c++/reflection: fix ICE with OMP_CLAUSE [PR124227]

This test crashes with -fopenmp -freflection because consteval_only_p
gets

  <omp_clause 0x7fffe99db120
    type <tree_vec 0x7fffe982b900 length:3>
    reduction
    op-0: <var_decl 0x7fffe99d6390 acc>
    op-1: <init_expr 0x7fffe99c9870>
    op-2: <bind_expr 0x7fffe982b8a0>
    op-3: <var_decl 0x7fffe99d6428 D.2864>
    op-4:>

so it takes its type, but complete_type crashes on a TREE_VEC.

So let's handle TREE_VEC in consteval_only_p.

	PR c++/124227

gcc/cp/ChangeLog:

	* reflect.cc (consteval_only_p): Handle TREE_VEC.

gcc/testsuite/ChangeLog:

	* g++.dg/reflect/pr124227.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/reflect.cc

@@ -8086,6 +8086,14 @@ consteval_only_p (tree t)
   if (!t)
     return false;
 
+  if (TREE_CODE (t) == TREE_VEC)
+    {
+      for (tree arg : tree_vec_range (t))
+	if (arg && consteval_only_p (arg))
+	  return true;
+      return false;
+    }
+
   /* We need the complete type otherwise we'd have no fields for class
      templates and thus come up with zilch for things like
        template<typename T>

gcc/testsuite/g++.dg/reflect/pr124227.C

@@ -0,0 +1,15 @@
+// PR c++/124227
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection -fopenmp" }
+// { dg-require-effective-target fopenmp }
+
+using b = float;
+template <typename> class c {};
+#pragma omp declare reduction(d : c<float> : omp_in)
+auto e = [] {
+c<b> acc;
+#pragma omp parallel reduction(d : acc)
+{
+}
+};
+