diff --git a/libcxx/include/numeric b/libcxx/include/numeric
--- a/libcxx/include/numeric
+++ b/libcxx/include/numeric
@@ -573,7 +573,7 @@
     return __fp_abs(__a) <= __hi && __fp_abs(__b) <= __hi ?  // typical case: overflow is impossible
       (__a + __b)/2 :                                        // always correctly rounded
       __fp_abs(__a) < __lo ? __a + __b/2 :                   // not safe to halve a
-      __fp_abs(__a) < __lo ? __a/2 + __b :                   // not safe to halve b
+      __fp_abs(__b) < __lo ? __a/2 + __b :                   // not safe to halve b
       __a/2 + __b/2;                                         // otherwise correctly rounded
 }