constexpr_lambda_example_1.cpp

#include <limits>
#include <utility>

template < class T >
constexpr
decltype(std::declval<T>()*2)
twice(T a) {
  return a*2;
}

static_assert([]{
    for(int i{0}; i < 1'000'000; ++i)
        if ( twice(i) != i*2 ) return  false;
    return true;
}());

int main() {}