Re: C++ PATCH for c++/78244 - narrowing conversion in template not detected, part 2

[Jason Merrill <jason@redhat.com>]

On Fri, Jan 25, 2019 at 4:20 PM Marek Polacek <polacek@redhat.com> wrote:
> On Fri, Jan 25, 2019 at 10:05:00AM -0500, Jason Merrill wrote:
> > On 1/24/19 7:17 PM, Marek Polacek wrote:
> > > On Wed, Jan 23, 2019 at 03:34:04PM -0500, Jason Merrill wrote:
> > > > On Wed, Jan 23, 2019 at 12:57 PM Marek Polacek <polacek@redhat.com> wrote:
> > > > >
> > > > > On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
> > > > > > I was talking about digest_init, not reshape_init.  digest_init calls
> > > > > > convert_for_initialization.
> > > > >
> > > > > /facepalm
> > > > >
> > > > > So yes, digest_init calls convert_for_initialization which will end up
> > > > > calling perform_implicit_conversion_flags which could call convert_like_real
> > > > > where the narrowing warnings are given, but it doesn't, we go to this case:
> > > > >
> > > > >    else if (processing_template_decl && conv->kind != ck_identity)
> > > > >      {
> > > > >        /* In a template, we are only concerned about determining the
> > > > >           type of non-dependent expressions, so we do not have to
> > > > >           perform the actual conversion.  But for initializers, we
> > > > >           need to be able to perform it at instantiation
> > > > >           (or instantiate_non_dependent_expr) time.  */
> > > > >        expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
> > > > >
> > > > > finish_decltype_type throws away the expression because it's not dependent, and
> > > > > only uses its type.  So narrowing remains undetected.  Not sure if I should mess
> > > > > with perform_implicit_conversion_flags.
> > > >
> > > > Let's try that; this is a situation where the comment is incorrect.
> > > > Perhaps just call check_narrowing here if appropriate, rather than go
> > > > through the whole conversion machinery.
> > >
> > > I have not been successful.
> > >
> > > First, I modified perform_implicit_conversion_flags to go the convert_like
> > > route when dealing with something non-dependent.  That breaks e.g. in
> > > build_value_init:
> > >   346   /* The AGGR_INIT_EXPR tweaking below breaks in templates.  */
> > >   347   gcc_assert (!processing_template_decl
> > >   348               || (SCALAR_TYPE_P (type) || TREE_CODE (type) == ARRAY_TYPE));
> > > Even if I restrict the convert_like way for non-dependent exprs in a template
> > > for scalars, it still breaks elsewhere, e.g. constexpr-template3.C where it
> > > complains about taking the address of an rvalue.
> > >
> > > Second, I added check_narrowing to the processing_template_decl case in
> > > perform_implicit_conversion_flags.  That works except it breaks
> > > constexpr-inst1.C -- we no longer get the error.  That's because currently
> > > check_narrowing in finish_compound_literal calls maybe_constant_init, which
> > > calls instantiate_constexpr_fns and we get the desired diagnostic.  But if
> > > I move check_narrowing to perform_implicit_conversion_flags, we no longer
> > > call it in this case -- processing_template_decl is 0 so we call convert_like
> > > but that doesn't do the trick.
> > >
> > > So, back to the patch that leaves check_narrowing in finish_compound_literal?
> >
> > That patch still needs a test for the aggregate case.
>
> Ok, this is a version with Wnarrowing16.C added.
>
> ...but we still don't warn for the TYPE_NON_AGGREGATE_CLASS case in
> finish_compound_literal, so the nightmare continues.

Alas.  Are you going to keep looking at that, or would you like me to take over?

> -  if (SCALAR_TYPE_P (type)
> -      && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
> +  if (!BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
>        && !check_narrowing (type, compound_literal, complain))
>      return error_mark_node;

Does this hunk actually make a difference?  It looks like
check_narrowing only does anything for arithmetic types.

OK with or without this hunk.

Jason