Re: [PATCH] c++: CTAD within alias template [PR91911]

[Jason Merrill <jason@redhat.com>]

On 6/30/21 11:58 AM, Patrick Palka wrote:
> On Wed, 30 Jun 2021, Patrick Palka wrote:
> 
>> On Fri, 25 Jun 2021, Jason Merrill wrote:
>>
>>> On 6/25/21 1:11 PM, Patrick Palka wrote:
>>>> On Fri, 25 Jun 2021, Jason Merrill wrote:
>>>>
>>>>> On 6/24/21 4:45 PM, Patrick Palka wrote:
>>>>>> In the first testcase below, during parsing of the alias template
>>>>>> ConstSpanType, transparency of alias template specializations means we
>>>>>> replace SpanType<T> with SpanType's substituted definition.  But this
>>>>>> substitution lowers the level of the CTAD placeholder for span(T()) from
>>>>>> 2 to 1, and so the later instantiantion of ConstSpanType<int>
>>>>>> erroneously substitutes this CTAD placeholder with the template argument
>>>>>> at level 1 index 0, i.e. with int, before we get a chance to perform the
>>>>>> CTAD.
>>>>>>
>>>>>> In light of this, it seems we should avoid level lowering when
>>>>>> substituting through through the type-id of a dependent alias template
>>>>>> specialization.  To that end this patch makes lookup_template_class_1
>>>>>> pass tf_partial to tsubst in this situation.
>>>>>
>>>>> This makes sense, but what happens if SpanType is a member template, so
>>>>> that
>>>>> the levels of it and ConstSpanType don't match?  Or the other way around?
>>>>
>>>> If SpanType<T> is a member template of say the class template A<U> (and
>>>> thus its level is greater than ConstSpanType):
>>>>
>>>>     template<class U>
>>>>     struct A {
>>>>       template<class T>
>>>>       using SpanType = decltype(span(T()));
>>>>     };
>>>>
>>>>     template<class T>
>>>>     using ConstSpanType = span<const typename
>>>> A<int>::SpanType<T>::value_type>;
>>>>
>>>>     using type = ConstSpanType<int>;
>>>>
>>>> then this case luckily works even without the patch because
>>>> instantiate_class_template now reuses the specialization A<int>::SpanType<T>
>>>> that was formed earlier during instantiation of A<int>, where we
>>>> substitute only a single level of template arguments, so the level of
>>>> the CTAD placeholder inside the defining-type-id of this specialization
>>>> dropped from 3 to 2, so still more than the level of ConstSpanType.
>>>>
>>>> This luck is short-lived though, because if we replace
>>>> A<int>::SpanType<T> with say A<int>::SpanType<const T> then the testcase
>>>> breaks again (without the patch) because we no longer can reuse that
>>>> specialization, so we instead form it on the spot by substituting two
>>>> levels of template arguments (U=int,T=T) into the defining-type-id,
>>>> causing the level of the placeholder to drop to 1.  I think the patch
>>>> causes its level to remain 3 (though I guess it should really be 2).
>>>>
>>>>
>>>> For the other way around, if ConstSpanType<T> is a member template of
>>>> say the class template B<V> (and thus its level is greater than
>>>> SpanType):
>>>>
>>>>     template<class T>
>>>>     using SpanType = decltype(span(T()));
>>>>
>>>>     template<class V>
>>>>     struct B {
>>>>       template<class T>
>>>>       using ConstSpanType = span<const typename SpanType<T>::value_type>;
>>>>     };
>>>>
>>>>     using type = B<char>::ConstSpanType<int>;
>>>>
>>>> then tf_partial doesn't help here at all; we end up substituting 'int'
>>>> for the CTAD placeholder...  What it seems we need is to _increase_ the
>>>> level of the CTAD placeholder from 2 to 3 during the dependent
>>>> substitution..
>>>>
>>>> Hmm, rather than messing with tf_partial, which is apparently only a
>>>> partial solution, maybe we should just make tsubst never substitute a
>>>> CTAD placeholder -- they should always be resolved from do_class_deduction,
>>>> and their level doesn't really matter otherwise.  (But we'd still want
>>>> to substitute into the CLASS_PLACEHOLDER_TEMPLATE of the placeholder in
>>>> case it's a template template parm.)  Something like:
>>>>
>>>> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
>>>> index 5107bfbf9d1..dead651ed84 100644
>>>> --- a/gcc/cp/pt.c
>>>> +++ b/gcc/cp/pt.c
>>>> @@ -15552,7 +15550,8 @@ tsubst (tree t, tree args, tsubst_flags_t complain,
>>>> tree in_decl)
>>>>      	levels = TMPL_ARGS_DEPTH (args);
>>>>    	if (level <= levels
>>>> -	    && TREE_VEC_LENGTH (TMPL_ARGS_LEVEL (args, level)) > 0)
>>>> +	    && TREE_VEC_LENGTH (TMPL_ARGS_LEVEL (args, level)) > 0
>>>> +	    && !template_placeholder_p (t))
>>>>    	  {
>>>>    	    arg = TMPL_ARG (args, level, idx);
>>>>    
>>>> seems to work better.
