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// Copyright (C) 2023 johannst
#include <type_traits>
// (1) Primary template.
template<typename T, typename = void>
struct is_valid : std::false_type {};
// (2) Partial template specialization.
template<typename T>
struct is_valid<T, std::void_t<decltype(std::declval<T>().some_fun1()),
decltype(std::declval<T>().some_fun2())>> : std::true_type {};
struct A {
void some_fun1() {}
void some_fun2() {}
};
struct B {};
static_assert(is_valid<A>::value, "is true");
// * Compare template arg list with primary template, we only supplied one
// arg, the second one will be defaulted as
// is_valid<A, void>
// * Compare template arg list against available specializations, this will
// try to match the pattern <A, void> against the patterns defined in the
// partial specializations.
// * Try specialization (2)
// * T -> A
// * Evaluate std::void_t -> decltype's are well-formed
// std::void_t<...> -> void
// * Specialization (2) matches <A, void>
// * Pick the most specialized version -> (2)
static_assert(!is_valid<B>::value, "is false");
// * Compare template arg list with primary template, we only supplied one
// arg, the second one will be defaulted as
// is_valid<A, void>
// * Compare template arg list against available specializations, this will
// try to match the pattern <B, void> against the patterns defined in the
// partial specializations.
// * Try specialization (2)
// * T -> B
// * Evaluate std::void_t -> decltype's are ill-formed
// * Specialization (2) is removed from candidate set, no hard error (SFINAE)
// * No specialization matches, take the primary template.
int main() {}
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