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

[Jason Merrill <jason@redhat.com>]

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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:

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From 0cd9dafcfa8cf69b3a1eae91bea82d388c5328d2 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Fri, 30 Sep 2022 10:04:22 -0400
Subject: [PATCH] c++: fix broken conversion in coroutines
To: gcc-patches@gcc.gnu.org

You can't use CONVERT_EXPR to convert between two class types.

VIEW_CONVERT_EXPR takes liberties with the C++ type system, but is probably
safe in this context.  Let's also only use it when the type isn't already
what we want.

gcc/cp/ChangeLog:

	* coroutines.cc (expand_one_await_expression): Change conversion
	to VIEW_CONVERT_EXPR.
	* cp-gimplify.cc (cp_genericize_r) [CONVERT_EXPR]: Add assert.
---
 gcc/cp/coroutines.cc  | 5 ++++-
 gcc/cp/cp-gimplify.cc | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/coroutines.cc b/gcc/cp/coroutines.cc
index eca01abcb7a..60b846600b9 100644
--- a/gcc/cp/coroutines.cc
+++ b/gcc/cp/coroutines.cc
@@ -1728,7 +1728,10 @@ expand_one_await_expression (tree *stmt, tree *await_expr, void *d)
     }
   else
     {
-      r = build1_loc (loc, CONVERT_EXPR, void_coro_handle_type, suspend);
+      r = suspend;
+      if (!same_type_ignoring_top_level_qualifiers_p (susp_type,
+						      void_coro_handle_type))
+	r = build1_loc (loc, VIEW_CONVERT_EXPR, void_coro_handle_type, r);
       r = build2_loc (loc, INIT_EXPR, void_coro_handle_type, data->conthand, r);
       r = build1 (CONVERT_EXPR, void_type_node, r);
       append_to_statement_list (r, &body_list);
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index b4599fc34d8..5d26e59d098 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1589,6 +1589,7 @@ cp_genericize_r (tree *stmt_p, int *walk_subtrees, void *data)
       break;
 
     case CONVERT_EXPR:
+      gcc_checking_assert (!AGGREGATE_TYPE_P (TREE_TYPE (stmt)));
       gcc_assert (!CONVERT_EXPR_VBASE_PATH (stmt));
       break;
 
-- 
2.31.1