On Wed, 6 Nov 2024 at 07:56, Halalaluyafail3 via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> N4993 Section 29.5.3.1 [rand.req.genl] Paragraph 1:
> > Throughout this subclause 29.5, the effect of instantiating a template:
> >
> > ...
> >
> > - that has a template type parameter named IntType is undefined unless
> the
> > corresponding template argument is cv-unqualified and is one of short,
> int,
> > long, long long, unsigned short, unsigned int, unsigned long, or
> unsigned
> > long long.
> >
> > - that has a template type parameter named UIntType is undefined unless
> the
> > corresponding template argument is cv-unqualified and is one of
> unsigned
> > short, unsigned int, unsigned long, or unsigned long long.
>
> This section requires that only standard integer types which are not
> ordinary
> character types are valid to be used with the standard engines.
>
> N4993 Section 29.5.6 [rand.predef] Before Paragraph 3:
> > using mt19937 =
> > mersenne_twister_engine<uint_fast32_t, 32, 624, 397, 31,
> 0x9908'b0df,
> > 11, 0xffff'ffff, 7, 0x9d2c'5680, 15, 0xefc6'0000, 18,
> 1'812'433'253>;
>
> N4993 Section 29.5.2 [rand.synopsis]:
> > template<class UIntType, size_t w, size_t n, size_t m, size_t r,
> > UIntType a, size_t u, UIntType d, size_t s,
> > UIntType b, size_t t,
> > UIntType c, size_t l, UIntType f>
> > class mersenne_twister_engine;
>
> The standard type alias std::mt19937 instantiates
> std::mersenne_twister_engine
> with the type std::uint_fast32_t for UIntType. However, there is no
> requirement
> that std::uint_fast32_t has to be one of those types. What should happen
> with an
> implementation that does not define std::uint_fast32_t to be one of those
> types
> when using std::mt19937 (e.g. defining it as unsigned char or a type wider
> than
> long long)? After discussion on discord I think there are 3 possible
> interpretations of what should happen here:
> 1. On such an implementation, std::mt19937 is not valid to use.
> 2. On such an implementation, std::uint_fast32_t must be valid for
> UIntType;
> i.e. it can be used anyways despite not being one of the listed types.
>

This. Implementations are free to make std::mt19937 work for additional
types.

It's a purely hypothetical problem anyway, no such implementation exists.


> 3. Such an implementation cannot be a conforming hosted implementation,
> this is effectively a requirement on what std::uint_fast32_t can be.
> In N4993 Section 29.5.6 [rand.predef] and N4993 Section 29.5.4.4
> [rand.eng.sub]
> Paragraph 7 there are several other uses of type aliases from cstdint as
> template type arguments for parameters named UIntType (and thus have the
> same
> problems).
>
> I'm thinking about making a LWG issue for this, though it seems like it is
> preferred to post to this mailing list first.
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>