Re: [PATCH] c++: don't substitute TEMPLATE_PARM_CONSTRAINT [PR100374]

[Jason Merrill <jason@redhat.com>]

On 5/31/22 08:56, Patrick Palka wrote:
> On Sun, 29 May 2022, Jason Merrill wrote:
> 
>> On 5/29/22 22:10, Jason Merrill wrote:
>>> On 5/27/22 14:05, Patrick Palka wrote:
>>>> This makes us avoid substituting into the TEMPLATE_PARM_CONSTRAINT of
>>>> each template parameter except as necessary for (friend) declaration
>>>> matching, like we already do for the overall TEMPLATE_PARMS_CONSTRAINTS
>>>> of a template parameter list.
>>>>
>>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
>>>> trunk and perhaps 12.2?  Also tested on range-v3 and cmcstl2.
>>>
>>> Are there already tests that cover the friend cases?
> 
> Yes, by cpp2a/concepts-friend{2,3,7}.C I think.
> 
>>
>> Also, don't you also need to handle specialization of partial instantiations?
> 
> Hmm, do you have an example?  IIUC we call tsubst_friend_function and
> tsubst_friend_class only from instantiate_class_template_1, which always
> uses the most general template and full template argument set to
> instantiate any friend declarations.  So friend declarations are never
> partially instantiated I think.  (And IIUC non-friends are irrelevant
> here since we don't ever want to substitute their constraints outside of
> satisfaction.)

 From C++20 CA104:

   template <class T> struct A {
     template <class U> U f(U) requires C<typename T::type>;
     template <class U> U f(U) requires C<T>;
   };

   // Substitute int for T in above requirements to find match.
   template <> template <class U> U A<int>::f(U) requires C<int>  { }

