On Mon, Dec 13, 2021 at 10:02:24AM -0500, Jason Merrill wrote:
> On 12/10/21 17:29, Marek Polacek wrote:
> > My r11-2202 was trying to enforce [dcl.type.auto.deduct]/4, which says
> > "If the placeholder-type-specifier is of the form type-constraint[opt]
> > decltype(auto), T shall be the placeholder alone." But this made us
> > reject 'constexpr decltype(auto)', which, after clarification from CWG,
> > should be valid. [dcl.type.auto.deduct]/4 is supposed to be a syntactic
> > constraint, not semantic, so it's OK that the constexpr marks the object
> > as const.
> >
> > As a consequence, checking TYPE_QUALS in do_auto_deduction is too late,
> > and we have a FIXME there anyway. So in this patch I'm attempting to
> > detect 'const decltype(auto)' earlier. If I'm going to use TYPE_QUALS,
> > it needs to happen before we mark the object as const due to constexpr,
> > that is, before grokdeclarator's
> >
> > /* A `constexpr' specifier used in an object declaration declares
> > the object as `const'. */
> > if (constexpr_p && innermost_code != cdk_function)
> > ...
> >
> > Constrained decltype(auto) was a little problem, hence the TYPENAME
> > check. But in a typename context you can't use decltype(auto) anyway,
> > I think.
>
> I wonder about checking even earlier, like in cp_parser_decl_specifier_seq?
That _almost_ works except it wouldn't detect things like 'decltype(auto)*'
because the '*' isn't parsed in cp_parser_decl_specifier_seq, only in
declarator. So the
if (a != type)
{
error_at (loc, "%qT as type rather than plain "
"%<decltype(auto)%>", type);
check wouldn't work. Maybe I could just check if the next token is * or &
and give an error then.
Marek
