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 = boost::hana;
using namespace hana::literals;

struct C1 {};

template < typename T,
           std::size_t size >
struct Array {};

constexpr auto array_ = [] (auto size) {
      return [=] (auto type) {
        return hana::type_c<Array<typename decltype(type)::type, size()>>;
      };
    };

int main() {

  constexpr auto c1 = hana::type_c<C1>;
  constexpr auto test = hana::type_c<Array<typename decltype(c1)::type, hana::size_c<100>()>>;
  constexpr auto test2 = array_(hana::size_c<100>)(c1);
}

```

I post a question earlier because I found a different minimal example, but it wasn't enough.

**Error:**

```
test2.cpp: In instantiation of ‘<lambda(auto:1)>::<lambda(auto:2)> [with auto:2 = boost::hana::type_impl<C1>::_; auto:1 = boost::hana::integral_constant<long unsigned int, 100>]’:
test2.cpp:31:54:   required from here
test2.cpp:20:16: error: ‘__closure’ is not a constant expression
         return hana::type_c<Array<typename decltype(type)::type, size()>>;
                ^~~~
test2.cpp:20:16: note: in template argument for type ‘long unsigned int’ 
test2.cpp: In function ‘int main()’:
test2.cpp:31:18: error: ‘constexpr const void test2’ has incomplete type
   constexpr auto test2 = array_(hana::size_c<100>)(c1);

```

`__closure is not a constant expression` : if someone could explain me this error that would be a great help. I ran into that error before but can't remember why.