Visibility bug in gcc-9 and below using concepts

[marehr]

The following code has unwanted side effects:

#include <range/v3/view/split.hpp>

class A {
  int i{5};
};

int main()
{
    return A{}.i; // should be illegal, because it is private
}

https://godbolt.org/z/rP1bGK

which makes any private/protected member public.

Expected output:

<source>: In function 'int main()':
<source>:9:16: error: 'int A::i' is private within this context
    9 |     return A{}.i; // should be illegal, because it is private
      |                ^
<source>:4:7: note: declared private here
    4 |   int i{5};
      |       ^

This is a known gcc-bug which was fixed in gcc-10. We internally have the following concept check to make sure we don't use any concept syntax that triggers this bug:

template <typename t> concept private_bug = requires(t a) { a.i; };
static_assert(!private_bug<A>, "See https://github.com/seqan/seqan3/issues/1317");

(I love this expression, because you can have self-warning code for concepts with concepts xD)

---

I tried to reduce the code and this came out:

#include <utility>
namespace ranges {
template <typename I1, typename I2 = I1> concept indirectly_swappable = true;
}
namespace ranges {
template <typename Rng>
using iterator_t = decltype(begin(std::declval<Rng &>()));
template <typename V, typename Pattern> struct split_view;
namespace detail {
template <typename JoinView, bool Const> struct split_inner_iterator;
template <typename V, typename Pattern, bool Const>
struct split_inner_iterator<split_view<V, Pattern>, Const> {
  using Base = V;
// <<<<<<<<<<<<<<<<<<<<<<<< misbehaving code
  template <bool = false>
  friend constexpr auto iter_swap(split_inner_iterator const &x,
                                  split_inner_iterator const &y)
      -> std::enable_if_t<indirectly_swappable<iterator_t<Base>> && true> {}
// >>>>>>>>>>>>>>>>>>>>>>>>
};
} // namespace detail
} // namespace ranges
class A {
  int i{5};
};
template <typename t> concept private_bug = requires(t a) { a.i; };
static_assert(!private_bug<A>,  "See https://github.com/seqan/seqan3/issues/1317");

https://godbolt.org/z/rP1bGK

Adding parenthesis around the && expression, solves this issue.

// <<<<<<<<<<<<<<<<<<<<<<<< misbehaving code
  template <bool = false>
  friend constexpr auto iter_swap(split_inner_iterator const &x,
                                  split_inner_iterator const &y)
      -> std::enable_if_t<(indirectly_swappable<iterator_t<Base>> && true)> {}
// >>>>>>>>>>>>>>>>>>>>>>>>

[marehr]

b52e3b5 is the first bad commit
commit b52e3b5
Author: Eric Niebler eniebler@boostorg
Date: Mon Aug 3 19:52:55 2020 -0500

Rework the concepts emulation later to do short-circuiting in requires clauses (#1539)