Re: [PATCH] c++: return-type-req in constraint using only outer tparms [PR104527]

[Jason Merrill <jason@redhat.com>]

On 3/10/22 16:57, Patrick Palka wrote:
> 
> On Thu, 10 Mar 2022, Jason Merrill wrote:
> 
>> On 2/16/22 15:56, Patrick Palka wrote:
>>> On Tue, 15 Feb 2022, Jason Merrill wrote:
>>>
>>>> On 2/14/22 11:32, Patrick Palka wrote:
>>>>> Here the template context for the atomic constraint has two levels of
>>>>> template arguments, but since it depends only on the innermost argument
>>>>> T we use a single-level argument vector during substitution into the
>>>>> constraint (built by get_mapped_args).  We eventually pass this vector
>>>>> to do_auto_deduction as part of checking the return-type-requirement
>>>>> inside the atom, but do_auto_deduction expects outer_targs to be a full
>>>>> set of arguments for sake of satisfaction.
>>>>
>>>> Could we note the current number of levels in the map and use that in
>>>> get_mapped_args instead of the highest level parameter we happened to use?
>>>
>>> Ah yeah, that seems to work nicely.  IIUC it should suffice to remember
>>> whether the atomic constraint expression came from a concept definition.
>>> If it did, then the depth of the argument vector returned by
>>> get_mapped_args must be one, otherwise (as in the testcase) it must be
>>> the same as the template depth of the constrained entity, which is the
>>> depth of ARGS.
>>>
>>> How does the following look?  Bootstrapped and regtested on
>>> x86_64-pc-linux-gnu and also on cmcstl2 and range-v3.
>>>
>>> -- >8 --
>>>
>>> Subject: [PATCH] c++: return-type-req in constraint using only outer tparms
>>>    [PR104527]
>>>
>>> Here the template context for the atomic constraint has two levels of
>>> template parameters, but since it depends only on the innermost parameter
>>> T we use a single-level argument vector (built by get_mapped_args) during
>>> substitution into the atom.  We eventually pass this vector to
>>> do_auto_deduction as part of checking the return-type-requirement within
>>> the atom, but do_auto_deduction expects outer_targs to be a full set of
>>> arguments for sake of satisfaction.
>>>
>>> This patch fixes this by making get_mapped_args always return an
>>> argument vector whose depth corresponds to the template depth of the
>>> context in which the atomic constraint expression was written, instead
>>> of the highest parameter level that the expression happens to use.
>>>
>>> 	PR c++/104527
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* constraint.cc (normalize_atom): Set
>>> 	ATOMIC_CONSTR_EXPR_FROM_CONCEPT_P appropriately.
>>> 	(get_mapped_args):  Make static, adjust parameters.  Always
>>> 	return a vector whose depth corresponds to the template depth of
>>> 	the context of the atomic constraint expression.  Micro-optimize
>>> 	by passing false as exact to safe_grow_cleared and by collapsing
>>> 	a multi-level depth-one argument vector.
>>> 	(satisfy_atom): Adjust call to get_mapped_args and
>>> 	diagnose_atomic_constraint.
>>> 	(diagnose_atomic_constraint): Replace map parameter with an args
>>> 	parameter.
>>> 	* cp-tree.h (ATOMIC_CONSTR_EXPR_FROM_CONCEPT_P): Define.
>>> 	(get_mapped_args): Remove declaration.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp2a/concepts-return-req4.C: New test.
>>> ---
>>>    gcc/cp/constraint.cc                          | 64 +++++++++++--------
>>>    gcc/cp/cp-tree.h                              |  7 +-
>>>    .../g++.dg/cpp2a/concepts-return-req4.C       | 24 +++++++
>>>    3 files changed, 69 insertions(+), 26 deletions(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-return-req4.C
>>>
>>> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
>>> index 12db7e5cf14..306e28955c6 100644
>>> --- a/gcc/cp/constraint.cc
>>> +++ b/gcc/cp/constraint.cc
>>> @@ -764,6 +764,8 @@ normalize_atom (tree t, tree args, norm_info info)
>>>      tree ci = build_tree_list (t, info.context);
>>>        tree atom = build1 (ATOMIC_CONSTR, ci, map);
>>> +  if (info.in_decl && concept_definition_p (info.in_decl))
>>> +    ATOMIC_CONSTR_EXPR_FROM_CONCEPT_P (atom) = true;
>>
>> I'm a bit nervous about relying on in_decl, given that we support normalizing
>> when it isn't set; I don't remember the circumstances for that.  Maybe make
>> the flag indicate that ctx_parms had depth 1?
> 
> in_decl gets reliably updated by norm_info::update_context whenever we
> recurse inside a concept-id during normalization.  And I think the only
> other situation we have to worry about is when starting out with a
> concept-id, which is handled by normalize_concept_definition where we
> also set in_decl appropriately.
> 
> AFAICT, in_decl is not set (at the start) only when normalizing a
> placeholder type constraint or nested-requirement, and from some
> subsumption entrypoints.  And we shouldn't see an atom that belongs to a
> concept in these cases unless we recurse into a concept-id, in which
> case norm_info::update_context will update in_decl appropriately.
