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

[Jason Merrill <jason at redhat dot com>]

On Tue, Jul 3, 2018 at 3:41 PM, Jason Merrill <jason@redhat.com> wrote:
> 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.

On the other hand, check_narrowing itself forces constexpr evaluation,
so this could be handled all in the conversion code; we just need to
tell check_narrowing to allow non-constant values in that case.

>> @@ -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?

Earlier you wrote,

> Checking complain doesn't work because that doesn't
> say if we have really issued an error.  If we have not, and we return
> NULL_TREE anyway, we hit this assert:
>  8517       gcc_assert (!(complain & tf_error) || seen_error ());

If (complain & tf_error), we shouldn't see error_mark_node without an
error having been issued.  Why is build_converted_constant_expr
returning error_mark_node without giving an error when (complain &
tf_error)?

>> +      /* 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