Index: llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
===================================================================
--- llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -287,7 +287,12 @@
 
 struct CallSiteInfo {
   std::vector<VirtualCallSite> CallSites;
+  bool HasTypeTestAssumeUsers;
   std::vector<FunctionSummary *> TypeCheckedLoadUsers;
+
+  bool isExported() const {
+    return HasTypeTestAssumeUsers || !TypeCheckedLoadUsers.empty();
+  }
 };
 
 struct VTableSlotInfo {
@@ -368,9 +373,11 @@
                             const std::set<TypeMemberInfo> &TypeMemberInfos,
                             uint64_t ByteOffset);
 
-  void applySingleImplDevirt(VTableSlotInfo &SlotInfo, Constant *TheFn);
+  void applySingleImplDevirt(VTableSlotInfo &SlotInfo, Constant *TheFn,
+                             bool &IsExported);
   bool trySingleImplDevirt(MutableArrayRef<VirtualCallTarget> TargetsForSlot,
-                           VTableSlotInfo &SlotInfo);
+                           VTableSlotInfo &SlotInfo,
+                           WholeProgramDevirtResolution *Res);
 
   bool tryEvaluateFunctionsWithArgs(
       MutableArrayRef<VirtualCallTarget> TargetsForSlot,
@@ -587,7 +594,7 @@
 }
 
 void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
-                                         Constant *TheFn) {
+                                         Constant *TheFn, bool &IsExported) {
   auto Apply = [&](CallSiteInfo &CSInfo) {
     for (auto &&VCallSite : CSInfo.CallSites) {
       if (RemarksEnabled)
@@ -598,6 +605,10 @@
       if (VCallSite.NumUnsafeUses)
         --*VCallSite.NumUnsafeUses;
     }
+    if (CSInfo.isExported()) {
+      IsExported = true;
+      CSInfo.TypeCheckedLoadUsers.clear();
+    }
   };
   Apply(SlotInfo.CSInfo);
   for (auto &P : SlotInfo.ConstCSInfo)
@@ -606,7 +617,7 @@
 
 bool DevirtModule::trySingleImplDevirt(
     MutableArrayRef<VirtualCallTarget> TargetsForSlot,
-    VTableSlotInfo &SlotInfo) {
+    VTableSlotInfo &SlotInfo, WholeProgramDevirtResolution *Res) {
   // See if the program contains a single implementation of this virtual
   // function.
   Function *TheFn = TargetsForSlot[0].Fn;
@@ -617,7 +628,23 @@
   // If so, update each call site to call that implementation directly.
   if (RemarksEnabled)
     TargetsForSlot[0].WasDevirt = true;
-  applySingleImplDevirt(SlotInfo, TheFn);
+
+  bool IsExported = false;
+  applySingleImplDevirt(SlotInfo, TheFn, IsExported);
+  if (!IsExported)
+    return false;
+
+  // If the only implementation has local linkage, we must promote to external
+  // to make it visible to thin LTO objects.
+  if (TheFn->hasLocalLinkage()) {
+    TheFn->setLinkage(GlobalValue::ExternalLinkage);
+    TheFn->setVisibility(GlobalValue::HiddenVisibility);
+    TheFn->setName(TheFn->getName() + "$merged");
+  }
+
+  Res->TheKind = WholeProgramDevirtResolution::SingleImpl;
+  Res->SingleImplName = TheFn->getName();
+
   return true;
 }
 
@@ -1056,11 +1083,19 @@
         if (!FS)
           continue;
         // FIXME: Only add live functions.
+        for (FunctionSummary::VFuncId VF : FS->type_test_assume_vcalls())
+          for (Metadata *MD : MetadataByGUID[VF.GUID])
+            CallSlots[{MD, VF.Offset}].CSInfo.HasTypeTestAssumeUsers = true;
         for (FunctionSummary::VFuncId VF : FS->type_checked_load_vcalls())
           for (Metadata *MD : MetadataByGUID[VF.GUID])
             CallSlots[{MD, VF.Offset}].CSInfo.TypeCheckedLoadUsers.push_back(
                 FS);
         for (const FunctionSummary::ConstVCall &VC :
+             FS->type_test_assume_const_vcalls())
+          for (Metadata *MD : MetadataByGUID[VC.VFunc.GUID])
+            CallSlots[{MD, VC.VFunc.Offset}]
+                .ConstCSInfo[VC.Args].HasTypeTestAssumeUsers = true;
+        for (const FunctionSummary::ConstVCall &VC :
              FS->type_checked_load_const_vcalls())
           for (Metadata *MD : MetadataByGUID[VC.VFunc.GUID])
             CallSlots[{MD, VC.VFunc.Offset}]
@@ -1079,7 +1114,14 @@
     std::vector<VirtualCallTarget> TargetsForSlot;
     if (tryFindVirtualCallTargets(TargetsForSlot, TypeIdMap[S.first.TypeID],
                                   S.first.ByteOffset)) {
-      if (!trySingleImplDevirt(TargetsForSlot, S.second) &&
+      WholeProgramDevirtResolution *Res = nullptr;
+      if (Action == PassSummaryAction::Export && isa<MDString>(S.first.TypeID))
+        Res =
+            &Summary
+                 ->getTypeIdSummary(cast<MDString>(S.first.TypeID)->getString())
+                 .WPDRes[S.first.ByteOffset];
+
+      if (!trySingleImplDevirt(TargetsForSlot, S.second, Res) &&
           tryVirtualConstProp(TargetsForSlot, S.second))
         DidVirtualConstProp = true;
 
