diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval-prop21.C b/gcc/testsuite/g++.dg/cpp2a/consteval-prop21.C
index debbda4f425..36da1bd91f3 100644
--- a/gcc/testsuite/g++.dg/cpp2a/consteval-prop21.C
+++ b/gcc/testsuite/g++.dg/cpp2a/consteval-prop21.C
@@ -5,7 +5,13 @@ template <typename T>
 constexpr int b(T) {
   return 0;
 }
-consteval __uint128_t operator"" _c(const char*) { return 0; }
+consteval
+#ifdef __SIZEOF_INT128__
+__uint128_t
+#else
+unsigned long long
+#endif
+operator"" _c(const char*) { return 0; }
 constexpr char e() {
   long f = true ? 0 : b(long(1));
   return b(f);