Index: lib/CodeGen/BackendUtil.cpp
===================================================================
--- lib/CodeGen/BackendUtil.cpp
+++ lib/CodeGen/BackendUtil.cpp
@@ -56,6 +56,7 @@
 #include "llvm/Transforms/Scalar/GVN.h"
 #include "llvm/Transforms/Utils/NameAnonGlobals.h"
 #include "llvm/Transforms/Utils/SymbolRewriter.h"
+#include <algorithm>
 #include <memory>
 using namespace clang;
 using namespace llvm;
@@ -1014,8 +1015,18 @@
       continue;
 
     auto GUID = GlobalList.first;
-    assert(GlobalList.second.SummaryList.size() == 1 &&
-           "Expected individual combined index to have one summary per GUID");
+    auto &SummaryList = GlobalList.second.SummaryList;
+    auto MultipleDefPred = [](std::unique_ptr<llvm::GlobalValueSummary> &S) {
+        auto Linkage = S->linkage();
+        return GlobalValue::isAvailableExternallyLinkage(Linkage) ||
+               GlobalValue::isLinkOnceODRLinkage(Linkage) ||
+               GlobalValue::isWeakODRLinkage(Linkage);
+    };
+    assert((SummaryList.size() == 1 || (SummaryList.size() > 1 &&
+            std::all_of(SummaryList.begin(), SummaryList.end(),
+                        MultipleDefPred))) &&
+           "Expected individual combined index to have one summary per GUID or "
+           "multiple entries if the allow multiple definitions.");
     auto &Summary = GlobalList.second.SummaryList[0];
     // Skip the summaries for the importing module. These are included to
     // e.g. record required linkage changes.