[PATCH] libstdc++: Make mangled name of generate_cannonical specialization consistient.

[Nathan Myers]

On 12/17/25 2:06 AM, Tomasz Kamiński wrote:
> The r16-6177-g866bc8a9214b1d introduced type-constraint on _Urbg template
> parameter in __glibcxx_concepts, which make the mangled inconsistient.
> This lead to the missing symbol errors in tests, from
> __gnu_cxx::random_condition::throw_conditionally.

TIL that changing a typename parameter to a concept-constrained type
changes the mangled name.

Thank you for solving this.

The last line below can be simplified, as uniform_random_bit_generator
forbids _Urbg::min() <= _Urbg::max().

> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/random.tcc (generate_canonical)
> 	[!_GLIBCXX_USE_OLD_GENERATE_CANONICAL]: Use static_assert
> 	instead of type-constraint on template parameter.
> ---
> I have not yet determined what is causing pre-C++20 version of
> __gnu_cxx::random_condition::throw_conditionally to be linked against
> post-C++20 version. I am concerned about it, because if we call
> generate_canonical from libstdc++ shared library, then _GLIBCXX_USE_OLD_GENERATE_CANONICAL
> flag will not properly restore old behavior. Posting anyway to
> aknowledge the issue.
> 
> 
>   libstdc++-v3/include/bits/random.tcc | 11 ++++-------
>   1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc
> index 38e8645c88c..29d50d553e2 100644
> --- a/libstdc++-v3/include/bits/random.tcc
> +++ b/libstdc++-v3/include/bits/random.tcc
> @@ -3619,12 +3619,6 @@ namespace __detail
>     template <> const bool __is_rand_dist_float_v<long double> = true;
>   #endif
>   
> -#ifdef __glibcxx_concepts
> -# define _Uniform_random_bit_generator uniform_random_bit_generator
> -#else
> -# define _Uniform_random_bit_generator typename
> -#endif
> -
>     // Note, this works even when (__range + 1) overflows:
>     template <typename _Rng>
>       constexpr bool __is_power_of_2_less_1(_Rng __range)
> @@ -3646,10 +3640,13 @@ namespace __detail
>      *  @since C++11
>      */
>     template<typename _RealT, size_t __digits,
> -	_Uniform_random_bit_generator _Urbg>
> +	   typename _Urbg>
>       _RealT
>       generate_canonical(_Urbg& __urng)
>       {
> +#ifdef __glibcxx_concepts
> +      static_assert(uniform_random_bit_generator<_Ubrg>);
> +#endif
>         static_assert(__is_rand_dist_float_v<_RealT>,
>   	"template argument must be floating point");
>         static_assert(__digits != 0 && _Urbg::max() > _Urbg::min(),