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

[Jason Merrill via Gcc-patches]

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>;