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Re: C++ PATCH for c++/57891, narrowing conversions in non-type template arguments


On Tue, Jul 3, 2018 at 2:58 PM, Marek Polacek <polacek@redhat.com> wrote:
> On Tue, Jul 03, 2018 at 12:40:51PM -0400, Jason Merrill wrote:
>> On Fri, Jun 29, 2018 at 3:58 PM, Marek Polacek <polacek@redhat.com> wrote:
>> > On Wed, Jun 27, 2018 at 07:35:15PM -0400, Jason Merrill wrote:
>> >> On Wed, Jun 27, 2018 at 12:53 PM, Marek Polacek <polacek@redhat.com> wrote:
>> >> > This PR complains about us accepting invalid code like
>> >> >
>> >> >   template<unsigned int> struct A {};
>> >> >   A<-1> a;
>> >> >
>> >> > Where we should detect the narrowing: [temp.arg.nontype] says
>> >> > "A template-argument for a non-type template-parameter shall be a converted
>> >> > constant expression ([expr.const]) of the type of the template-parameter."
>> >> > and a converted constant expression can contain only
>> >> > - integral conversions other than narrowing conversions,
>> >> > - [...]."
>> >> > It spurred e.g.
>> >> > <https://stackoverflow.com/questions/28184888/how-implicit-conversion-works-for-non-type-template-parameters>
>> >> > and has >=3 dups so it has some visibility.
>> >> >
>> >> > I think build_converted_constant_expr needs to set check_narrowing.
>> >> > check_narrowing also always mentions that it's in { } but that is no longer
>> >> > true; in the future it will also apply to <=>.  We'd probably have to add a new
>> >> > flag to struct conversion if wanted to distinguish between these.
>> >> >
>> >> > This does not yet fix detecting narrowing in function templates (78244).
>> >> >
>> >> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
>> >> >
>> >> > 2018-06-27  Marek Polacek  <polacek@redhat.com>
>> >> >
>> >> >         PR c++/57891
>> >> >         * call.c (build_converted_constant_expr): Set check_narrowing.
>> >> >         * decl.c (compute_array_index_type): Add warning sentinel.  Use
>> >> >         input_location.
>> >> >         * pt.c (convert_nontype_argument): Return NULL_TREE if any errors
>> >> >         were reported.
>> >> >         * typeck2.c (check_narrowing): Don't mention { } in diagnostic.
>> >> >
>> >> >         * g++.dg/cpp0x/Wnarrowing6.C: New test.
>> >> >         * g++.dg/cpp0x/Wnarrowing7.C: New test.
>> >> >         * g++.dg/cpp0x/Wnarrowing8.C: New test.
>> >> >         * g++.dg/cpp0x/constexpr-data2.C: Add dg-error.
>> >> >         * g++.dg/init/new43.C: Adjust dg-error.
>> >> >         * g++.dg/other/fold1.C: Likewise.
>> >> >         * g++.dg/parse/array-size2.C: Likewise.
>> >> >         * g++.dg/other/vrp1.C: Add dg-error.
>> >> >         * g++.dg/template/char1.C: Likewise.
>> >> >         * g++.dg/ext/builtin12.C: Likewise.
>> >> >         * g++.dg/template/dependent-name3.C: Adjust dg-error.
>> >> >
>> >> > diff --git gcc/cp/call.c gcc/cp/call.c
>> >> > index 209c1fd2f0e..956c7b149dc 100644
>> >> > --- gcc/cp/call.c
>> >> > +++ gcc/cp/call.c
>> >> > @@ -4152,7 +4152,10 @@ build_converted_constant_expr (tree type, tree expr, tsubst_flags_t complain)
>> >> >      }
>> >> >
>> >> >    if (conv)
>> >> > -    expr = convert_like (conv, expr, complain);
>> >> > +    {
>> >> > +      conv->check_narrowing = !processing_template_decl;
>> >>
>> >> Why !processing_template_decl?  This needs a comment.
>> >
>> > Otherwise we'd warn for e.g.
>> >
>> > template<int N> struct S { char a[N]; };
>> > S<1> s;
>> >
>> > where compute_array_index_type will try to convert the size of the array (which
>> > is a template_parm_index of type int when parsing the template) to size_type.
>> > So I guess I can say that we need to wait for instantiation?
>>
>> We certainly shouldn't give a narrowing diagnostic about a
>> value-dependent expression.  It probably makes sense to check that at
