Make __add_output_unless() a freestanding function, __set_intersection_add_output_unless(), because the lambda [tripped the "MacOS with C++03" test run](https://buildkite.com/llvm-project/libcxx-ci/builds/35055#018f173c-f155-4fbb-b6d7-a7aba01cec9e):

```
```

Author: ichaer

libcxx/include/__algorithm/set_intersection.h

@@ -43,6 +43,24 @@ struct __set_intersection_result {
       : __in1_(std::move(__in_iter1)), __in2_(std::move(__in_iter2)), __out_(std::move(__out_iter)) {}
 };
 
+// Helper for __set_intersection() with one-sided binary search: populate result and advance input iterators if they
+// haven't advanced in the last 2 calls. This function is very intimately related to the way it is used and doesn't
+// attempt to abstract that, it's not appropriate for general usage outside of its context. It would be a lambda of
+// __set_intersection() if that hadn't stumped the compiler in c++03 mode in some platforms.
+template <class _InForwardIter1, class _InForwardIter2, class _OutIter>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __set_intersection_add_output_unless(
+    bool __advanced, _InForwardIter1& __first1, _InForwardIter2& __first2, _OutIter& __result, bool& __prev_advanced) {
+  if (__advanced | __prev_advanced) {
+    __prev_advanced = __advanced;
+  } else {
+    *__result = *__first1;
+    ++__result;
+    ++__first1;
+    ++__first2;
+    __prev_advanced = true;
+  }
+}
+
 // With forward iterators we can make multiple passes over the data, allowing the use of one-sided binary search to
 // reduce best-case complexity to log(N). Understanding how we can use binary search and still respect complexity
 // guarantees is _not_ straightforward: the guarantee is "at most 2*(N+M)-1 comparisons", and one-sided binary search
@@ -76,30 +94,18 @@ __set_intersection(
   _LIBCPP_CONSTEXPR std::__identity __proj;
   bool __prev_advanced = true;
 
-  auto __add_output_unless = [&](bool __advanced) {
-    if (__advanced | __prev_advanced) {
-      __prev_advanced = __advanced;
-    } else {
-      *__result = *__first1;
-      ++__result;
-      ++__first1;
-      ++__first2;
-      __prev_advanced = true;
-    }
-  };
-
   while (__first2 != __last2) {
     _InForwardIter1 __first1_next =
         std::__lower_bound_onesided<_AlgPolicy>(__first1, __last1, *__first2, __comp, __proj);
     std::swap(__first1_next, __first1);
-    __add_output_unless(__first1 != __first1_next);
+    std::__set_intersection_add_output_unless(__first1 != __first1_next, __first1, __first2, __result, __prev_advanced);
     if (__first1 == __last1)
       break;
 
     _InForwardIter2 __first2_next =
         std::__lower_bound_onesided<_AlgPolicy>(__first2, __last2, *__first1, __comp, __proj);
     std::swap(__first2_next, __first2);
-    __add_output_unless(__first2 != __first2_next);
+    std::__set_intersection_add_output_unless(__first2 != __first2_next, __first1, __first2, __result, __prev_advanced);
   }
   return __set_intersection_result<_InForwardIter1, _InForwardIter2, _OutIter>(
       _IterOps<_AlgPolicy>::next(std::move(__first1), std::move(__last1)),