Re: [PATCH] c++: variable template and targ deduction [PR108550]

[Jason Merrill <jason@redhat.com>]

On 2/23/23 10:54, Marek Polacek wrote:
> On Thu, Feb 23, 2023 at 10:17:22AM -0500, Patrick Palka wrote:
>> On Wed, 22 Feb 2023, Marek Polacek wrote:
>>
>>> In this test, we get a bogus error because we failed to deduce the auto in
>>> constexpr auto is_pointer_v = is_pointer<Tp>::value;
>>> to bool.  Then ensure_literal_type_for_constexpr_object thinks the object
>>> isn't literal and an error is reported.
>>>
>>> This is another case of the interaction between tf_partial and 'auto',
>>> where the auto was not reduced so the deduction failed.  In more detail:
>>> we have
>>>
>>>    Wrap1<int>()
>>>
>>> in the code and we need to perform OR -> fn_type_unification.  The targ
>>> list is incomplete, so we do
>>>        tsubst_flags_t ecomplain = complain | tf_partial | tf_fndecl_type;
>>>        fntype = tsubst (TREE_TYPE (fn), explicit_targs, ecomplain, NULL_TREE);
>>> where TREE_TYPE (fn) is struct integral_constant <T402> (void).  Then
>>> we substitute the return type, which results in tsubsting is_pointer_v<int>.
>>> is_pointer_v is a variable template with a placeholder type:
>>>
>>>    template <class Tp>
>>>    constexpr auto is_pointer_v = is_pointer<Tp>::value;
>>>
>>> so we find ourselves in lookup_and_finish_template_variable.  tf_partial is
>>> still set, so finish_template_variable -> instantiate_template -> tsubst
>>> won't reduce the level of auto.  But then we do mark_used which eventually
>>> calls do_auto_deduction which clears tf_partial, because we want to replace
>>> the auto now.  But we hadn't reduced auto's level so this fails.  And
>>> since we're not in an immediate context, we emit a hard error.
>>>
>>> I suppose that when we reach lookup_and_finish_template_variable it's
>>> probably time to clear tf_partial.  (I added an assert and our testsuite
>>> doesn't have a test whereby we get to lookup_and_finish_template_variable
>>> while tf_partial is still active.)
>>>
>>> Does this make *any* sense to you?
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> 	PR c++/108550
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* pt.cc (lookup_and_finish_template_variable): Clear tf_partial.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp1y/var-templ70.C: New test.
>>> 	* g++.dg/cpp1y/var-templ71.C: New test.
>>> ---
>>>   gcc/cp/pt.cc                             |  6 ++++++
>>>   gcc/testsuite/g++.dg/cpp1y/var-templ70.C | 25 +++++++++++++++++++++++
>>>   gcc/testsuite/g++.dg/cpp1y/var-templ71.C | 26 ++++++++++++++++++++++++
>>>   3 files changed, 57 insertions(+)
>>>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ70.C
>>>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ71.C
>>>
>>> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
>>> index 1a071e95004..f636bac5413 100644
>>> --- a/gcc/cp/pt.cc
>>> +++ b/gcc/cp/pt.cc
>>> @@ -10355,6 +10355,12 @@ lookup_and_finish_template_variable (tree templ, tree targs,
>>>     if (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (templ)) == 1
>>>         && !any_dependent_template_arguments_p (targs))
>>>       {
>>> +      /* We may be called while doing a partial substitution, but the
>>> +	 type of the variable template may be auto, in which case we
>>> +	 will call do_auto_deduction in mark_used (which clears tf_partial)
>>> +	 and the auto must be properly reduced at that time for the
>>> +	 deduction to work.  */
>>> +      complain &= ~tf_partial;
>>
>> LGTM.  I can't think of a reason to keep tf_partial set at this point
>> since we know there's only a single level of template arguments left to
>> substitute and the args we have are non-dependent, so the substitution
>> is in no way partial.
> 
> Right, once we get to finish_template_variable I suppose we're really
> committed to the var tmpl.  So the fix should be safe, albeit it feels
> a bit ad hoc.  FWIW, I'd tested adding alias templates to the mix and
> it still worked.
>   
>> Though I wonder if ideally we should clear tf_partial more generally
>> from instantiate_template?  Modulo diagnostics, the result of
>> instantiate_template shouldn't depend on complain, in particular because
>> we cache the result in the specializations table is which keyed off of
>> only the given tmpl and args.  Not sure if that'd be a suitable change
>> for stage4/backports though...
> 
> This makes sense; I guess anytime you have the full set of targs and are
> going to instantiate, tf_partial should be cleared otherwise it can
> wreak havoc.  I'd be nervous about applying such a patch now but maybe
> we could experiment with that in GCC 14.

