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

[Jason Merrill <jason@redhat.com>]

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.

Note that I was wrong to mention copy elision above; it's irrelevant to 
codegen here since the handle type returns in a register.

> so, I suppose the equivalent could be:
> 
> conthand = coroutine_handle::from_address (suspend.address())

That sounds more correct, yes.

Jason