Observation: the proposed detection feature does not make sense without
`userexpr`. Even if you can determine, at compile time, a `bool` that tells
you whether `42_km` is a valid user-defined literal, the only way you could
possibly make use of that information is to avoid instantiating a template
that contains `42_km` because you know that it's not valid. But that
template, having not been instantiated, still makes the program IFNDR under
[temp.res.general]/6 <http://eel.is/c++draft/temp.res.general#6>, bullets 1
and 5.

On Tue, Aug 15, 2023 at 6:50 PM veronica alphonso via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I'm aware about the potential pitfalls of userexpr, the main one being how
> to determine whether t`s value comes from a literal, but i think this could
> be sorted in a similar way as how
> std::reference_constructs/converts_from_temporary work from the compiler's
> perspective; if t's type isn't one of the list declared at
> https://eel.is/c++draft/over.literal#3, is constructed from a
> non-literal/temporary, and it's accessed after its lifetime begins(dynamic
> branches included) and before its usage in the userexpr(the concept of
> access being defined by https://eel.is/c++draft/intro.defs#defns.access),
> then the t's usage on a userexpr is ill.formed.
>
> And sure, userexpr could be used for this endeavour, but i don't want to
> promote it as the main solution until i fully study all of its potential
> details, that's why my intention was to submit an intermediate
> detection-feature
>
> El mar, 15 ago 2023 a las 22:19, Jens Maurer (<jens.maurer_at_[hidden]>)
> escribió:
>
>>
>>
>> On 15/08/2023 20.05, veronica alphonso wrote:
>> > An example of using this proposed feature(and userexpr, which is
>> another feature yet to propose) would be:
>>
>> So, you're asking for a new feature to support yet another novel feature.
>>
>> > template<class R, class T, const char* suffix, bool sized = true>
>> > requires std::user_defined_literal_r<R, types_to_params<T,
>> std::conditional_t<sized, std::size_t, void>>, suffix>
>> > constexpr R instantiate_usl(T t)
>> > {
>> > return userexpr(t, sized, suffix);
>> > // userexpr converts the given instance, sized or not, and suffix to
>> its literal representation
>> > // in the form of an expression
>> > }
>>
>> Why can't you use
>>
>> requires { userexpr(t, sized, suffix }
>>
>> in your concept, then? All you want to check is whether the given
>> "userexpr"
>> is valid or not.
>>
>> (I'm not offering an opinion whether "userexpr" is a good idea to start
>> with.)
>>
>> Jens
>>
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-- 
*Brian Bi*