On Tue, Jul 3, 2018 at 2:58 PM, Marek Polacek <pola...@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 <pola...@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 <pola...@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 <pola...@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 <pola...@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
