Index: lld/trunk/COFF/Driver.cpp
===================================================================
--- lld/trunk/COFF/Driver.cpp
+++ lld/trunk/COFF/Driver.cpp
@@ -971,6 +971,32 @@
   Config->PDBAltPath = Buf;
 }
 
+/// Check that at most one resource obj file was used.
+/// Call after ObjFile::Instances is complete.
+static void diagnoseMultipleResourceObjFiles() {
+  // The .rsrc$01 section in a resource obj file contains a tree description
+  // of resources.  Merging multiple resource obj files would require merging
+  // the trees instead of using usual linker section merging semantics.
+  // Since link.exe disallows linking more than one resource obj file with
+  // LNK4078, mirror that.  The normal use of resource files is to give the
+  // linker many .res files, which are then converted to a single resource obj
+  // file internally, so this is not a big restriction in practice.
+  ObjFile *ResourceObjFile = nullptr;
+  for (ObjFile *F : ObjFile::Instances) {
+    if (!F->IsResourceObjFile)
+      continue;
+
+    if (!ResourceObjFile) {
+      ResourceObjFile = F;
+      continue;
+    }
+
+    error(toString(F) +
+          ": more than one resource obj file not allowed, already got " +
+          toString(ResourceObjFile));
+  }
+}
+
 // In MinGW, if no symbols are chosen to be exported, then all symbols are
 // automatically exported by default. This behavior can be forced by the
 // -export-all-symbols option, so that it happens even when exports are
@@ -1812,6 +1838,9 @@
   if (Config->DoGC)
     markLive(Symtab->getChunks());
 
+  // Needs to happen after the last call to addFile().
+  diagnoseMultipleResourceObjFiles();
+
   // Identify identical COMDAT sections to merge them.
   if (Config->DoICF) {
     findKeepUniqueSections();
Index: lld/trunk/COFF/DriverUtils.cpp
===================================================================
--- lld/trunk/COFF/DriverUtils.cpp
+++ lld/trunk/COFF/DriverUtils.cpp
@@ -697,6 +697,7 @@
 }
 
 // Convert Windows resource files (.res files) to a .obj file.
+// Does what cvtres.exe does, but in-process and cross-platform.
 MemoryBufferRef convertResToCOFF(ArrayRef<MemoryBufferRef> MBs) {
   object::WindowsResourceParser Parser;
 
Index: lld/trunk/COFF/InputFiles.h
===================================================================
--- lld/trunk/COFF/InputFiles.h
+++ lld/trunk/COFF/InputFiles.h
@@ -162,10 +162,13 @@
   // precompiled object. Any difference indicates out-of-date objects.
   llvm::Optional<uint32_t> PCHSignature;
 
-  // Tells whether this file was compiled with /hotpatch
+  // Whether this is an object file created from .res files.
+  bool IsResourceObjFile = false;
+
+  // Whether this file was compiled with /hotpatch.
   bool HotPatchable = false;
 
-  // Whether the object was already merged into the final PDB or not
+  // Whether the object was already merged into the final PDB.
   bool MergedIntoPDB = false;
 
   // If the OBJ has a .debug$T stream, this tells how it will be handled.
Index: lld/trunk/COFF/InputFiles.cpp
===================================================================
--- lld/trunk/COFF/InputFiles.cpp
+++ lld/trunk/COFF/InputFiles.cpp
@@ -206,6 +206,10 @@
   if (Def)
     C->Checksum = Def->CheckSum;
 