> 
> So IMHO it should be safe to rely on in_decl here to detect if the atom
> belongs to a concept, at least given the current entrypoints to
> subsumption/satisfaction..

Sounds good; please put a bit of that explanation in a comment where you 
set the flag.  OK with that change.

>>
>>>      if (!info.generate_diagnostics ())
>>>        {
>>>          /* Cache the ATOMIC_CONSTRs that we return, so that
>>> sat_hasher::equal
>>> @@ -2826,33 +2828,37 @@ satisfaction_value (tree t)
>>>        return boolean_true_node;
>>>    }
>>>    -/* Build a new template argument list with template arguments
>>> corresponding
>>> -   to the parameters used in an atomic constraint.  */
>>> +/* Build a new template argument vector according to the parameter
>>> +   mapping of the atomic constraint T, using arguments from ARGS.  */
>>>    -tree
>>> -get_mapped_args (tree map)
>>> +static tree
>>> +get_mapped_args (tree t, tree args)
>>>    {
>>> +  tree map = ATOMIC_CONSTR_MAP (t);
>>> +
>>>      /* No map, no arguments.  */
>>>      if (!map)
>>>        return NULL_TREE;
>>>    -  /* Find the mapped parameter with the highest level.  */
>>> -  int count = 0;
>>> -  for (tree p = map; p; p = TREE_CHAIN (p))
>>> -    {
>>> -      int level;
>>> -      int index;
>>> -      template_parm_level_and_index (TREE_VALUE (p), &level, &index);
>>> -      if (level > count)
>>> -        count = level;
>>> -    }
>>> +  /* Determine the depth of the resulting argument vector.  */
>>> +  int depth;
>>> +  if (ATOMIC_CONSTR_EXPR_FROM_CONCEPT_P (t))
>>> +    /* The expression of this atomic constraint comes from a concept
>>> definition,
>>> +       whose template depth is always one, so the resulting argument vector
>>> +       will also have depth one.  */
>>> +    depth = 1;
>>> +  else
>>> +    /* Otherwise, the expression of this atomic constraint was written in
>>> +       the context of the constrained entity, whose template depth is that
>>> +       of ARGS.  */
>>> +    depth = TMPL_ARGS_DEPTH (args);
>>>        /* Place each argument at its corresponding position in the argument
>>>         list. Note that the list will be sparse (not all arguments supplied),
>>>         but instantiation is guaranteed to only use the parameters in the
>>>         mapping, so null arguments would never be used.  */
>>> -  auto_vec< vec<tree> > lists (count);
>>> -  lists.quick_grow_cleared (count);
>>> +  auto_vec< vec<tree> > lists (depth);
>>> +  lists.quick_grow_cleared (depth);
>>>      for (tree p = map; p; p = TREE_CHAIN (p))
>>>        {
>>>          int level;
>>> @@ -2862,12 +2868,12 @@ get_mapped_args (tree map)
>>>          /* Insert the argument into its corresponding position.  */
>>>          vec<tree> &list = lists[level - 1];
>>>          if (index >= (int)list.length ())
>>> -	list.safe_grow_cleared (index + 1, true);
>>> +	list.safe_grow_cleared (index + 1, /*exact=*/false);
>>>          list[index] = TREE_PURPOSE (p);
>>>        }
>>>        /* Build the new argument list.  */
>>> -  tree args = make_tree_vec (lists.length ());
>>> +  args = make_tree_vec (lists.length ());
>>>      for (unsigned i = 0; i != lists.length (); ++i)
>>>        {
>>>          vec<tree> &list = lists[i];
>>> @@ -2879,6 +2885,16 @@ get_mapped_args (tree map)
>>>        }
>>>      SET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (args, 0);
>>>    +  if (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (args)
>>> +      && TMPL_ARGS_DEPTH (args) == 1)
>>> +    {
>>> +      /* Micro-optimization: represent a depth-one argument vector
>>> +	 using a single level.  */
>>> +      tree level = TMPL_ARGS_LEVEL (args, 1);
>>> +      ggc_free (args);
>>> +      args = level;
>>> +    }
>>> +
>>>      return args;
>>>    }
>>>    @@ -2933,7 +2949,7 @@ satisfy_atom (tree t, tree args, sat_info info)
>>>        }
>>>        /* Rebuild the argument vector from the parameter mapping.  */
>>> -  args = get_mapped_args (map);
>>> +  args = get_mapped_args (t, args);
>>>        /* Apply the parameter mapping (i.e., just substitute).  */
>>>      tree expr = ATOMIC_CONSTR_EXPR (t);
>>> @@ -2955,7 +2971,7 @@ satisfy_atom (tree t, tree args, sat_info info)
>>>      if (!same_type_p (TREE_TYPE (result), boolean_type_node))
>>>        {
>>>          if (info.noisy ())
>>> -	diagnose_atomic_constraint (t, map, result, info);
>>> +	diagnose_atomic_constraint (t, args, result, info);
>>>          return cache.save (inst_cache.save (error_mark_node));
>>>        }
>>>    @@ -2974,7 +2990,7 @@ satisfy_atom (tree t, tree args, sat_info info)
>>>        }
>>>      result = satisfaction_value (result);
>>>      if (result == boolean_false_node && info.diagnose_unsatisfaction_p ())
>>> -    diagnose_atomic_constraint (t, map, result, info);
>>> +    diagnose_atomic_constraint (t, args, result, info);
>>>        return cache.save (inst_cache.save (result));
>>>    }
>>> @@ -3642,11 +3658,10 @@ diagnose_trait_expr (tree expr, tree args)
>>>        }
>>>    }
>>>    -/* Diagnose a substitution failure in the atomic constraint T when
>>> applied
>>> -   with the instantiated parameter mapping MAP.  */
>>> +/* Diagnose a substitution failure in the atomic constraint T using ARGS.
