[PATCH] c++: top-level cv-quals on type of NTTP [PR100893]

Jason Merrill jason@redhat.com
Thu Jun 3 20:34:58 GMT 2021


On 6/3/21 2:40 PM, Patrick Palka wrote:
> On Thu, 3 Jun 2021, Patrick Palka wrote:
> 
>> Here, we're rejecting the specialization of g<T,F> with T=A, F=&f in the
>> first testcase due to a spurious constness mismatch between the type of
>> the template argument &f and the substituted type of F (the substituted
>> type has a top-level const).  Note that this mismatch doesn't occur with
>> object pointers because in that case a call to
>> perform_qualification_conversions from convert_nontype_argument
>> implicitly adds a top-level const to the argument (via a cast) to match.
>>
>> This however seems to be a manifestation of a more general conformance
>> issue -- that we're not dropping top-level cv-quals after substituting
>> into the type of an NTTP as per [temp.param]/6 (we only do so at parse
>> time in process_template_parm).  This patch makes convert_template_argument
>> drop top-level cv-quals accordingly.
>>
>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
>> trunk?
>>
>> 	PR c++/100893
>>
>> gcc/cp/ChangeLog:
>>
>> 	* pt.c (convert_template_argument): Strip top-level cv-quals
>> 	on the substituted type of a non-type template parameter.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	* g++.dg/template/param4.C: New test.
>> 	* g++.dg/template/param5.C: New test.
>> ---
>>   gcc/cp/pt.c                            |  7 ++++++-
>>   gcc/testsuite/g++.dg/template/param4.C | 10 ++++++++++
>>   gcc/testsuite/g++.dg/template/param5.C |  7 +++++++
>>   3 files changed, 23 insertions(+), 1 deletion(-)
>>   create mode 100644 gcc/testsuite/g++.dg/template/param4.C
>>   create mode 100644 gcc/testsuite/g++.dg/template/param5.C
>>
>> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
>> index 7211bdc5bbc..66cc88a331f 100644
>> --- a/gcc/cp/pt.c
>> +++ b/gcc/cp/pt.c
>> @@ -8494,7 +8494,12 @@ convert_template_argument (tree parm,
>>   	    return error_mark_node;
>>   	}
>>         else
>> -	t = tsubst (t, args, complain, in_decl);
>> +	{
>> +	  t = tsubst (t, args, complain, in_decl);
>> +	  /* Ignore top-level qualifiers on the substituted type of this
>> +	     non-type template parameter, as per [temp.param]/6.  */
>> +	  t = cv_unqualified (t);
>> +	}
> 
> Err, shortly after posting I realized we could get top-level cv-quals not
> only after substitution, but also after deduction of a decltype(auto)
> NTTP (as in nontype-auto19.C below), so the call to cv_unqualified would
> need to happen after do_auto_deduction too, like so.  Testing in
> progress.
> 
> -- >8 --
> 
> Subject: [PATCH] c++: top-level cv-quals on type of NTTP [PR100893]
> 
> Here, we're rejecting the specialization of g<T,F> with T=A, F=&f in
> param4.C below due to a spurious constness mismatch between the type of
> the template argument &f and the substituted type of F (the substituted
> type has a top-level const).  Note that this mismatch doesn't occur with
> object pointers because in that case a call to
> perform_qualification_conversions from convert_nontype_argument
> implicitly adds a top-level const to the argument (via a cast) to match.
> 
> This however seems to be a manifestation of a more general conformance
> issue -- that we're not dropping top-level cv-quals after substituting
> into the type of an NTTP as per [temp.param]/6 (we only do so at parse
> time in process_template_parm).  So this patch makes
> convert_template_argument drop top-level cv-quals accordingly.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
> 
> 	PR c++/100893
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.c (convert_template_argument): Strip top-level cv-quals
> 	on the substituted type of a non-type template parameter.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/template/param4.C: New test.
> 	* g++.dg/template/param5.C: New test.
> 	* g++.dg/cpp1z/nontype-auto19.C: New test.
> 	* g++.dg/cpp2a/concepts-decltype.C: Don't expect that the
> 	deduced type of a decltype(auto) NTTP has top-level cv-quals.
> ---
>   gcc/cp/pt.c                                    |  4 ++++
>   gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C    |  8 ++++++++
>   gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C |  2 +-
>   gcc/testsuite/g++.dg/template/param4.C         | 10 ++++++++++
>   gcc/testsuite/g++.dg/template/param5.C         |  7 +++++++
>   5 files changed, 30 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
>   create mode 100644 gcc/testsuite/g++.dg/template/param4.C
>   create mode 100644 gcc/testsuite/g++.dg/template/param5.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 7211bdc5bbc..3cac073ed50 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -8496,6 +8496,10 @@ convert_template_argument (tree parm,
>         else
>   	t = tsubst (t, args, complain, in_decl);
>   
> +      /* Drop top-level cv-qualifiers on the substituted/deduced type of
> +	 this non-type template parameter, as per [temp.param]/6.  */
> +      t = cv_unqualified (t);

Might move this after...

>         if (invalid_nontype_parm_type_p (t, complain))
>   	return error_mark_node;

...this test.  OK either way.

> diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C b/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
> new file mode 100644
> index 00000000000..5b292a6a1e6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
> @@ -0,0 +1,8 @@
> +// Verify top-level cv-qualifiers are dropped from the deduced
> +// type of a non-type-template parameter, as per [temp.param]/6.
> +// { dg-do compile { target c++17 } }
> +
> +constexpr int x = 42;
> +template<decltype(auto) V> decltype(V)& f();
> +using type = decltype(f<x>());
> +using type = int&;
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
> index 13733c645f3..b375f74eb8a 100644
> --- a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
> @@ -61,7 +61,7 @@ constexpr int Z = 10;
>   
>   static_assert(deduced_as<0, int>);
>   static_assert(deduced_as<0, int&>); // { dg-error "invalid variable template" }
> -static_assert(deduced_as<Z, const int>);
> +static_assert(deduced_as<Z, int>);
>   static_assert(deduced_as<(Z), const int>); // { dg-error "invalid variable template" }
>   static_assert(deduced_as<(Z), const int&>);
>   
> diff --git a/gcc/testsuite/g++.dg/template/param4.C b/gcc/testsuite/g++.dg/template/param4.C
> new file mode 100644
> index 00000000000..8061ff72ba2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/param4.C
> @@ -0,0 +1,10 @@
> +// PR c++/100893
> +
> +template<class T, typename T::type F> void g() { }
> +
> +struct A { typedef void (*const type)(); };
> +void f();
> +template void g<A, &f>();
> +
> +struct B { typedef void (B::*const type)(); void f(); };
> +template void g<B, &B::f>();
> diff --git a/gcc/testsuite/g++.dg/template/param5.C b/gcc/testsuite/g++.dg/template/param5.C
> new file mode 100644
> index 00000000000..b8c92f4c217
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/param5.C
> @@ -0,0 +1,7 @@
> +// Verify top-level cv-qualifiers are dropped when determining the substituted
> +// type of a non-type-template parameter, as per [temp.param]/6.
> +// { dg-do compile { target c++11 } }
> +
> +template<class T, T V> decltype(V)& f();
> +using type = decltype(f<const volatile int, 0>());
> +using type = int&;
> 



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