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### 2784. Resolution to LWG 2484 is missing "otherwise, no effects" and is hard to parse

Section: 17.9.8 [except.nested] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-15 Last modified: 2017-07-30

Priority: 0

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Discussion:

The discussion notes for LWG 2484 point out there should be an "else, no effects" at the end, which didn't make it into the resolution. Without this it's unclear whether it should do nothing, or be ill-formed, or undefined.

Additionally, the precise effects of the Effects are hard to determine, because the conditions on the static type and dynamic type are intermingled, but must actually be checked separately (the static checks must be done statically and the dynamic checks must be done dynamically!) Furthermore, the obvious way to know if "the dynamic type of `e` is `nested_exception` or is publicly and unambiguously derived from `nested_exception`" is to use `dynamic_cast`, so we have to use `dynamic_cast` to find out whether to perform the `dynamic_cast` expression specified in the Effects. It would make more sense to specify it in terms of a `dynamic_cast` to a pointer type, and only call `rethrow_nested()` if the result is not null.

The entire spec can be expressed in C++17 as:

>     if constexpr(is_polymorphic_v<E> && (!is_base_of_v<nested_exception, E> || is_convertible_v<E*, nested_exception*>))
>       if (auto p = dynamic_cast<const nested_exception*>(addressof(e)))
>         p->rethrow_nested();

This uses traits to perform checks on the static type, then uses `dynamic_cast` to perform the checks on the dynamic type. I think the spec would be clearer if it had the same structure.

[2016-11-12, Issaquah]

Sat AM: Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify 17.9.8 [except.nested] p9:

   >     template <class E> void rethrow_if_nested(const E& e);

   > > -9- Effects: If `E` is not a polymorphic class type, or if `nested_exception` is an inaccessible or ambiguous base class of `E`, there is no effect. Otherwise, if the static type or the dynamic type of `e` is `nested_exception` or is publicly and unambiguously derived from `nested_exception`, calls performs:

   > > >     dynamic_cast<const nested_exception&>(e).rethrow_nested();
   > > >     if (auto p = dynamic_cast<const nested_exception*>(addressof(e)))
   > > >       p->rethrow_nested();