[Bug c++/92268] New: Constraint normalization substitutes parameter too early

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92268

            Bug ID: 92268
           Summary: Constraint normalization substitutes parameter too
                    early
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 67491
  Target Milestone: ---

The following program fails to compile with -std=gnu++2a

template<typename T, typename U>
  struct common_reference { };

template<typename T, typename U>
  using common_reference_t = typename common_reference<T, U>::type;

template<typename T, typename U> concept foo = true;
template<typename T, typename U> concept bar = true;
template<typename T, typename U> concept baz = true;

template<typename T, typename U>
  concept common_reference_with
    =  foo<common_reference_t<T, U>, common_reference_t<U, T>>
    && bar<common_reference_t<T, U>, common_reference_t<U, T>>
    && baz<common_reference_t<T, U>, common_reference_t<U, T>>;

template<typename T>
  using iter_reference_t = decltype(((T*)0)->f());

template<typename I>
  concept forward_iterator
    = common_reference_with<iter_reference_t<I>&&, typename I::value_type&>;

struct test_range
{
  struct iterator
  {
    using value_type = int;

    char f() const;
  };

  iterator begin();
};

template<typename T>
concept F = requires (T& t) { { t.begin() } -> forward_iterator; };

static_assert( !F<test_range> );


Checking the F<test_range> concept fails:

bug.cc:37:33: error: no type named 'type' in 'struct common_reference<char&&,
int&>'
   37 | concept F = requires (T& t) { { t.begin() } -> forward_iterator; };
      |                               ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bug.cc:37:33: error: no type named 'type' in 'struct common_reference<char&&,
int&>'
bug.cc:37:33: error: no type named 'type' in 'struct common_reference<char&&,
int&>'



On IRC Jason said:

[02:36] <jason> jwakely: you're hitting an error during normalization, which
means the constraints could never be satisfied
[02:40] <jason> but it looks like when we normalize the forward_iterator
constraint on the compound-requirement we're normalizing
forward_iterator<iterator>, which is wrong -- we should be normalizing
forward_iterator<T>, and then checking whether that's satisfied with iterator
[02:40] <jason> so it's a compiler bug
[02:40] <jason> as well as the diagnostic issue
[02:40] <jason> Andrew sent me a patch to improve handling of nested
requirements that I haven't looked at yet, it may help with this


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67491
[Bug 67491] [meta-bug] concepts issues