On Mon, 29 Jan 2024, Patrick Palka wrote:
> On Mon, 29 Jan 2024, Patrick Palka wrote:
>
> > On Fri, 26 Jan 2024, Jason Merrill wrote:
> >
> > > On 1/26/24 17:11, Jason Merrill wrote:
> > > > On 1/26/24 16:52, Jason Merrill wrote:
> > > > > On 1/25/24 14:18, Patrick Palka wrote:
> > > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> > > > > > OK for trunk/13? This isn't a very satisfactory fix, but at least
> > > > > > it safely fixes these testcases I guess. Note that there's
> > > > > > implementation disagreement about the second testcase, GCC always
> > > > > > accepted it but Clang/MSVC/icc reject it.
> > > > >
> > > > > Because of trying to initialize int& from {c}; removing the extra
> > > > > braces
> > > > > makes it work everywhore.
> > > > >
> > > > > https://eel.is/c++draft/dcl.init#list-3.10 says that we always
> > > > > generate a
> > > > > prvalue in this case, so perhaps we shouldn't recalculate if the
> > > > > initializer is an init-list?
> > > >
> > > > ...but it seems bad to silently bind a const int& to a prvalue instead
> > > > of
> > > > directly to the reference returned by the operator, as clang does if we
> > > > add
> > > > const to the second testcase, so I think there's a defect in the
> > > > standard
> > > > here.
> > >
> > > Perhaps bullet 3.9 should change to "...its referenced type is
> > > reference-related to E <ins>or scalar</ins>, ..."
> > >
> > > > Maybe for now also disable the maybe_valid heuristics in the case of an
> > > > init-list?
> > > >
> > > > > The first testcase is special because it's a C-style cast; seems like
> > > > > the
> > > > > maybe_valid = false heuristics should be disabled if c_cast_p.
> >
> > Thanks a lot for the pointers. IIUC c_cast_p and LOOKUP_SHORTCUT_BAD_CONVS
> > should already be mutually exclusive, since the latter is set only when
> > computing argument conversions, so it shouldn't be necessary to check
> > c_cast_p.
> >
> > I suppose we could disable the heuristic for init-lists, but after some
> > digging I noticed that the heuristics were originally in same spot they
> > are now until r5-601-gd02f620dc0bb3b moved them to get checked after
> > the recursive recalculation case in reference_binding, returning a bad
> > conversion instead of NULL. (Then in r13-1755-g68f37670eff0b872 I moved
> > them back; IIRC that's why I felt confident that moving the checks was
> > safe.)
> > Thus we didn't always accept the second testcase, we only started doing so
> > in
> > GCC 5: https://godbolt.org/z/6nsEW14fh (sorry for missing this and saying we
> > always accepted it)
> >
> > And indeed the current order of checks seems consistent with that of
> > [dcl.init.ref]/5. So I wonder if we don't instead want to "complete"
> > the NULL-to-bad-conversion adjustment in r5-601-gd02f620dc0bb3b and
> > do:
> >
> > gcc/cp/ChangeLog:
> >
> > * call.cc (reference_binding): Set bad_p according to
> > maybe_valid_p in the recursive case as well. Remove
> > redundant gcc_assert.
> >
> > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > index 9de0d77c423..c4158b2af37 100644
> > --- a/gcc/cp/call.cc
> > +++ b/gcc/cp/call.cc
> > @@ -2033,8 +2033,8 @@ reference_binding (tree rto, tree rfrom, tree expr,
> > bool c_cast_p, int flags,
> > sflags, complain);
> > if (!new_second)
> > return bad_direct_conv ? bad_direct_conv : nullptr;
> > + t->bad_p = !maybe_valid_p;
>
> Oops, that should be |= not =.
>
> > > Perhaps bullet 3.9 should change to "...its referenced type is
> > > reference-related to E <ins>or scalar</ins>, ..."
> > conv = merge_conversion_sequences (t, new_second);
> > - gcc_assert (maybe_valid_p || conv->bad_p);
> > return conv;
> > }
> > }
> >
> > This'd mean we'd go back to rejecting the second testcase (only the
> > call, not the direct-init, interestingly enough), but that seems to be
>
> In the second testcase, with the above fix initialize_reference silently
> returns error_mark_node for the direct-init without issuing a
> diagnostic, because in the error path convert_like doesn't find anything
> wrong with the bad conversion. So more changes need to be made if we
> want to set bad_p in the recursive case of reference_binding it seems;
> dunno if that's the path we want to go down?
>
> On the other hand, disabling the badness checks in certain cases seems
> to be undesirable as well, since AFAICT their current position is
> consistent with [dcl.init.ref]/5?
>
> So I wonder if we should just go with the safest thing at this stage,
> which would be the original patch that removes the problematic assert?
Ping.
>
> > the correct behavior anyway IIUC. The testsuite is otherwise happy
> > with this change.