>>>
>>> Makes sense.
>>
>> Here's a patch that implements that.  I reckon it's good to have both
>> workarounds in place because the tf_partial workaround is necessary to
>> accept class-deduction93a.C below, and the tsubst workaround is
>> necessary to accept class-deduction-92b.C below.
> 
> Whoops, forgot to git-add class-deduction93a.C:
> 
> -- >8 --
> 
> Subject: [PATCH] c++: CTAD within alias template [PR91911]
> 
> In the first testcase below, during parsing of the alias template
> ConstSpanType, transparency of alias template specializations means we
> replace SpanType<T> with SpanType's substituted definition.  But this
> substitution lowers the level of the CTAD placeholder for span{T()} from
> 2 to 1, and so the later instantiation of ConstSpanType<int> erroneously
> substitutes this CTAD placeholder with the template argument at level 1
> index 0, i.e. with int, before we get a chance to perform the CTAD.
> 
> In light of this, it seems we should avoid level lowering when
> substituting through the type-id of a dependent alias template
> specialization.  To that end this patch makes lookup_template_class_1
> pass tf_partial to tsubst in this situation.
> 
> Unfortunately, using tf_partial alone isn't sufficient because the
> template context in which we perform the dependent substitution may
> have more levels than the substituted alias template and so we
> end up substituting the CTAD placeholder anyway, as in
> class-deduction92b.C below.  (There, it seems we'd need to _increase_
> the level of the placeholder for span{T()} from 2 to 3 during the
> dependent substitution.)  Since we never want to resolve a CTAD
> placeholder outside of CTAD proper, this patch takes the relatively
> ad-hoc approach of making tsubst explicitly avoid doing so.
> 
> This tsubst workaround doesn't obviate the tf_partial workaround because
> it's still desirable to avoid prematurely level lowering a CTAD placeholder:
> it's less work for the compiler, and it gives us a chance to substitute
> a template placeholder that's a template template parameter with a
> concrete template template argument, as in the last testcase below.

Hmm, what if we combine the template template parameter with the level 
mismatch?

> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
> 
> 	PR c++/91911
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.c (lookup_template_class_1): When looking up a dependent
> 	alias template specialization, pass tf_partial to tsubst.
> 	(tsubst) <case TEMPLATE_TYPE_PARM>: Avoid substituting a CTAD
> 	placeholder.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1z/class-deduction92.C: New test.
> 	* g++.dg/cpp1z/class-deduction92a.C: New test.
> 	* g++.dg/cpp1z/class-deduction92b.C: New test.
> 	* g++.dg/cpp1z/class-deduction93.C: New test.
> 	* g++.dg/cpp1z/class-deduction93a.C: New test.
> ---
>   gcc/cp/pt.c                                   | 15 +++++++++--
>   .../g++.dg/cpp1z/class-deduction92.C          | 17 ++++++++++++
>   .../g++.dg/cpp1z/class-deduction92a.C         | 22 +++++++++++++++
>   .../g++.dg/cpp1z/class-deduction92b.C         | 22 +++++++++++++++
>   .../g++.dg/cpp1z/class-deduction93.C          | 25 +++++++++++++++++
>   .../g++.dg/cpp1z/class-deduction93a.C         | 27 +++++++++++++++++++
>   6 files changed, 126 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction92.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction92a.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction92b.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction93.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction93a.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index f2039e09cd7..db769d59951 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -9954,7 +9954,12 @@ lookup_template_class_1 (tree d1, tree arglist, tree in_decl, tree context,
>   		template-arguments for the template-parameters in the
>   		type-id of the alias template.  */
>   
> -	  t = tsubst (TREE_TYPE (gen_tmpl), arglist, complain, in_decl);
> +	  /* When substituting a dependent alias template specialization,
> +	     we pass tf_partial to avoid lowering the level of any 'auto's
> +	     within its type-id (91911).  */
> +	  t = tsubst (TREE_TYPE (gen_tmpl), arglist,
> +		      complain | (tf_partial * is_dependent_type),
> +		      in_decl);
>   	  /* Note that the call above (by indirectly calling
>   	     register_specialization in tsubst_decl) registers the
>   	     TYPE_DECL representing the specialization of the alias
> @@ -15544,9 +15549,15 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>   	gcc_assert (TREE_VEC_LENGTH (args) > 0);
