Re: [PATCH] c++: non-dependent .* folding [PR112427]

[Patrick Palka <ppalka@redhat.com>]

On Fri, 10 Nov 2023, Patrick Palka wrote:

> On Thu, 9 Nov 2023, Jason Merrill wrote:
> 
> > On 11/8/23 16:59, Patrick Palka wrote:
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > > trunk?
> > > 
> > > -- >8 --
> > > 
> > > Here when building up the non-dependent .* expression, we crash from
> > > fold_convert on 'b.a' due to this (templated) COMPONENT_REF having an
> > > IDENTIFIER_NODE instead of FIELD_DECL operand that middle-end routines
> > > expect.  Like in r14-4899-gd80a26cca02587, this patch fixes this by
> > > replacing the problematic piecemeal folding with a single call to
> > > cp_fully_fold.
> > > 
> > > 	PR c++/112427
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > > 	* typeck2.cc (build_m_component_ref): Use cp_convert, build2 and
> > > 	cp_fully_fold instead of fold_build_pointer_plus and fold_convert.
> > 
> > > gcc/testsuite/ChangeLog:
> > > 
> > > 	* g++.dg/template/non-dependent29.C: New test.
> > > ---
> > >   gcc/cp/typeck2.cc                               |  5 ++++-
> > >   gcc/testsuite/g++.dg/template/non-dependent29.C | 13 +++++++++++++
> > >   2 files changed, 17 insertions(+), 1 deletion(-)
> > >   create mode 100644 gcc/testsuite/g++.dg/template/non-dependent29.C
> > > 
> > > diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
> > > index 309903afed8..208004221da 100644
> > > --- a/gcc/cp/typeck2.cc
> > > +++ b/gcc/cp/typeck2.cc
> > > @@ -2378,7 +2378,10 @@ build_m_component_ref (tree datum, tree component,
> > > tsubst_flags_t complain)
> > >         /* Build an expression for "object + offset" where offset is the
> > >   	 value stored in the pointer-to-data-member.  */
> > >         ptype = build_pointer_type (type);
> > > -      datum = fold_build_pointer_plus (fold_convert (ptype, datum),
> > > component);
> > > +      datum = cp_convert (ptype, datum, complain);
> > > +      datum = build2 (POINTER_PLUS_EXPR, ptype,
> > > +		      datum, convert_to_ptrofftype (component));
> > 
> > We shouldn't need to build the POINTER_PLUS_EXPR at all in template context.
> > OK with that change.
> 
> Hmm, that seems harmless at first glance, but I noticed
> build_min_non_dep (called from build_x_binary_op in this case) is
> careful to propagate TREE_SIDE_EFFECTS of the given tree, and so eliding
> POINTER_PLUS_EXPR here could potentially mean that the tree we
> ultimately return from build_x_binary_op when in a template context has
> TREE_SIDE_EFFECTS not set when it used to.  Shall we still elide the
> POINTER_PLUS_EXPR in a template context despite this?
> 
> (The TREE_SIDE_EFFECTS propagation in build_min_non_dep was added in
> r71108 to avoid bogus ahead of time -Wunused-value warnings.  But then
> r105273 later made us stop issuing -Wunused-value warnings ahead of time
> altogether.  So perhaps we don't need to maintain the TREE_SIDE_EFFECTS
> flag on templated trees at all anymore?)

IMO it'd be nice to restore ahead of time -Wunused-value warnings;
it seems the original motivation for r105273 / PR8057 was to avoid
redundantly issuing a warning twice, once ahead of time and once at
instantiation time, which we now could do in a better way with
warning_suppressed_p etc.  If so, then IIUC eliding the POINTER_PLUS_EXPR
could mean we'd incorrectly issue a -Wunused-value warning for e.g.
'a.*f()' in a template context?

> 
> > 
> > Jason
> > 
> > 
>