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

[Jason Merrill via Gcc-patches]

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?

Also, don't you also need to handle specialization of partial 
instantiations?

    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>;