c++: constrained class template friend [PR93467]

This fixes two issues in our handling of constrained class template
friend declarations.

The first issue is that we fail to set the constraints on the injected
class template declaration during tsubst_friend_class.

The second issue is that the template parameter levels within the parsed
constraints of a class template friend declaration are shifted if the
enclosing class is a template, and this shift leads to spurious
constraint mismatch errors in associate_classtype_constraints if the
friend declaration refers to an already declared class template.

gcc/cp/ChangeLog:

	PR c++/93467
	* constraint.cc (associate_classtype_constraints): If there is a
	discrepancy between the current template depth and the template
	depth of the original declaration, then adjust the template
	parameter depth within the current constraints appropriately.
	* pt.c (tsubst_friend_class): Substitute into and set the
	constraints on the injected declaration.

gcc/testsuite/ChangeLog:

	PR c++/93467
	* g++.dg/cpp2a/concepts-friend6.C: New test.
	* g++.dg/cpp2a/concepts-friend7.C: New test.

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 92ff283013eca1e394c10ff40e2d23a5deccd8f0..d0da2300ba99e2825db59fd5acbdc43bf6aa9cf3 100644 (file)
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1075,6 +1075,19 @@ associate_classtype_constraints (tree type)
         original declaration.  */
       if (tree orig_ci = get_constraints (decl))
         {
+         if (int extra_levels = (TMPL_PARMS_DEPTH (current_template_parms)
+                                 - TMPL_ARGS_DEPTH (TYPE_TI_ARGS (type))))
+           {
+             /* If there is a discrepancy between the current template depth
+                and the template depth of the original declaration, then we
+                must be redeclaring a class template as part of a friend
+                declaration within another class template.  Before matching
+                constraints, we need to reduce the template parameter level
+                within the current constraints via substitution.  */
+             tree outer_gtargs = template_parms_to_args (current_template_parms);
+             TREE_VEC_LENGTH (outer_gtargs) = extra_levels;
+             ci = tsubst_constraint_info (ci, outer_gtargs, tf_none, NULL_TREE);
+           }
           if (!equivalent_constraints (ci, orig_ci))
             {
              error ("%qT does not match original declaration", type);

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index c07a48f1261b60c8c21960cb305358560f262846..cdf6a3eeaf33f05526f5aa618dbfec439dd7377e 100644 (file)
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -11213,6 +11213,16 @@ tsubst_friend_class (tree friend_tmpl, tree args)
          DECL_ANTICIPATED (tmpl)
            = DECL_ANTICIPATED (DECL_TEMPLATE_RESULT (tmpl)) = true;
 
+         /* Substitute into and set the constraints on the new declaration.  */
+         if (tree ci = get_constraints (friend_tmpl))
+           {
+             ++processing_template_decl;
+             ci = tsubst_constraint_info (ci, args, tf_warning_or_error,
+                                          DECL_FRIEND_CONTEXT (friend_tmpl));
+             --processing_template_decl;
+             set_constraints (tmpl, ci);
+           }
+
          /* Inject this template into the enclosing namspace scope.  */
          tmpl = pushdecl_namespace_level (tmpl, true);
        }

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend6.C b/gcc/testsuite/g++.dg/cpp2a/concepts-friend6.C
new file mode 100644 (file)
index 0000000..11e8313
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend6.C
@@ -0,0 +1,19 @@
+// PR c++/93467
+// { dg-do compile { target c++20 } }
+
+template<bool B> requires B
+  class C;
+
+template<typename>
+class S1
+{
+  template<bool B> requires B
+    friend class ::C;
+};
+
+template<typename>
+class S2
+{
+  template<bool B> requires (!B)
+    friend class ::C; // { dg-error "does not match original declaration" }
+};

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend7.C b/gcc/testsuite/g++.dg/cpp2a/concepts-friend7.C
new file mode 100644 (file)
index 0000000..4481b5c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend7.C
@@ -0,0 +1,18 @@
+// PR c++/93467
+// { dg-do compile { target c++20 } }
+
+template<typename T> concept True = true;
+
+template<typename U>
+struct S1 {
+    template<True T> friend struct S2; // friend declaration for S2
+};
+
+S1<int> s; // instantiate S1
+
+template<True T> struct S2; // another declaration for S2
+
+template<typename U>
+struct S3 {
+    template<True T> friend struct ::S2; // a third declaration for S2
+};