Re: [PATCH] c++: Fix noexcept with unevaluated operand [PR101087]

[Marek Polacek <polacek@redhat.com>]

On Thu, Jul 08, 2021 at 09:30:27AM -0400, Jason Merrill wrote:
> On 7/7/21 9:40 PM, Marek Polacek wrote:
> > It sounds plausible that this assert
> > 
> >    int f();
> >    static_assert(noexcept(sizeof(f())));
> > 
> > should pass: sizeof produces a std::size_t and its operand is not
> > evaluated, so it can't throw.  noexcept should only evaluate to
> > false for potentially evaluated operands.  Therefore I think that
> > check_noexcept_r shouldn't walk into operands of sizeof/decltype/
> > alignof/typeof.  Only checking cp_unevaluated_operand therein does
> > not work, because expr_noexcept_p can be called in an unevaluated
> > context, so I resorted to the following cp_evaluated hack.  Does
> > that seem acceptable?
> 
> I suppose, but why not check for SIZEOF_EXPR/ALIGNOF_EXPR/NOEXCEPT_EXPR
> directly?

I thought I would, but then it occurred to me that it might be better to
rely on cp_walk_subtrees which ++/--s cp_unevaluated_operand for those
codes.  I'd be happy to change the patch to check those codes directly;
maybe I'm overthinking things here.

--
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA