Index: include/llvm/LTO/LTO.h
===================================================================
--- include/llvm/LTO/LTO.h
+++ include/llvm/LTO/LTO.h
@@ -387,7 +387,7 @@
                    ArrayRef<SymbolResolution> Res);
 
   Error runRegularLTO(AddOutputFn AddOutput);
-  Error runThinLTO(AddOutputFn AddOutput);
+  Error runThinLTO(AddOutputFn AddOutput, bool HasRegularLTO);
 
   mutable bool CalledGetMaxTasks = false;
 };
Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -410,11 +410,15 @@
 }
 
 Error LTO::run(AddOutputFn AddOutput) {
+  // Save the status of having a regularLTO combined module, as
+  // this is needed for generating the ThinLTO Task ID, and
+  // the CombinedModule will be moved at the end of runRegularLTO.
+  bool HasRegularLTO = RegularLTO.CombinedModule != nullptr;
   // Invoke regular LTO if there was a regular LTO module to start with.
-  if (RegularLTO.CombinedModule)
+  if (HasRegularLTO)
     if (auto E = runRegularLTO(AddOutput))
       return E;
-  return runThinLTO(AddOutput);
+  return runThinLTO(AddOutput, HasRegularLTO);
 }
 
 Error LTO::runRegularLTO(AddOutputFn AddOutput) {
@@ -696,7 +700,7 @@
   };
 }
 
-Error LTO::runThinLTO(AddOutputFn AddOutput) {
+Error LTO::runThinLTO(AddOutputFn AddOutput, bool HasRegularLTO) {
   if (ThinLTO.ModuleMap.empty())
     return Error();
 
@@ -753,9 +757,8 @@
   // ParallelCodeGenParallelismLevel if an LTO module is present, as tasks 0
   // through ParallelCodeGenParallelismLevel-1 are reserved for parallel code
   // generation partitions.
-  unsigned Task = RegularLTO.CombinedModule
-                      ? RegularLTO.ParallelCodeGenParallelismLevel
-                      : 0;
+  unsigned Task =
+      HasRegularLTO ? RegularLTO.ParallelCodeGenParallelismLevel : 0;
   unsigned Partition = 1;
 
   for (auto &Mod : ThinLTO.ModuleMap) {
Index: test/LTO/Resolution/X86/Inputs/mixed_lto.ll
===================================================================
--- /dev/null
+++ test/LTO/Resolution/X86/Inputs/mixed_lto.ll
@@ -0,0 +1,6 @@
+target triple = "x86_64-unknown-linux-gnu"
+declare i32 @g()
+define i32 @main() {
+  call i32 @g()
+  ret i32 0
+}
Index: test/LTO/Resolution/X86/mixed_lto.ll
===================================================================
--- /dev/null
+++ test/LTO/Resolution/X86/mixed_lto.ll
@@ -0,0 +1,19 @@
+; Test mixed-mode LTO (mix of regular and thin LTO objects)
+; RUN: opt %s -o %t1.o
+; RUN: opt -module-summary %p/Inputs/mixed_lto.ll -o %t2.o
+
+; RUN: llvm-lto2 -o %t3.o %t2.o %t1.o -r %t2.o,main,px -r %t2.o,g, -r %t1.o,g,px
+
+; Task 0 is the regular LTO file (this file)
+; RUN: nm %t3.o.0 | FileCheck %s --check-prefix=NM0
+; NM0: T g
+
+; Task 1 is the (first) ThinLTO file (Inputs/mixed_lto.ll)
+; RUN: nm %t3.o.1 | FileCheck %s --check-prefix=NM1
+; NM1-DAG: T main
+; NM1-DAG: U g
+
+target triple = "x86_64-unknown-linux-gnu"
+define i32 @g() {
+  ret i32 0
+}
Index: test/tools/gold/X86/Inputs/mixed_lto.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/Inputs/mixed_lto.ll
@@ -0,0 +1,6 @@
+target triple = "x86_64-unknown-linux-gnu"
+declare i32 @g()
+define i32 @main() {
+  call i32 @g()
+  ret i32 0
+}
Index: test/tools/gold/X86/mixed_lto.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/mixed_lto.ll
@@ -0,0 +1,18 @@
+; Test mixed-mode LTO (mix of regular and thin LTO objects)
+; RUN: opt %s -o %t.o
+; RUN: opt -module-summary %p/Inputs/mixed_lto.ll -o %t2.o
+
+; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:     -shared \
+; RUN:     --plugin-opt=thinlto \
+; RUN:     --plugin-opt=-import-instr-limit=0 \
+; RUN:     -o %t3.o %t2.o %t.o
+; RUN: nm %t3.o | FileCheck %s
+
+; CHECK-DAG: T main
+; CHECK-DAG: T g
+
+target triple = "x86_64-unknown-linux-gnu"
+define i32 @g() {
+  ret i32 0
+}