template for expansion statements usage

[githubuser6000]

Is your feature request related to a problem? Please describe.

I was trying to use the template for syntax, but it wasn't clear to me from the proposal how and in what contexts it is supposed to work.

I am trying this example ...

struct test {
    int one;
    int two;
    int three;
};

template<typename T>
consteval void template_for_expansion_stmt() {
    template for(constexpr auto member : std::meta::define_static_array(
                                 std::meta::nonstatic_data_members_of(^T)))
    {
        // ...
    }
}

int main() {
    template_for_expansion_stmt<test>();
}

I get this error

clang++ -std=c++26 -stdlib=libc++ -freflection -fparameter-reflection -fconsteval-blocks -fexpansion-statements main.cxx -o main
src/main.cxx:43:42: error: cannot decompose private member '__data_' of 'std::span<const meta::info>'
   43 |     template for(constexpr auto member : std::meta::define_static_array(
      |                                          ^
src/main.cxx:51:5: note: in instantiation of function template specialization 'template_for_expansion_stmt<test>' requested here
   51 |     template_for_expansion_stmt<test>();
      |     ^
/home/user/.local/llvm-reflect/bin/../include/c++/v1/span:568:11: note: declared private here
  568 |   pointer __data_;
      |           ^
src/main.cxx:43:42: error: cannot decompose private member '__size_' of 'std::span<const meta::info>'
   43 |     template for(constexpr auto member : std::meta::define_static_array(
      |                                          ^
/home/user/.local/llvm-reflect/bin/../include/c++/v1/span:569:13: note: declared private here
  569 |   size_type __size_;
      |             ^
2 errors generated.

Thank you for the help!

[Tsche]

From a quick look at the code it seems to me that expansion statements are only implemented over init-lists and decomposable types. std::vector is not decomposable because of its private members (that's the error you're seeing). The same is the case for std::span<T>, which is why the workaround with define_static_array didn't work either.

A simple workaround would be something along the lines of

template <typename T, T... Vs>
constexpr inline T static_array[sizeof...(Vs)] = {Vs...};

consteval auto intern(std::ranges::input_range auto&& r){
    std::vector args = {^^std::ranges::range_value_t<decltype(r)>};
    for (auto e : r) {
        args.push_back(std::meta::reflect_value(e));
    }
    return substitute(^^static_array, args);
}

struct Foo {
    int a;
    int b;
};

int main(){
    consteval {
        // error: constexpr variable '[, , , , , ]' must be initialized by a constant expression
        // error: cannot decompose private member '__begin_' of 'std::vector<meta::info>'
        //template for (constexpr auto member : nonstatic_data_members_of(^^Foo)) { /* ... */ }

        template for (constexpr auto member : [:intern(nonstatic_data_members_of(^^Foo)):]) { 
            /* ... */
        }
    }
}

run on compiler explorer

[githubuser6000]

Oh that's pretty neat, thanks!