[Bug c++/79503] New: [new inheriting ctors] spurious bad candidate from same or derived type

[lucdanton at free dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79503

            Bug ID: 79503
           Summary: [new inheriting ctors] spurious bad candidate from
                    same or derived type
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lucdanton at free dot fr
  Target Milestone: ---

My understanding of the new inheriting constructor rules is a bit shaky so I
may be off here. The testcase:

//--------------------
struct payload {};

struct base: private payload {
    base(payload) {}
};

struct derived: base {
    using base::base;
};

int main()
{
    payload data;
    // error: no matching function for call to 'derived::derived(payload&)'
    // note: candidate: base::base(payload)
    // note:   an inherited constructor is not a candidate for initialization
from an expression of the same or derived type
    derived demo(data);
}
//--------------------

Compilation fails on recent trunk with e.g.:
    g++-trunk -std=c++14 main.cpp
but succeeds with -fno-new-inheriting-ctors. A hint that something is wrong is
that there is no error if base derives from some other class than payload.

At the very least the last note is misleading: all inherited candidates are
from base whereas the expression has type payload which is neither the same nor
derived from it.