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
