Re: [PATCH] c++: C++20 DR 2237, disallow simple-template-id in cdtor.

[Jason Merrill <jason@redhat.com>]

On 4/4/20 7:30 PM, Marek Polacek wrote:
> This patch implements DR 2237 which says that a simple-template-id is
> no longer valid as the declarator-id of a constructor or destructor;
> see <https://eel.is/c++draft/diff.cpp17.class#2>.  It is not explicitly
> stated but out-of-line destructors with a simple-template-id are also
> meant to be ill-formed now.  (Out-of-line constructors like that are
> invalid since DR1435 I think.)  This change only applies to C++20; it
> is not a DR against C++17.
> 
> I'm not crazy about the diagnostic in constructors but ISTM that
> cp_parser_constructor_declarator_p shouldn't print errors.
> 
> Does it seem reasonable to apply this now or should I defer to GCC 11?

A new error should wait for GCC 11.

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 2020-04-04  Marek Polacek  <polacek@redhat.com>
> 
> 	DR 2237
> 	* parser.c (cp_parser_unqualified_id): Reject simple-template-id as
> 	the declarator-id of a destructor.
> 	(cp_parser_constructor_declarator_p): Reject simple-template-id as
> 	the declarator-id of a constructor.
> 
> 	* g++.dg/DRs/dr2237.C: New test.
> 	* g++.dg/parse/constructor2.C: Add dg-error for C++20.
> 	* g++.dg/parse/dtor12.C: Likewise.
> 	* g++.dg/parse/dtor4.C: Likewise.
> 	* g++.dg/template/dtor4.C: Adjust dg-error.
> 	* g++.dg/template/error34.C: Likewise.
> 	* g++.old-deja/g++.other/inline15.C: Only run for C++17 and lesses.
> 	* g++.old-deja/g++.pt/ctor2.C: Add dg-error for C++20.
> ---
>   gcc/cp/parser.c                                | 16 ++++++++++++++++
>   gcc/testsuite/g++.dg/DRs/dr2237.C              | 18 ++++++++++++++++++
>   gcc/testsuite/g++.dg/parse/constructor2.C      |  4 ++--
>   gcc/testsuite/g++.dg/parse/dtor12.C            |  2 +-
>   gcc/testsuite/g++.dg/parse/dtor4.C             |  2 +-
>   gcc/testsuite/g++.dg/template/dtor4.C          |  2 +-
>   gcc/testsuite/g++.dg/template/error34.C        | 10 +++++-----
>   .../g++.old-deja/g++.other/inline15.C          |  2 +-
>   gcc/testsuite/g++.old-deja/g++.pt/ctor2.C      |  2 +-
>   9 files changed, 46 insertions(+), 12 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/DRs/dr2237.C
> 
> diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> index 7e5921e039f..810edfa87a9 100644
> --- a/gcc/cp/parser.c
> +++ b/gcc/cp/parser.c
> @@ -6114,6 +6114,16 @@ cp_parser_unqualified_id (cp_parser* parser,
>   	    return build_min_nt_loc (loc, BIT_NOT_EXPR, make_auto ());
>   	  }
>   
> +	/* DR 2237 (C++20 only): A simple-template-id is no longer valid as the
> +	   declarator-id of a constructor or destructor.  */
> +	if (token->type == CPP_TEMPLATE_ID && cxx_dialect >= cxx2a)
> +	  {
> +	    if (!cp_parser_uncommitted_to_tentative_parse_p (parser))
> +	      error_at (tilde_loc, "template-id not allowed for destructor");
> +	    cp_parser_simulate_error (parser);

The usual pattern is

if (!cp_parser_simulate_error (parser))
   error...

> +	    return error_mark_node;
> +	  }
> +
>   	/* If there was an explicit qualification (S::~T), first look
>   	   in the scope given by the qualification (i.e., S).
>   
> @@ -28675,6 +28685,12 @@ cp_parser_constructor_declarator_p (cp_parser *parser, cp_parser_flags flags,
>         if (!constructor_name_p (id, nested_name_specifier))
>   	constructor_p = false;
>       }
> +  /* DR 2237 (C++20 only): A simple-template-id is no longer valid as the
> +     declarator-id of a constructor or destructor.  */
> +  else if (constructor_p
> +	   && cxx_dialect >= cxx2a
> +	   && cp_lexer_next_token_is (parser->lexer, CPP_TEMPLATE_ID))
> +    constructor_p = false;
>     /* If we still think that this might be a constructor-declarator,
>        look for a class-name.  */
>     else if (constructor_p)

Do you also want to exclude CPP_TEMPLATE_ID from the test at the top of 
the function for C++20?

>   if (next_token->type != CPP_NAME
>       && next_token->type != CPP_SCOPE
>       && next_token->type != CPP_NESTED_NAME_SPECIFIER
>       && next_token->type != CPP_TEMPLATE_ID)
>     return false;

Jason