[gcc r12-8996] c++: pack in requires-expr parm list [PR107417]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:b428bb449be1bdbbd4000b51bb7c665981dc8c8f

commit r12-8996-gb428bb449be1bdbbd4000b51bb7c665981dc8c8f
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sun Dec 4 10:47:24 2022 -0500

    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.
    
    (cherry picked from commit 079add3ad39d6620d34665dd9c26c21951eb657c)

Diff:
---
 gcc/cp/pt.cc                                     | 15 ---------------
 gcc/cp/tree.cc                                   | 21 ++++++++++++---------
 gcc/testsuite/g++.dg/cpp2a/concepts-requires33.C | 10 ++++++++++
 3 files changed, 22 insertions(+), 24 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index cb674cc2258..a5f14ef98e8 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10648,21 +10648,6 @@ 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 3b37567cbd7..bc521f8ae9c 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -5488,15 +5488,18 @@ 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
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires33.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires33.C
new file mode 100644
index 00000000000..1ff237ac382
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires33.C
@@ -0,0 +1,10 @@
+// PR c++/107417
+// { dg-do compile { target c++20 } }
+
+template<class... T>
+void f() requires (requires (T x) { true; } && ...);
+
+int main() {
+  f<int>();
+  f<int, void>(); // { dg-error "no match" }
+}