[Bug c++/87584] New: [8 regression] const overload ignored on operator[] in captured by value object in template function

[jlegg at feralinteractive dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87584

            Bug ID: 87584
           Summary: [8 regression] const overload ignored on operator[] in
                    captured by value object in template function
           Product: gcc
           Version: 8.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlegg at feralinteractive dot com
  Target Milestone: ---

The following c++ code incorrectly produces the error "passing ‘const C’ as
‘this’ argument discards qualifiers", instead of using the C::operator[](int)
const overload:

struct C
{
        int operator[](int)
        {
                return 0;
        }

        int operator[](int) const
        {
                return 1;
        }
};

template <typename T>
int f(T)
{
        C c;
        return [=] { return c[0]; }();
}

int main()
{
        return f(0);
}

The bug reproduces on GCC 8.1 and 8.2. However, this is handled correctly by
GCC 7.3.