Re: C++ PATCH for c++/88538 - braced-init-list in template-argument-list

[Jason Merrill <jason@redhat.com>]

On 1/8/19 10:42 AM, Marek Polacek wrote:
> On Mon, Jan 07, 2019 at 09:52:55PM -0500, Jason Merrill wrote:
>> On 1/7/19 6:56 PM, Marek Polacek wrote:
>>> At the risk of seeming overly eager, I thought it would be reasonable to
>>> go with the following: enabling braced-init-list as a template-argument.
>>> As the discussion on the reflector clearly indicates, this was the intent
>>> from the get-go.
>>>
>>> I know, it's not a regression.  But I restricted the change to C++20, and it
>>> should strictly allow code that wasn't accepted before -- when a template
>>> argument starts with {.  Perhaps we could even drop the C++20 check.
>>>
>>> What's your preference?
>>
>> Let's keep the C++20 check for now at least.  I'd suggest moving the change
>> further down, with this code:
> 
> Okay.  I've experimented with checking expr_non_constant_p, but this version
> gives better diagnostics.
> 
> Bootstrapped/regtested running on x86_64-linux, ok for trunk if it passes?
> 
> 2019-01-08  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/88538 - braced-init-list in template-argument-list.
> 	* parser.c (cp_parser_template_argument): Handle braced-init-list when
> 	in C++20.
> 
> 	* g++.dg/cpp2a/nontype-class11.C: New test.
> 
> diff --git gcc/cp/parser.c gcc/cp/parser.c
> index bca1739ace3..87f37d8ab2b 100644
> --- gcc/cp/parser.c
> +++ gcc/cp/parser.c
> @@ -17020,6 +17020,18 @@ cp_parser_template_argument (cp_parser* parser)
>       argument = cp_parser_constant_expression (parser);
>     else
>       {
> +      /* In C++20, we can encounter a braced-init-list.  */
> +      if (cxx_dialect >= cxx2a
> +	  && cp_lexer_next_token_is (parser->lexer, CPP_OPEN_BRACE))
> +	{
> +	  cp_parser_parse_tentatively (parser);

Hmm, I wonder if we would get better diagnostics for an ill-formed 
braced-init-list without tentative parsing here.  OK either way.

Jason