This sounds right to me; it would never make sense to have tf_partial 
set in instantiate_template, so we should be able to clear it there.  We 
might even do

  complain = complain & tf_warning_or_error

since none of the other flags are relevant at that level either.

But this patch is OK for GCC 13.

> Thanks for taking a look!
>   
>>>         var = finish_template_variable (var, complain);
>>>         mark_used (var);
>>>       }
>>> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ70.C b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
>>> new file mode 100644
>>> index 00000000000..1d35c38c7cc
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
>>> @@ -0,0 +1,25 @@
>>> +// PR c++/108550
>>> +// { dg-do compile { target c++14 } }
>>> +
>>> +template<class T>
>>> +struct is_pointer
>>> +{
>>> +  static constexpr bool value = false;
>>> +};
>>> +
>>> +template<class T, T T1>
>>> +struct integral_constant
>>> +{
>>> +  static constexpr T value = T1;
>>> +};
>>> +
>>> +
>>> +template <class Tp>
>>> +constexpr auto is_pointer_v = is_pointer<Tp>::value;
>>> +
>>> +template <class Tp, int = 0>
>>> +integral_constant<bool, is_pointer_v<int>> Wrap1();
>>
>> It might be good to have a version of this testcase that uses a
>> dependent is_pointer_v<Tp> here instead of is_pointer_v<int>.
> 
> True, added (and I threw an alias template in the mix too).
> 
> -- >8 --
> In this test, we get a bogus error because we failed to deduce the auto in
> constexpr auto is_pointer_v = is_pointer<Tp>::value;
> to bool.  Then ensure_literal_type_for_constexpr_object thinks the object
> isn't literal and an error is reported.
> 
> This is another case of the interaction between tf_partial and 'auto',
> where the auto was not reduced so the deduction failed.  In more detail:
> we have
> 
>    Wrap1<int>()
> 
> in the code and we need to perform OR -> fn_type_unification.  The targ
> list is incomplete, so we do
>        tsubst_flags_t ecomplain = complain | tf_partial | tf_fndecl_type;
>        fntype = tsubst (TREE_TYPE (fn), explicit_targs, ecomplain, NULL_TREE);
> where TREE_TYPE (fn) is struct integral_constant <T402> (void).  Then
> we substitute the return type, which results in tsubsting is_pointer_v<int>.
> is_pointer_v is a variable template with a placeholder type:
> 
>    template <class Tp>
>    constexpr auto is_pointer_v = is_pointer<Tp>::value;
> 
> so we find ourselves in lookup_and_finish_template_variable.  tf_partial is
> still set, so finish_template_variable -> instantiate_template -> tsubst
> won't reduce the level of auto.  But then we do mark_used which eventually
> calls do_auto_deduction which clears tf_partial, because we want to replace
> the auto now.  But we hadn't reduced auto's level so this fails.  And
> since we're not in an immediate context, we emit a hard error.
> 
> I suppose that when we reach lookup_and_finish_template_variable it's
> probably time to clear tf_partial.  (I added an assert and our testsuite
> doesn't have a test whereby we get to lookup_and_finish_template_variable
> while tf_partial is still active.)
> 
> 	PR c++/108550
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (lookup_and_finish_template_variable): Clear tf_partial.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/var-templ70.C: New test.
> 	* g++.dg/cpp1y/var-templ71.C: New test.
> 	* g++.dg/cpp1y/var-templ72.C: New test.
> ---
>   gcc/cp/pt.cc                             |  6 ++++++