>>>>      PR c++/100374
>>>>
>>>> gcc/cp/ChangeLog:
>>>>
>>>>      * pt.cc (tsubst_each_template_parm_constraint): Define.
>>>>      (tsubst_friend_function): Use it.
>>>>      (tsubst_friend_class): Use it.
>>>>      (tsubst_template_parm): Don't substitute TEMPLATE_PARM_CONSTRAINT.
>>>>
>>>> gcc/testsuite/ChangeLog:
>>>>
>>>>      * g++.dg/cpp2a/concepts-template-parm11.C: New test.
>>>> ---
>>>>    gcc/cp/pt.cc                                  | 35 ++++++++++++++++---
>>>>    .../g++.dg/cpp2a/concepts-template-parm11.C   | 16 +++++++++
>>>>    2 files changed, 47 insertions(+), 4 deletions(-)
>>>>    create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-template-parm11.C
>>>>
>>>> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
>>>> index 24bbe2f4060..ec168234325 100644
>>>> --- a/gcc/cp/pt.cc
>>>> +++ b/gcc/cp/pt.cc
>>>> @@ -184,6 +184,7 @@ static int unify_pack_expansion (tree, tree, tree,
>>>>                     tree, unification_kind_t, bool, bool);
>>>>    static tree copy_template_args (tree);
>>>>    static tree tsubst_template_parms (tree, tree, tsubst_flags_t);
>>>> +static void tsubst_each_template_parm_constraint (tree, tree,
>>>> tsubst_flags_t);
>>>>    tree most_specialized_partial_spec (tree, tsubst_flags_t);
>>>>    static tree tsubst_aggr_type (tree, tree, tsubst_flags_t, tree, int);
>>>>    static tree tsubst_arg_types (tree, tree, tree, tsubst_flags_t, tree);
>>>> @@ -11254,7 +11255,12 @@ tsubst_friend_function (tree decl, tree args)
>>>>          tree parms = DECL_TEMPLATE_PARMS (new_friend);
>>>>          tree treqs = TEMPLATE_PARMS_CONSTRAINTS (parms);
>>>>          treqs = maybe_substitute_reqs_for (treqs, new_friend);
>>>> -      TEMPLATE_PARMS_CONSTRAINTS (parms) = treqs;
>>>> +      if (treqs != TEMPLATE_PARMS_CONSTRAINTS (parms))
>>>> +    {
>>>> +      TEMPLATE_PARMS_CONSTRAINTS (parms) = treqs;
>>>> +      /* As well as each TEMPLATE_PARM_CONSTRAINT.  */
>>>> +      tsubst_each_template_parm_constraint (parms, args,
>>>> tf_warning_or_error);
>>>> +    }
>>>>        }
>>>>      /* The mangled name for the NEW_FRIEND is incorrect.  The function
>>>> @@ -11500,6 +11506,8 @@ tsubst_friend_class (tree friend_tmpl, tree args)
>>>>        {
>>>>          tree parms = tsubst_template_parms (DECL_TEMPLATE_PARMS
>>>> (friend_tmpl),
>>>>                              args, tf_warning_or_error);
>>>> +      tsubst_each_template_parm_constraint (parms, args,
>>>> +                        tf_warning_or_error);
>>>>              location_t saved_input_location = input_location;
>>>>              input_location = DECL_SOURCE_LOCATION (friend_tmpl);
>>>>              tree cons = get_constraints (tmpl);
>>>> @@ -11534,6 +11542,8 @@ tsubst_friend_class (tree friend_tmpl, tree args)
>>>>                           DECL_FRIEND_CONTEXT (friend_tmpl));
>>>>              --processing_template_decl;
>>>>              set_constraints (tmpl, ci);
>>>> +          tsubst_each_template_parm_constraint (DECL_TEMPLATE_PARMS
>>>> (tmpl),
>>>> +                            args, tf_warning_or_error);
>>>>            }
>>>>          /* Inject this template into the enclosing namspace scope.  */
>>>> @@ -13656,7 +13666,6 @@ tsubst_template_parm (tree t, tree args,
>>>> tsubst_flags_t complain)
>>>>      default_value = TREE_PURPOSE (t);
>>>>      parm_decl = TREE_VALUE (t);
>>>> -  tree constraint = TEMPLATE_PARM_CONSTRAINTS (t);
>>>>      parm_decl = tsubst (parm_decl, args, complain, NULL_TREE);
>>>>      if (TREE_CODE (parm_decl) == PARM_DECL
>>>> @@ -13664,13 +13673,31 @@ tsubst_template_parm (tree t, tree args,
>>>> tsubst_flags_t complain)
>>>>        parm_decl = error_mark_node;
>>>>      default_value = tsubst_template_arg (default_value, args,
>>>>                           complain, NULL_TREE);
>>>> -  constraint = tsubst_constraint (constraint, args, complain, NULL_TREE);
>>>>      tree r = build_tree_list (default_value, parm_decl);
>>>> -  TEMPLATE_PARM_CONSTRAINTS (r) = constraint;
>>>> +  TEMPLATE_PARM_CONSTRAINTS (r) = TEMPLATE_PARM_CONSTRAINTS (t);
>>>>      return r;
>>>>    }
>>>> +/* Substitute in-place the TEMPLATE_PARM_CONSTRAINT of each template
>>>> +   parameter in PARMS for sake of declaration matching.  */
>>>> +
>>>> +static void
>>>> +tsubst_each_template_parm_constraint (tree parms, tree args,
>>>> +                      tsubst_flags_t complain)
>>>> +{
>>>> +  ++processing_template_decl;
>>>> +  for (; parms; parms = TREE_CHAIN (parms))
>>>> +    {
>>>> +      tree level = TREE_VALUE (parms);
>>>> +      for (tree parm : tree_vec_range (level))
>>>> +    TEMPLATE_PARM_CONSTRAINTS (parm)
>>>> +      = tsubst_constraint (TEMPLATE_PARM_CONSTRAINTS (parm), args,
>>>> +                   complain, NULL_TREE);
>>>> +    }
>>>> +  --processing_template_decl;
>>>> +}
>>>> +
>>>>    /* Substitute the ARGS into the indicated aggregate (or enumeration)
>>>>       type T.  If T is not an aggregate or enumeration type, it is
>>>>       handled as if by tsubst.  IN_DECL is as for tsubst.  If
>>>> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-template-parm11.C
>>>> b/gcc/testsuite/g++.dg/cpp2a/concepts-template-parm11.C
>>>> new file mode 100644
>>>> index 00000000000..f70de341a07
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-template-parm11.C
>>>> @@ -0,0 +1,16 @@
>>>> +// PR c++/100374
>>>> +// { dg-do compile { target c++20 } }
>>>> +
>>>> +template<class T, class U>
>>>> +concept C = true;
>>>> +
>>>> +template<class T>
>>>> +struct A {
>>>> +  template<C<typename T::value_type> U>
>>>> +  void f();
>>>> +
>>>> +  template<C<typename T::value_type> U>
>>>> +  struct B;
>>>> +};
>>>> +
>>>> +template struct A<float>;
>>>
>>