c++: partially inst requires-expr in noexcept-spec [PR101043]

Here we're ICEing from strip_typedefs for the partially instantiated
requires-expression when walking its REQUIRES_EXPR_EXTRA_ARGS which
in this case is a TREE_LIST with non-empty TREE_PURPOSE (to hold the
captured local specialization 't' as per build_extra_args) which
strip_typedefs doesn't expect.

We can probably skip walking REQUIRES_EXPR_EXTRA_ARGS at all since it
shouldn't contain any typedefs in the first place, but it seems safer
and more generally useful to just teach strip_typedefs to handle non-empty
TREE_PURPOSE the obvious way.  (The code asserts TREE_PURPOSE was empty
even since since its inception i.e. r189298.)

	PR c++/101043

gcc/cp/ChangeLog:

	* tree.cc (strip_typedefs_expr) <case TREE_LIST>: Handle
	non-empty TREE_PURPOSE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires37.C: New test.

1 files changed, 12 insertions, 7 deletions

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index dc4126f..0736e8d 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -1911,19 +1911,24 @@ strip_typedefs_expr (tree t, bool *remove_attributes, unsigned int flags)
     case TREE_LIST:
       {
 	bool changed = false;
-	releasing_vec vec;
+	auto_vec<tree_pair, 4> vec;
 	r = t;
 	for (; t; t = TREE_CHAIN (t))
 	  {
-	    gcc_assert (!TREE_PURPOSE (t));
-	    tree elt = strip_typedefs (TREE_VALUE (t),
-				       remove_attributes, flags);
-	    if (elt != TREE_VALUE (t))
+	    tree purpose = strip_typedefs (TREE_PURPOSE (t),
+					   remove_attributes, flags);
+	    tree value = strip_typedefs (TREE_VALUE (t),
+					 remove_attributes, flags);
+	    if (purpose != TREE_PURPOSE (t) || value != TREE_VALUE (t))
 	      changed = true;
-	    vec_safe_push (vec, elt);
+	    vec.safe_push ({purpose, value});
 	  }
 	if (changed)
-	  r = build_tree_list_vec (vec);
+	  {
+	    r = NULL_TREE;
+	    for (int i = vec.length () - 1; i >= 0; i--)
+	      r = tree_cons (vec[i].first, vec[i].second, r);
+	  }
 	return r;
       }