"Ouch"};
    throw leaf::exception(exc);
  },
  [] (leaf::catch_<std::runtime_error>) {}, // [1]
  [] (const std::runtime_error&) {}); // [2]

The [1] handler will not be invoked, but the [2] will.

Removing cvref qualifiers in the check causes exception to behave "as expected:"

template <typename Ex>
  requires std::is_base_of<std::exception, std::remove_cvref_t<Ex>>
exception(Ex&& ex) {
  // Throws a class derived from 'std::remove_cvref_t<Ex>' ...
}

I'm thinking a simple fix is possible, but want to confirm that this is actually an unintended behavior, and whether there might be other code locations that could also be affected.

[zajo]

Thank you. There is another bug, in that I have a static_assert if you attempt to pass an object of a type that derives from std::exception as anything but the first argument to leaf::exception, which also doesn't trigger for the same reason.

[zajo]

FYI the fix for this and catching abstract exception types will ship with the upcoming Boost release.