+  // link.exe uses the presence of .rsrc$01 for LNK4078, so match that.
+  if (Name == ".rsrc$01")
+    IsResourceObjFile = true;
+
   // CodeView sections are stored to a different vector because they are not
   // linked in the regular manner.
   if (C->isCodeView())
@@ -667,7 +671,8 @@
 // types of external files: Precomp/PCH OBJs, when compiling with /Yc and /Yu.
 // And PDB type servers, when compiling with /Zi. This function extracts these
 // dependencies and makes them available as a TpiSource interface (see
-// DebugTypes.h).
+// DebugTypes.h). Both cases only happen with cl.exe: clang-cl produces regular
+// output even with /Yc and /Yu and with /Zi.
 void ObjFile::initializeDependencies() {
   if (!Config->Debug)
     return;
Index: lld/trunk/test/CMakeLists.txt
===================================================================
--- lld/trunk/test/CMakeLists.txt
+++ lld/trunk/test/CMakeLists.txt
@@ -34,8 +34,8 @@
 set(LLD_TEST_DEPS lld)
 if (NOT LLD_BUILT_STANDALONE)
   list(APPEND LLD_TEST_DEPS
-    FileCheck count llc llvm-ar llvm-as llvm-bcanalyzer llvm-config llvm-dis
-    llvm-dwarfdump llvm-lib llvm-mc llvm-nm llvm-objcopy llvm-objdump
+    FileCheck count llc llvm-ar llvm-as llvm-bcanalyzer llvm-config llvm-cvtres
+    llvm-dis llvm-dwarfdump llvm-lib llvm-mc llvm-nm llvm-objcopy llvm-objdump
     llvm-pdbutil llvm-readelf llvm-readobj not obj2yaml opt yaml2obj
     )
 endif()
Index: lld/trunk/test/COFF/multiple-resource-objs.test
===================================================================
--- lld/trunk/test/COFF/multiple-resource-objs.test
+++ lld/trunk/test/COFF/multiple-resource-objs.test
@@ -0,0 +1,11 @@
+# RUN: llvm-cvtres /out:%t_resource.obj %S/Inputs/resource.res
+# RUN: llvm-cvtres /out:%t_id.obj %S/Inputs/id.res
+# RUN: not lld-link /out:%t.exe /dll /noentry %t_id.obj %t_resource.obj 2>&1 | \
+# RUN:     FileCheck --check-prefix=TWOOBJ %s
+
+TWOOBJ: error: {{.*}}_resource.obj: more than one resource obj file not allowed, already got {{.*}}_id.obj
+
+# RUN: not lld-link /out:%t.exe /dll /noentry %S/Inputs/id.res %t_resource.obj 2>&1 | \
+# RUN:     FileCheck --check-prefix=OBJRES %s
+
+OBJRES: error: internal .obj file created from .res files: more than one resource obj file not allowed, already got {{.*}}_resource.obj
Index: llvm/trunk/lib/Object/WindowsResource.cpp
===================================================================
--- llvm/trunk/lib/Object/WindowsResource.cpp
+++ llvm/trunk/lib/Object/WindowsResource.cpp
@@ -442,7 +442,8 @@
       Data(Parser.getData()), StringTable(Parser.getStringTable()) {
   performFileLayout();
 
-  OutputBuffer = WritableMemoryBuffer::getNewMemBuffer(FileSize);
+  OutputBuffer = WritableMemoryBuffer::getNewMemBuffer(
+      FileSize, "internal .obj file created from .res files");
 }
 
 void WindowsResourceCOFFWriter::performFileLayout() {
@@ -521,7 +522,7 @@
   Header->NumberOfSections = 2;
   Header->TimeDateStamp = TimeDateStamp;
   Header->PointerToSymbolTable = SymbolTableOffset;
-  // One symbol for every resource plus 2 for each section and @feat.00
+  // One symbol for every resource plus 2 for each section and 1 for @feat.00
   Header->NumberOfSymbols = Data.size() + 5;
   Header->SizeOfOptionalHeader = 0;
   // cvtres.exe sets 32BIT_MACHINE even for 64-bit machine types. Match it.
Index: llvm/trunk/utils/gn/secondary/lld/test/BUILD.gn
===================================================================
--- llvm/trunk/utils/gn/secondary/lld/test/BUILD.gn
+++ llvm/trunk/utils/gn/secondary/lld/test/BUILD.gn
@@ -82,6 +82,7 @@
     "//llvm/tools/llvm-ar:symlinks",
     "//llvm/tools/llvm-as",
     "//llvm/tools/llvm-bcanalyzer",
+    "//llvm/tools/llvm-cvtres",
     "//llvm/tools/llvm-dis",
     "//llvm/tools/llvm-dwarfdump",
     "//llvm/tools/llvm-mc",