[Bug libstdc++/80064] [7 Regression] _Iter_cmp_iter instantiates bogus type

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80064

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think the testcase is invalid. std::make_heap is declared like so:

  template<class RandomAccessIterator>
  void make_heap(RandomAccessIterator first, RandomAccessIterator last);

  template<class RandomAccessIterator, class Compare>
  void make_heap(RandomAccessIterator first, RandomAccessIterator last,
                 Compare comp);

The explicit instantiation matches the second overload, then argument
substitition creates the function:

  void make_heap(vector<int>::iterator first, vector<int>::iterator last,
                 bool(comp)(int, int));

You can't pass functions by value so the third argument decays to:

  void make_heap(vector<int>::iterator first, vector<int>::iterator last,
                 bool(*comp)(int, int));

However, this means the type of the function parameter comp is no longer of
type Compare, and so if the implementation tries to use that type it will fail, 

e.g. GCC 7 does something like:

  struct wrapper {
    Compare cmp;
  } wrapped_comp = { comp };

And libc++ does something like:

  Compare& ref = comp;

Both fail, because Compare is a function type, but comp is a pointer to 
function.

The test should use:

  std::make_heap<std::vector<int>::iterator, bool(*)(int, int)>

That is the actual type that is deduced for the greater argument.