Getting the compiler to tell which part of concept conjunction failed

The fold expression is hiding which type failed. If you split the check into a per-type concept, most compilers will report the specific instantiation that doesn't satisfy the constraint.

For example:

template
concept hasupdate =
    requires(updatefunct& cb) {
        { cb = &UpdateTraits::update };
    };
template
concept updatetraits = (hasupdate && ...);

Then if double is missing, the compiler is much more likely to point at hasupdate instead of just saying the whole fold expression evaluated to false.

Here is one solution. The idea is keep an alias of the type that failed, and try to use a function that is deleted for everything except a sentinel (in this case void):

#include 
#include 
#include 
template
using updatefunct = UpdateResultType ()(Sink&, T);
template
struct has_applicable_update
{
    using result = void;
};
template
struct has_applicable_update
{
    static constexpr auto current_value = requires(update_func_t& cb) {
        { cb = &UpdateTraits::update };
    };
    using result = std::conditional_t<
        current_value,
        typename has_applicable_update::result,
        T
    >;
};
template
consteval bool concept_satisfied_for_type() = delete;
template
consteval bool concept_satisfied_for_type()
{ return true; }
template
concept update_traits = concept_satisfied_for_type();
template
void do_stuff();
struct my_traits{
    static int update(std::vector&, int);
    //static double* update(std::vector&, double);
};
int main()
{
    dostuff();
}

If the concept is satisfied, typename hasapplicableupdate::result is void, and conceptsatisfiedfortype returns true. Otherwise, it tried to call the deleted specialization, and substitution fails:

: In function 'int main()':
:53:24: error: no matching function for call to 'dostuff()'
   53 |     dostuff();
      |     ~~~~~~~~~~~~~~~~~~~^~
:53:24: note: there is 1 candidate
:39:6: note: candidate 1: 'template  requires  updatetraits void dostuff()'
   39 | void dostuff();
      |      ^~~~~~~~
:39:6: note: template argument deduction/substitution failed:
:39:6: note: constraints not satisfied
: In substitution of 'template  requires  updatetraits void dostuff() [with UpdateTraits = mytraits]':
:53:24:   required from here
   53 |     dostuff();
      |     ~~~~~~~~~~~~~~~~~~~^~
:36:9:   required for the satisfaction of 'updatetraits' [with UpdateTraits = mytraits]
:36:139: error: use of deleted function 'consteval bool conceptsatisfiedfortype() [with T = double]'
   36 | concept updatetraits = conceptsatisfiedfortype();

GCC 16 emits a god error message when using std::issamev. This one works also for GCC 15.