bindgen disregards c++ using directives.

Input C/C++ Header

#include <compare>, found in gcc 10.0+, see link:

namespace std _GLIBCXX_VISIBILITY(default)
{
  // [cmp.categories], comparison category types

  namespace __cmp_cat
  {
    using type = signed char;

    enum class _Ord : type { equivalent = 0, less = -1, greater = 1 };

    enum class _Ncmp : type { _Unordered = 2 };

    struct __unspec
    {
      constexpr __unspec(__unspec*) noexcept { }
    };
  }
  // snip
}

Bindgen Invocation

bindgen flags:

        opaque_types = [
            "std::.*",
        ],

bindgen output:

        pub mod __cmp_cat {
            #[allow(unused_imports)]
            use self::super::super::super::root;
            pub type type_ = u8;
            pub const _Ord_equivalent: root::std::__cmp_cat::_Ord = 0;
            pub const _Ord_less: root::std::__cmp_cat::_Ord = -1;
            pub const _Ord_greater: root::std::__cmp_cat::_Ord = 1;
            pub type _Ord = root::std::__cmp_cat::type_;
            pub const _Ncmp__Unordered: root::std::__cmp_cat::_Ncmp = 2;
            pub type _Ncmp = root::std::__cmp_cat::type_;
            #[repr(C)]
            #[repr(align(1))]
            #[derive(Debug, Copy, Clone)]
            pub struct __unspec {
                pub _bindgen_opaque_blob: u8,
            }
        }

Actual Results

error[E0600]: cannot apply unary operator `-` to type `u8`
    --> xxx/lib.rs:1763:63
     |
1763 |             pub const _Ord_less: root::std::__cmp_cat::_Ord = -1;
     |                                                               ^^
     |                                                               |
     |                                                               cannot apply unary operator `-`
     |                                                               help: you may have meant the maximum value of `u8`: `u8::MAX`
     |
     = note: unsigned values cannot be negated

Expected Results

Bindgen should use pub tye type_ = i8 (instead of u8), because c++ has: using type = signed char;, hence the code should compile.

Start by reproducing the provided C++ header and bindgen configuration, then trace how the using type = signed char alias is translated into the generated Rust type. Add a regression test if the repository has a suitable bindgen test location; done means the alias becomes i8 and the negative _Ord_less constant compiles.