Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -583,23 +583,28 @@
   if (!LTOInfo)
     return LTOInfo.takeError();
 
-  if (EnableSplitLTOUnit.hasValue()) {
-    // If only some modules were split, flag this in the index so that
-    // we can skip or error on optimizations that need consistently split
-    // modules (whole program devirt and lower type tests).
-    if (EnableSplitLTOUnit.getValue() != LTOInfo->EnableSplitLTOUnit)
-      ThinLTO.CombinedIndex.setPartiallySplitLTOUnits();
-  } else
-    EnableSplitLTOUnit = LTOInfo->EnableSplitLTOUnit;
-
   BitcodeModule BM = Input.Mods[ModI];
   auto ModSyms = Input.module_symbols(ModI);
   addModuleToGlobalRes(ModSyms, {ResI, ResE},
                        LTOInfo->IsThinLTO ? ThinLTO.ModuleMap.size() + 1 : 0,
                        LTOInfo->HasSummary);
 
-  if (LTOInfo->IsThinLTO)
+  if (LTOInfo->IsThinLTO) {
+    // We only care about having consistently split ThinLTO modules, as regular
+    // LTO modules are always handled properly by whole program devirtualization
+    // and lower type tests, and we don't want to issue spurious errors linking
+    // with regular LTO modules.
+    if (EnableSplitLTOUnit.hasValue()) {
+      // If only some modules were split, flag this in the index so that
+      // we can skip or error on optimizations that need consistently split
+      // modules (whole program devirt and lower type tests).
+      if (EnableSplitLTOUnit.getValue() != LTOInfo->EnableSplitLTOUnit)
+        ThinLTO.CombinedIndex.setPartiallySplitLTOUnits();
+    } else
+      EnableSplitLTOUnit = LTOInfo->EnableSplitLTOUnit;
+
     return addThinLTO(BM, ModSyms, ResI, ResE);
+  }
 
   Expected<RegularLTOState::AddedModule> ModOrErr =
       addRegularLTO(BM, ModSyms, ResI, ResE);
Index: test/ThinLTO/X86/Inputs/nonthin_empty.ll
===================================================================
--- /dev/null
+++ test/ThinLTO/X86/Inputs/nonthin_empty.ll
@@ -0,0 +1,7 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+^0 = module: (path: "nonthin.o", hash: (0, 0, 0, 0, 0))
+
+!0 = !{i32 1, !"ThinLTO", i32 0}
+!llvm.module.flags = !{ !0 }
Index: test/ThinLTO/X86/Inputs/thin_empty.ll
===================================================================
--- /dev/null
+++ test/ThinLTO/X86/Inputs/thin_empty.ll
@@ -0,0 +1,4 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+^0 = module: (path: "thin.o", hash: (0, 0, 0, 0, 0))
Index: test/ThinLTO/X86/cfi-devirt.ll
===================================================================
--- test/ThinLTO/X86/cfi-devirt.ll
+++ test/ThinLTO/X86/cfi-devirt.ll
@@ -44,7 +44,7 @@
 ; containing an llvm.type.checked.load (with a split LTO Unit) with one
 ; that does not have a split LTO Unit. Use -thinlto-distributed-indexes
 ; to ensure it is being caught in the thin link.
-; RUN: opt -thinlto-bc -o %t2.o %S/Inputs/empty.ll
+; RUN: opt -thinlto-bc -o %t2.o %S/Inputs/thin_empty.ll
 ; RUN: not llvm-lto2 run %t.o %t2.o -thinlto-distributed-indexes \
 ; RUN:   -o %t3 \
 ; RUN:   -r=%t.o,test,px \
@@ -61,6 +61,26 @@
 ; RUN:   -r=%t.o,_ZTV1C,px 2>&1 | FileCheck %s --check-prefix=ERROR
 ; ERROR: failed: inconsistent LTO Unit splitting (recompile with -fsplit-lto-unit)
 
+; Simulate linking instead with a regular LTO module containing a summary
+; (not marked as split). This should succeed as we only care about
+; inconsistently split ThinLTO modules.
+; RUN: opt -module-summary -o %t4.o %S/Inputs/nonthin_empty.ll
+; RUN: llvm-lto2 run %t.o %t4.o -thinlto-distributed-indexes \
+; RUN:   -o %t3 \
+; RUN:   -r=%t.o,test,px \
+; RUN:   -r=%t.o,_ZN1A1nEi,p \
+; RUN:   -r=%t.o,_ZN1B1fEi,p \
+; RUN:   -r=%t.o,_ZN1C1fEi,p \
+; RUN:   -r=%t.o,empty,p \
+; RUN:   -r=%t.o,_ZTV1B, \
+; RUN:   -r=%t.o,_ZTV1C, \
+; RUN:   -r=%t.o,_ZN1A1nEi, \
+; RUN:   -r=%t.o,_ZN1B1fEi, \
+; RUN:   -r=%t.o,_ZN1C1fEi, \
+; RUN:   -r=%t.o,_ZTV1B,px \
+; RUN:   -r=%t.o,_ZTV1C,px 2>&1 | FileCheck %s --check-prefix=NOERROR --allow-empty
+; NOERROR-NOT: failed: inconsistent LTO Unit splitting (recompile with -fsplit-lto-unit)
+
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-grtev4-linux-gnu"