Re: [PATCH RFC] c++: fix broken conversion in coroutines

[Iain Sandoe <iain@sandoe.co.uk>]

> On 6 Oct 2022, at 22:44, Jason Merrill <jason@redhat.com> wrote:
> 
> On 10/3/22 23:53, Jason Merrill wrote:
>> On 9/30/22 18:50, Iain Sandoe wrote:
>>> Hi Jason,
>>> 
>>>> On 30 Sep 2022, at 23:06, Jason Merrill <jason@redhat.com> wrote:
>>>> 
>>>> You can't use CONVERT_EXPR to convert between two class types, and it was
>>>> breaking copy elision.
>>>> 
>>>> Unfortunately, this patch breaks symmetric-transfer-00-basic.C, where
>>>> susp_type is Loopy<int>::handle_type.  How is this supposed to work?
>>> 
>>> We are trying to save a type-erased handle (which the symmetric transfer makes
>>> and indirect call through, nothing else).
>> The problem is you're treating one class directly as another class here, without the indirection involved in usual type-erasure idioms.
>> It does seem that the gimplifier handles this fine, but it doesn't correspond to anything in the language and much of the front end assumes that CONVERT_EXPR is only used for scalars.  VIEW_CONVERT_EXPR would better express that we're not doing anything to the value, just cheating the type system.  That's still dodgy from a language perspective, but probably safe enough in this case.
> 
> So I'm applying this:<0001-c-fix-broken-conversion-in-coroutines.patch>

thanks, I have not had any cycles to look at this.

however, when I next do - was planning on looking at the:
cont = handle.from_address(await_suspend().address())
approach, since both .address() and .from_address() are constexpr, cp_fold_function should turn that into essentially a NOP.
Iain