diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -1472,8 +1472,15 @@
           StringRef symbolName = defined->getName();
           if (config->exportedSymbols.match(symbolName)) {
             if (defined->privateExtern) {
-              warn("cannot export hidden symbol " + symbolName +
-                   "\n>>> defined in " + toString(defined->getFile()));
+              // If it's a weak, it's not "hidden".
+              if (!defined->isWeakDef())
+                warn("cannot export hidden symbol " + symbolName +
+                     "\n>>> defined in " + toString(defined->getFile()));
+              else
+                // When a weakdef private extern symbol is exported,
+                // it becomes "non-private.
+                // This matches LD64's behaviour.
+                defined->privateExtern = false;
             }
           } else {
             defined->privateExtern = true;
diff --git a/lld/MachO/MarkLive.cpp b/lld/MachO/MarkLive.cpp
--- a/lld/MachO/MarkLive.cpp
+++ b/lld/MachO/MarkLive.cpp
@@ -67,7 +67,7 @@
         // FIXME: Instead of doing this here, maybe the Driver code doing
         // the matching should add them to explicitUndefineds? Then the
         // explicitUndefineds code below would handle this automatically.
-        assert(!defined->privateExtern &&
+        assert((!defined->privateExtern || defined->isWeakDef()) &&
                "should have been rejected by driver");
         addSym(defined);
         continue;