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

[Patrick Palka <ppalka@redhat.com>]

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..

> 
> >     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" }
> > +}
> 
>