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

[Jason Merrill <jason@redhat.com>]

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?

Jason