c++: pack in requires-expr parm list [PR107417]

Here find_parameter_packs_r isn't detecting the pack T inside the
requires-expr's parameter list ultimately because cp_walk_trees
deliberately avoids walking the list so as to avoid false positives in
the unexpanded pack checker.

But it should still be fine to walk the TREE_TYPE of each parameter,
which we already need to do from for_each_template_parm_r, and is
sufficient to fix the testcase.

	PR c++/107417

gcc/cp/ChangeLog:

	* pt.cc (for_each_template_parm_r) <case REQUIRES_EXPR>: Move
	walking of the TREE_TYPE of each parameter to ...
	* tree.cc (cp_walk_subtrees) <case REQUIRES_EXPR>: ... here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires33.C: New test.

2 files changed, 12 insertions, 24 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index bc8ea06..80110da 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10573,21 +10573,6 @@ for_each_template_parm_r (tree *tp, int *walk_subtrees, void *d)
 	return error_mark_node;
       break;
 
-    case REQUIRES_EXPR:
-      {
-	if (!fn)
-	  return error_mark_node;
-
-	/* Recursively walk the type of each constraint variable.  */
-	tree p = TREE_OPERAND (t, 0);
-	while (p)
-	  {
-	    WALK_SUBTREE (TREE_TYPE (p));
-	    p = TREE_CHAIN (p);
-	  }
-      }
-      break;
-
     default:
       break;
     }
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 04a055d..33bde16 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -5603,15 +5603,18 @@ cp_walk_subtrees (tree *tp, int *walk_subtrees_p, walk_tree_fn func,
       break;
  
     case REQUIRES_EXPR:
-      // Only recurse through the nested expression. Do not
-      // walk the parameter list. Doing so causes false
-      // positives in the pack expansion checker since the
-      // requires parameters are introduced as pack expansions.
-      ++cp_unevaluated_operand;
-      result = cp_walk_tree (&REQUIRES_EXPR_REQS (*tp), func, data, pset);
-      --cp_unevaluated_operand;
-      *walk_subtrees_p = 0;
-      break;
+      {
+	cp_unevaluated u;
+	for (tree parm = REQUIRES_EXPR_PARMS (*tp); parm; parm = DECL_CHAIN (parm))
+	  /* Walk the types of each parameter, but not the parameter itself,
+	     since doing so would cause false positives in the unexpanded pack
+	     checker if the requires-expr introduces a function parameter pack,
+	     e.g. requires (Ts... ts) { }.   */
+	  WALK_SUBTREE (TREE_TYPE (parm));
+	WALK_SUBTREE (REQUIRES_EXPR_REQS (*tp));
+	*walk_subtrees_p = 0;
+	break;
+      }
 
     case DECL_EXPR:
       /* User variables should be mentioned in BIND_EXPR_VARS