>   	template_parm_level_and_index (t, &level, &idx);
>   
> +	/* Retrieve the argument for this template parameter.  */
>   	levels = TMPL_ARGS_DEPTH (args);
>   	if (level <= levels
> -	    && TREE_VEC_LENGTH (TMPL_ARGS_LEVEL (args, level)) > 0)
> +	    && TREE_VEC_LENGTH (TMPL_ARGS_LEVEL (args, level)) > 0
> +	    /* Avoid substituting CTAD placeholders; they get
> +	       resolved only during CTAD proper.  We can get here
> +	       after dependent substitution of an alias template
> +	       whose defining-type-id uses CTAD (91911).  */
> +	    && !template_placeholder_p (t))
>   	  {
>   	    arg = TMPL_ARG (args, level, idx);
>   
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C
> new file mode 100644
> index 00000000000..379eb960da6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C
> @@ -0,0 +1,17 @@
> +// PR c++/91911
> +// { dg-do compile { target c++17 } }
> +
> +template<class T>
> +struct span {
> +  using value_type = T;
> +  span(T);
> +};
> +
> +template<class T>
> +using SpanType = decltype(span{T()});
> +
> +template<class T>
> +using ConstSpanType = span<const typename SpanType<T>::value_type>;
> +
> +using type = ConstSpanType<int>;
> +using type = span<const int>;
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction92a.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction92a.C
> new file mode 100644
> index 00000000000..b9aa8f3bbf0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction92a.C
> @@ -0,0 +1,22 @@
> +// PR c++/91911
> +// { dg-do compile { target c++17 } }
> +// A variant of class-deduction92.C where SpanType has more levels than
> +// ConstSpanType.
> +
> +template<class T>
> +struct span {
> +  using value_type = T;
> +  span(T);
> +};
> +
> +template<class>
> +struct A {
> +  template<class T>
> +  using SpanType = decltype(span{T()});
> +};
> +
> +template<class T>
> +using ConstSpanType = span<const typename A<int>::SpanType<const T>::value_type>;
> +
> +using type = ConstSpanType<int>;
> +using type = span<const int>;
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction92b.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction92b.C
> new file mode 100644
> index 00000000000..0ea0cef0238
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction92b.C
> @@ -0,0 +1,22 @@
> +// PR c++/91911
> +// { dg-do compile { target c++17 } }
> +// A variant of class-deduction92.C where SpanType has fewer levels than
> +// ConstSpanType.
> +
> +template<class T>
> +struct span {
> +  using value_type = T;
> +  span(T);
> +};
> +
> +template<class T>
> +using SpanType = decltype(span{T()});
> +
> +template<class>
> +struct B {
> +  template<class T>
> +  using ConstSpanType = span<const typename SpanType<T>::value_type>;
> +};
> +
> +using type = B<int>::ConstSpanType<int>;
> +using type = span<const int>;
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C
> new file mode 100644
> index 00000000000..20504780d32
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C
> @@ -0,0 +1,25 @@
> +// PR c++/98077
> +// { dg-do compile { target c++17 } }
> +
> +template<class R>
> +struct function {
> +  template<class T> function(T);
> +  using type = R;
> +};
> +
> +template<class T> function(T) -> function<decltype(T()())>;
> +
> +template<class T>
> +struct CallableTrait;
> +
> +template<class R>
> +struct CallableTrait<function<R>> { using ReturnType = R; };
> +
> +template<class F>
> +using CallableTraitT = CallableTrait<decltype(function{F()})>;
> +
> +template<class F>
> +using ReturnType = typename CallableTraitT<F>::ReturnType;
> +
> +using type = ReturnType<int(*)()>;
> +using type = int;
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction93a.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction93a.C
> new file mode 100644
> index 00000000000..7656e7845fe
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction93a.C
> @@ -0,0 +1,27 @@
> +// PR c++/98077
> +// { dg-do compile { target c++17 } }
> +// A variant of class-deduction93.C where the template placeholder is a template
> +// template parameter.
> +
> +template<class R>
> +struct function {
> +  template<class T> function(T);
> +  using type = R;
> +};
> +
> +template<class T> function(T) -> function<decltype(T()())>;
> +
> +template<class T>
> +struct CallableTrait;
> +
> +template<class R>
> +struct CallableTrait<function<R>> { using ReturnType = R; };
> +
> +template<class F, template<class> class Tmpl>
> +using CallableTraitT = CallableTrait<decltype(Tmpl{F()})>;
> +
> +template<class F, template<class> class Tmpl>
> +using ReturnType = typename CallableTraitT<F, Tmpl>::ReturnType;
> +
> +using type = ReturnType<int(*)(), function>;
> +using type = int;
>