Re: [PATCH RFC] c++: static_assert (false) in template [DR2518]

[Marek Polacek <polacek@redhat.com>]

On Fri, Feb 17, 2023 at 03:32:09PM -0500, Jason Merrill via Gcc-patches wrote:
> Tested x86_64-pc-linux-gnu.  This isn't really a regression fix, but it's very
> safe, and fixes a longstanding annoyance, so I'm leaning toward putting it into
> GCC 13.  Thoughts?

LGTM.
 
> -- 8< --
> 
> For a long time, people have expected to be able to write
> static_assert (false) in a template and only have it diagnosed if the
> template is instantiated, but we (and other implementations) gave an error
> about the uninstantiated template because the standard says that if no valid
> instantiation of the template is possible, the program is ill-formed, no
> diagnostic required, and we try to diagnose IFNDR things when feasible.
> 
> At the meeting last week we were looking at CWG2518, which wanted to specify
> that an implementation must not accept a program containing a failing #error
> or static_assert.  We also looked at P2593, which proposed allowing
> static_assert in an uninstantiated template.  We ended up combining these
> two in order to avoid requiring implementations to reject programs with
> static_assert (false) in uninstantiated templates.
> 
> The committee accepted this as a DR, so I'm making the change to all
> standard modes.  This behavior was also conformant previously, since no
> diagnostic was required in this case.
> 
> We continue to diagnose non-constant or otherwise ill-formed conditions, so
> no changes to existing tests were needed.
> 
> 	DR 2518
> 	PR c++/52809
> 	PR c++/53638
> 	PR c++/87389
> 	PR c++/89741
> 	PR c++/92099
> 	PR c++/104041
> 	PR c++/104691
> 
> gcc/cp/ChangeLog:
> 
> 	* semantics.cc (finish_static_assert): Don't diagnose in
> 	template context.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/DRs/dr2518.C: New test.
> ---
>  gcc/cp/semantics.cc               | 17 ++++++++++-------
>  gcc/testsuite/g++.dg/DRs/dr2518.C |  7 +++++++
>  2 files changed, 17 insertions(+), 7 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/DRs/dr2518.C
> 
> diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> index c2df0b69b30..79b7cc72f21 100644
> --- a/gcc/cp/semantics.cc
> +++ b/gcc/cp/semantics.cc
> @@ -11232,14 +11232,16 @@ finish_static_assert (tree condition, tree message, location_t location,
>    if (check_for_bare_parameter_packs (condition))
>      condition = error_mark_node;
>  
> +  /* Save the condition in case it was a concept check.  */
> +  tree orig_condition = condition;
> +
>    if (instantiation_dependent_expression_p (condition))
>      {
>        /* We're in a template; build a STATIC_ASSERT and put it in
>           the right place. */
> -      tree assertion;
> -
> -      assertion = make_node (STATIC_ASSERT);
> -      STATIC_ASSERT_CONDITION (assertion) = condition;
> +    defer:
> +      tree assertion = make_node (STATIC_ASSERT);
> +      STATIC_ASSERT_CONDITION (assertion) = orig_condition;
>        STATIC_ASSERT_MESSAGE (assertion) = message;
>        STATIC_ASSERT_SOURCE_LOCATION (assertion) = location;
>  
> @@ -11253,9 +11255,6 @@ finish_static_assert (tree condition, tree message, location_t location,
>        return;
>      }
>  
> -  /* Save the condition in case it was a concept check.  */
> -  tree orig_condition = condition;
> -
>    /* Fold the expression and convert it to a boolean value. */
>    condition = contextual_conv_bool (condition, complain);
>    condition = fold_non_dependent_expr (condition, complain,
> @@ -11270,6 +11269,10 @@ finish_static_assert (tree condition, tree message, location_t location,
>  
>        if (integer_zerop (condition))
>  	{
> +	  /* CWG2518: static_assert failure in a template is not IFNDR.  */
> +	  if (processing_template_decl)
> +	    goto defer;
> +
>  	  int sz = TREE_INT_CST_LOW (TYPE_SIZE_UNIT
>  				     (TREE_TYPE (TREE_TYPE (message))));
>  	  int len = TREE_STRING_LENGTH (message) / sz - 1;
> diff --git a/gcc/testsuite/g++.dg/DRs/dr2518.C b/gcc/testsuite/g++.dg/DRs/dr2518.C
> new file mode 100644
> index 00000000000..240186211e6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/DRs/dr2518.C
> @@ -0,0 +1,7 @@
> +// CWG 2518
> +// { dg-do compile { target c++11 } }
> +
> +template <class T> void f()
> +{
> +  static_assert (false, "");
> +}
> 
> base-commit: 07f497c2da3600cc99cd7d1b5c6726972fb2b5a1
> -- 
> 2.31.1
> 

Marek