c++: ICE with requires-expr and -Wsequence-point [PR105304]

Here we're crashing from verify_sequence_points for this requires-expr
condition because it contains a templated CAST_EXPR with empty operand,
and verify_tree doesn't ignore this empty operand only because the
manual tail recursion that it performs for unary expression trees skips
the NULL test.

	PR c++/105304

gcc/c-family/ChangeLog:

	* c-common.cc (verify_tree) [restart]: Move up to before the
	NULL test.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires30.C: New test.

2 files changed, 11 insertions, 1 deletions

diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index 70f55f3..bb0544e 100644
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -2009,12 +2009,12 @@ verify_tree (tree x, struct tlist **pbefore_sp, struct tlist **pno_sp,
   enum tree_code code;
   enum tree_code_class cl;
 
+ restart:
   /* X may be NULL if it is the operand of an empty statement expression
      ({ }).  */
   if (x == NULL)
     return;
 
- restart:
   code = TREE_CODE (x);
   cl = TREE_CODE_CLASS (code);
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires30.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires30.C
new file mode 100644
index 0000000..f500af3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires30.C
@@ -0,0 +1,10 @@
+// PR c++/105304
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-Wall -Wsequence-point" }
+
+struct A { };
+
+int main() {
+  if (requires { A(); })
+    ;
+}