Index: llvm/test/Transforms/WholeProgramDevirt/export-single-impl.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/WholeProgramDevirt/export-single-impl.ll
@@ -0,0 +1,72 @@
+; RUN: opt -wholeprogramdevirt -wholeprogramdevirt-summary-action=export -wholeprogramdevirt-read-summary=%S/Inputs/export.yaml -wholeprogramdevirt-write-summary=%t -S -o - %s | FileCheck %s
+; RUN: FileCheck --check-prefix=SUMMARY %s < %t
+
+; SUMMARY:      TypeIdMap:
+; SUMMARY-NEXT:   typeid1:
+; SUMMARY-NEXT:     TTRes:
+; SUMMARY-NEXT:       Kind:            Unsat
+; SUMMARY-NEXT:       SizeM1BitWidth:  0
+; SUMMARY-NEXT:     WPDRes:
+; SUMMARY-NEXT:       0:
+; SUMMARY-NEXT:         Kind:            SingleImpl
+; SUMMARY-NEXT:         SingleImplName:  vf1
+; SUMMARY-NEXT:         ResByArg:
+; SUMMARY-NEXT:   typeid2:
+; SUMMARY-NEXT:     TTRes:
+; SUMMARY-NEXT:       Kind:            Unsat
+; SUMMARY-NEXT:       SizeM1BitWidth:  0
+; SUMMARY-NEXT:     WPDRes:
+; SUMMARY-NEXT:       0:
+; SUMMARY-NEXT:         Kind:            SingleImpl
+; SUMMARY-NEXT:         SingleImplName:  vf2
+; SUMMARY-NEXT:         ResByArg:
+; SUMMARY-NEXT:   typeid3:
+; SUMMARY-NEXT:     TTRes:
+; SUMMARY-NEXT:       Kind:            Unsat
+; SUMMARY-NEXT:       SizeM1BitWidth:  0
+; SUMMARY-NEXT:     WPDRes:
+; SUMMARY-NEXT:       0:
+; SUMMARY-NEXT:         Kind:            SingleImpl
+; SUMMARY-NEXT:         SingleImplName:  vf3
+; SUMMARY-NEXT:         ResByArg:
+; SUMMARY-NEXT:   typeid4:
+; SUMMARY-NEXT:     TTRes:
+; SUMMARY-NEXT:       Kind:            Unsat
+; SUMMARY-NEXT:       SizeM1BitWidth:  0
+; SUMMARY-NEXT:     WPDRes:
+; SUMMARY-NEXT:       0:
+; SUMMARY-NEXT:         Kind:            SingleImpl
+; SUMMARY-NEXT:         SingleImplName:  'vf4$merged'
+; SUMMARY-NEXT:         ResByArg:
+; SUMMARY-NEXT: ...
+
+; CHECK: @vt1 = constant void (i8*)* @vf1
+@vt1 = constant void (i8*)* @vf1, !type !0
+
+; CHECK: @vt2 = constant void (i8*)* @vf2
+@vt2 = constant void (i8*)* @vf2, !type !1
+
+@vt3 = constant void (i8*)* @vf3, !type !2
+
+; CHECK: @vt4 = constant void (i8*)* @"vf4$merged"
+@vt4 = constant void (i8*)* @vf4, !type !3
+
+; CHECK: declare void @vf1(i8*)
+declare void @vf1(i8*)
+
+; CHECK: define void @vf2(i8*)
+define void @vf2(i8*) {
+  ret void
+}
+
+declare void @vf3(i8*)
+
+; CHECK: define hidden void @"vf4$merged"
+define internal void @vf4(i8*) {
+  ret void
+}
+
+!0 = !{i32 0, !"typeid1"}
+!1 = !{i32 0, !"typeid2"}
+!2 = !{i32 0, !"typeid3"}
+!3 = !{i32 0, !"typeid4"}