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From: Jason Merrill <>
To: Iain Sandoe <>
Cc: GCC Patches <>
Subject: Re: [PATCH RFC] c++: fix broken conversion in coroutines
Date: Mon, 3 Oct 2022 23:53:06 -0400	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 9/30/22 18:50, Iain Sandoe wrote:
> Hi Jason,
>> On 30 Sep 2022, at 23:06, Jason Merrill <> 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.


  reply	other threads:[~2022-10-04  3:53 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-30 22:06 Jason Merrill
2022-09-30 22:50 ` Iain Sandoe
2022-10-04  3:53   ` Jason Merrill [this message]
2022-10-06 21:44     ` Jason Merrill
2022-10-06 22:47       ` Iain Sandoe

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