Is there a reason to use std::conjunction/std::disjunction instead of a fold expression over “&&”/“||”?

Question

Is there any specific cases you cannot correctly do with std::conjunction/std::disjunction and not using the more \"fundamental\" (i.e. language featur

Answer 1

std::conjunction short-circuits ::value instantiation, while the fold expression doesn't. This means that, given:

template  
struct valid_except_void : std::false_type { };

template <> 
struct valid_except_void { };

The following will compile:

template 
constexpr auto test = std::conjunction_v...>;

constexpr auto inst = test;

But the following won't:

template 
constexpr auto test = (valid_except_void::value && ...);

constexpr auto inst = test;

live example on godbolt.org

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From cppreference:

Conjunction is short-circuiting: if there is a template type argument Bi with bool(Bi::value) == false, then instantiating conjunction::value does not require the instantiation of Bj::value for j > i.