>   gcc/testsuite/g++.dg/cpp1y/var-templ70.C | 25 ++++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp1y/var-templ71.C | 26 +++++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp1y/var-templ72.C | 27 ++++++++++++++++++++++++
>   4 files changed, 84 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ70.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ71.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ72.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 1a071e95004..f636bac5413 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -10355,6 +10355,12 @@ lookup_and_finish_template_variable (tree templ, tree targs,
>     if (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (templ)) == 1
>         && !any_dependent_template_arguments_p (targs))
>       {
> +      /* We may be called while doing a partial substitution, but the
> +	 type of the variable template may be auto, in which case we
> +	 will call do_auto_deduction in mark_used (which clears tf_partial)
> +	 and the auto must be properly reduced at that time for the
> +	 deduction to work.  */
> +      complain &= ~tf_partial;
>         var = finish_template_variable (var, complain);
>         mark_used (var);
>       }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ70.C b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
> new file mode 100644
> index 00000000000..1d35c38c7cc
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
> @@ -0,0 +1,25 @@
> +// PR c++/108550
> +// { dg-do compile { target c++14 } }
> +
> +template<class T>
> +struct is_pointer
> +{
> +  static constexpr bool value = false;
> +};
> +
> +template<class T, T T1>
> +struct integral_constant
> +{
> +  static constexpr T value = T1;
> +};
> +
> +
> +template <class Tp>
> +constexpr auto is_pointer_v = is_pointer<Tp>::value;
> +
> +template <class Tp, int = 0>
> +integral_constant<bool, is_pointer_v<int>> Wrap1();
> +
> +int main() {
> +  static_assert(!decltype(Wrap1<int>())::value, "");
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ71.C b/gcc/testsuite/g++.dg/cpp1y/var-templ71.C
> new file mode 100644
> index 00000000000..e0cf55230d9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ71.C
> @@ -0,0 +1,26 @@
> +// PR c++/108550
> +// { dg-do compile { target c++14 } }
> +
> +template<class T, T T1>
> +struct integral_constant
> +{
> +  static constexpr T value = T1;
> +};
> +
> +template <typename T>
> +struct S {
> +  template <typename U, typename V>
> +  static constexpr void foo(V) { }
> +
> +  constexpr bool bar () const { foo<int>(10); return false; }
> +};
> +
> +template <class Tp>
> +constexpr auto is_pointer_v = S<Tp>{}.bar();
> +
> +template <class Tp, int = 0>
> +integral_constant<bool, is_pointer_v<int>> Wrap1();
> +
> +int main() {
> +  static_assert(!decltype(Wrap1<int>())::value, "");
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ72.C b/gcc/testsuite/g++.dg/cpp1y/var-templ72.C
> new file mode 100644
> index 00000000000..7426bad4a6c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ72.C
> @@ -0,0 +1,27 @@
> +// PR c++/108550
> +// { dg-do compile { target c++14 } }
> +
> +template<class T>
> +struct is_pointer
> +{
> +  static constexpr bool value = false;
> +};
> +
> +template<class T, T T1>
> +struct integral_constant
> +{
> +  static constexpr T value = T1;
> +};
> +
> +template<typename T>
> +using PTR_P = is_pointer<T>;
> +
> +template <class Tp>
> +constexpr auto is_pointer_v = PTR_P<Tp>::value;
> +
> +template <class Tp, int = 0>
> +integral_constant<bool, is_pointer_v<Tp>> Wrap1();
> +
> +int main() {
> +  static_assert(!decltype(Wrap1<int>())::value, "");
> +}
> 
> base-commit: 426b0ae103894d1f1bd82e5f049ff8a53bd82a8d