Why is this nested lambda not considered constexpr?

Question

I\'m trying to create a curried interface using nested constexpr lambdas, but the compiler does not consider it to be a constant expression.

namespace hana =         


        

Answer 1

I reduced your test case to this:

#include 

constexpr auto f = [](auto size) {
  return [=](){
    constexpr auto s = size();
    return 1;
  };
};

static_assert(f(std::integral_constant{})(), "");

int main() { }

As said in the comments above, this happens because size is not a constant expression from within the function body. This is not specific to Hana. As a workaround, you can use

constexpr auto f = [](auto size) {
  return [=](){
    constexpr auto s = decltype(size)::value;
    return 1;
  };
};

or anything similar.