Re: [PATCH] c++: Allow constexpr decltype(auto) [PR102229]

[Marek Polacek <polacek@redhat.com>]

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