>> the top of check_narrowing, with all the other early exit conditions.
>> But if we do know the constant value in the template, it's good to
>> complain then rather than wait for instantiation.
>
> Makes sense; how about this then?  (Regtest/bootstrap running.)
>
> 2018-07-03  Marek Polacek  <polacek@redhat.com>
>
>         PR c++/57891
>         * call.c (build_converted_constant_expr): Set check_narrowing.
>         * decl.c (compute_array_index_type): Add warning sentinel.  Use
>         input_location.
>         * pt.c (convert_nontype_argument): Return NULL_TREE if any errors
>         were reported.
>         * typeck2.c (check_narrowing): Don't warn for instantiation-dependent
>         expressions or non-constants in a template.  Don't mention { } in
>         diagnostic.
>
>         * g++.dg/cpp0x/Wnarrowing6.C: New test.
>         * g++.dg/cpp0x/Wnarrowing7.C: New test.
>         * g++.dg/cpp0x/Wnarrowing8.C: New test.
>         * g++.dg/cpp0x/Wnarrowing9.C: New test.
>         * g++.dg/cpp0x/Wnarrowing10.C: New test.
>         * g++.dg/cpp0x/constexpr-data2.C: Add dg-error.
>         * g++.dg/init/new43.C: Adjust dg-error.
>         * g++.dg/other/fold1.C: Likewise.
>         * g++.dg/parse/array-size2.C: Likewise.
>         * g++.dg/other/vrp1.C: Add dg-error.
>         * g++.dg/template/char1.C: Likewise.
>         * g++.dg/ext/builtin12.C: Likewise.
>         * g++.dg/template/dependent-name3.C: Adjust dg-error.
>
> diff --git gcc/cp/call.c gcc/cp/call.c
> index 209c1fd2f0e..4fb0fa8774b 100644
> --- gcc/cp/call.c
> +++ gcc/cp/call.c
> @@ -4152,7 +4152,10 @@ build_converted_constant_expr (tree type, tree expr, tsubst_flags_t complain)
>      }
>
>    if (conv)
> -    expr = convert_like (conv, expr, complain);
> +    {
> +      conv->check_narrowing = true;
> +      expr = convert_like (conv, expr, complain);
> +    }
>    else
>      expr = error_mark_node;
>
> diff --git gcc/cp/decl.c gcc/cp/decl.c
> index c04b9b7d457..8da63fa2aaa 100644
> --- gcc/cp/decl.c
> +++ gcc/cp/decl.c
> @@ -9508,6 +9508,8 @@ compute_array_index_type (tree name, tree size, tsubst_flags_t complain)
>        else
>         {
>           size = instantiate_non_dependent_expr_sfinae (size, complain);
> +         /* Don't warn about narrowing for VLAs.  */
> +         warning_sentinel s (warn_narrowing, !TREE_CONSTANT (osize));
>           size = build_converted_constant_expr (size_type_node, size, complain);

Hmm, perhaps the underlying issue is that we only want
build_converted_constant_expr to check for narrowing of constant
values; if the value isn't constant, it isn't any kind of constant
expression.  So perhaps the checking needs to happen as part of
constexpr evaluation.

> @@ -6669,9 +6669,12 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>           /* C++17: A template-argument for a non-type template-parameter shall
>              be a converted constant expression (8.20) of the type of the
>              template-parameter.  */
> +         int errs = errorcount;
>           expr = build_converted_constant_expr (type, expr, complain);
>           if (expr == error_mark_node)
> -           return error_mark_node;
> +           /* Make sure we return NULL_TREE only if we have really issued
> +              an error, as described above.  */
> +           return errorcount > errs ? NULL_TREE : error_mark_node;

Is this still needed?

> +      /* If we're in a template and we know the constant value, we can
> +        warn.  Otherwise wait for instantiation.  */
> +      || (processing_template_decl && !TREE_CONSTANT (init)))

I don't think we want this condition.  If the value is non-constant
but also not dependent, it'll never be constant, so we can go ahead
and complain.

Jason


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