>>> */
>>>      static void
>>> -diagnose_atomic_constraint (tree t, tree map, tree result, sat_info info)
>>> +diagnose_atomic_constraint (tree t, tree args, tree result, sat_info info)
>>>    {
>>>      /* If the constraint is already ill-formed, we've previously diagnosed
>>>         the reason. We should still say why the constraints aren't satisfied.
>>> */
>>> @@ -3667,7 +3682,6 @@ diagnose_atomic_constraint (tree t, tree map, tree
>>> result, sat_info info)
>>>      /* Generate better diagnostics for certain kinds of expressions.  */
>>>      tree expr = ATOMIC_CONSTR_EXPR (t);
>>>      STRIP_ANY_LOCATION_WRAPPER (expr);
>>> -  tree args = get_mapped_args (map);
>>>      switch (TREE_CODE (expr))
>>>        {
>>>        case TRAIT_EXPR:
>>> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
>>> index f253b32c3f2..dc2429a8406 100644
>>> --- a/gcc/cp/cp-tree.h
>>> +++ b/gcc/cp/cp-tree.h
>>> @@ -466,6 +466,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
>>>          IMPLICIT_CONV_EXPR_NONTYPE_ARG (in IMPLICIT_CONV_EXPR)
>>>          BASELINK_FUNCTIONS_MAYBE_INCOMPLETE_P (in BASELINK)
>>>          BIND_EXPR_VEC_DTOR (in BIND_EXPR)
>>> +      ATOMIC_CONSTR_EXPR_FROM_CONCEPT_P (in ATOMIC_CONSTR)
>>>       2: IDENTIFIER_KIND_BIT_2 (in IDENTIFIER_NODE)
>>>          ICS_THIS_FLAG (in _CONV)
>>>          DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (in VAR_DECL)
>>> @@ -1679,6 +1680,11 @@ check_constraint_info (tree t)
>>>    #define ATOMIC_CONSTR_MAP_INSTANTIATED_P(NODE) \
>>>      TREE_LANG_FLAG_0 (ATOMIC_CONSTR_CHECK (NODE))
>>>    +/* Whether the expression for this atomic constraint belongs to a
>>> +   concept definition.  */
>>> +#define ATOMIC_CONSTR_EXPR_FROM_CONCEPT_P(NODE) \
>>> +  TREE_LANG_FLAG_1 (ATOMIC_CONSTR_CHECK (NODE))
>>> +
>>>    /* The expression of an atomic constraint. */
>>>    #define ATOMIC_CONSTR_EXPR(NODE) \
>>>      CONSTR_EXPR (ATOMIC_CONSTR_CHECK (NODE))
>>> @@ -8306,7 +8312,6 @@ extern tree evaluate_requires_expr
>>> (tree);
>>>    extern tree tsubst_constraint                   (tree, tree,
>>> tsubst_flags_t, tree);
>>>    extern tree tsubst_constraint_info              (tree, tree,
>>> tsubst_flags_t, tree);
>>>    extern tree tsubst_parameter_mapping		(tree, tree,
>>> tsubst_flags_t, tree);
>>> -extern tree get_mapped_args			(tree);
>>>      struct processing_constraint_expression_sentinel
>>>    {
>>> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-return-req4.C
>>> b/gcc/testsuite/g++.dg/cpp2a/concepts-return-req4.C
>>> new file mode 100644
>>> index 00000000000..471946bc8eb
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-return-req4.C
>>> @@ -0,0 +1,24 @@
>>> +// PR c++/104527
>>> +// { dg-do compile { target c++20 } }
>>> +
>>> +template<class T, class U>
>>> +concept is_same = __is_same(T, U);
>>> +
>>> +template<class T>
>>> +struct A {
>>> +  template<class...>
>>> +    requires requires { { 0 } -> is_same<T>; }
>>> +  struct B {};
>>> +
>>> +  template<class...>
>>> +    requires requires { { 1 } -> is_same<T>; }
>>> +  static void f();
>>> +};
>>> +
>>> +A<int>::B<> a1;
>>> +A<bool>::B<> a2; // { dg-error "constraint" }
>>> +
>>> +int main() {
>>> +  A<int>::f();
>>> +  A<bool>::f(); // { dg-error "no match" }
>>> +}
>>
>>
>