[Bug c++/82311] New: prototype don't match

["serge dot guelton at telecom-bretagne dot eu" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82311

            Bug ID: 82311
           Summary: prototype don't match
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: serge.guelton@telecom-bretagne.eu
  Target Milestone: ---

The following code

```
template <class Arg>
struct my_type {

  long fast(long i) const;
  long fast(long i) ;

  auto operator[](long i) const -> decltype(this->fast(i));

  template <class E>
  auto fast(E const &expr) const -> decltype(expr.any());
};


template <class Arg>
auto my_type<Arg>::operator[](long i) const -> decltype(this->fast(i))
{
  return fast(i);
}
```

does not compile with gcc 7

$> g++ --version
g++-7 (Debian 7.2.0-5) 7.2.0

The error message is the following

bug.cc:15:6: error: prototype for ‘decltype (((const
my_type<Arg>*)this)->my_type<Arg>::fast(i)) my_type<Arg>::operator
 auto my_type<Arg>::operator[](long i) const -> decltype(this->fast(i))
      ^~~~~~~~~~~~
bug.cc:7:8: error: candidate is: decltype (((const
my_type<Arg>*)this)->my_type<Arg>::fast(i)) my_type<Arg>::operator[]
   auto operator[](long i) const -> decltype(this->fast(i));
        ^~~~~~~~

And surprisingly enough, swaping the two last declarations in the structure
solve the issue.