[PATCH] libstdc++: Work around modules ICE in <charconv> [PR105297]

[Jonathan Wakely]

On Thu, 21 Apr 2022 at 04:38, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> This makes the initializer for __table in __from_chars_alnum_to_val
> dependent in an artificial way, which works around the modules testsuite
> ICE reported in PR105297 by preventing the initializer from getting
> evaluated at parse time.
>
> Compared to the alternative workaround of using a non-local class type
> for __table, this workaround has the advantage of slightly speeding up
> compilation of the <charconv> header, since now the table will not get
> built (via constexpr evaluation) until one of the integer std::from_chars
> overloads is actually instantiated.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

OK, thanks.

>
>         PR c++/105297
>         PR c++/105322
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/charconv (__from_chars_alnum_to_val): Make
>         initializer for __table dependent in an artificial way.
> ---
>  libstdc++-v3/include/std/charconv | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv
> index f1ace406017..561234cb2fc 100644
> --- a/libstdc++-v3/include/std/charconv
> +++ b/libstdc++-v3/include/std/charconv
> @@ -445,7 +445,9 @@ namespace __detail
>         return __c - '0';
>        else
>         {
> -         static constexpr auto __table = __from_chars_alnum_to_val_table();
> +         // This initializer is deliberately made dependent in order to work
> +         // around modules bug PR105322.
> +         static constexpr auto __table = (_DecOnly, __from_chars_alnum_to_val_table());
>           return __table.__data[__c];
>         }
>      }
> --
> 2.36.0.rc2.10.g1ac7422e39
>