On Wed, Dec 15, 2021 at 04:17:37PM -0500, Jason Merrill wrote:
> On 12/15/21 15:20, Marek Polacek wrote:
> > 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
>
> Ah, right.
>
> > 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.
>
> No, checking in grokdeclarator makes sense.
>
> > 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.
>
> Maybe check PLACEHOLDER_TYPE_CONSTRAINTS in check_decltype_auto instead?
I've tried that, but that is also true for
const constexpr C decltype(auto) x2 = 0;
const constexpr C decltype(auto) fn4() { return 0; }
where we do want to check if the auto has quals. Therefore the not very
pretty TYPENAME check :/.
Marek
