Index: lld/COFF/DriverUtils.cpp
===================================================================
--- lld/COFF/DriverUtils.cpp
+++ lld/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/COFF/InputFiles.h
===================================================================
--- lld/COFF/InputFiles.h
+++ lld/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 Any difference indicates out-of-date objects.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/COFF/InputFiles.cpp
===================================================================
--- lld/COFF/InputFiles.cpp
+++ lld/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/COFF/SymbolTable.h
===================================================================
--- lld/COFF/SymbolTable.h
+++ lld/COFF/SymbolTable.h
@@ -119,6 +119,8 @@
 
   llvm::DenseMap<llvm::CachedHashStringRef, Symbol *> SymMap;
   std::unique_ptr<BitcodeCompiler> LTO;
+
+  ObjFile *ResourceObjFile = nullptr;
 };
 
 extern SymbolTable *Symtab;
Index: lld/COFF/SymbolTable.cpp
===================================================================
--- lld/COFF/SymbolTable.cpp
+++ lld/COFF/SymbolTable.cpp
@@ -44,6 +44,15 @@
   }
 
   if (auto *F = dyn_cast<ObjFile>(File)) {
+    if (F->IsResourceObjFile) {
+      if (!ResourceObjFile)
+        ResourceObjFile = F;
+      else
+        error(toString(File) +
+              ": more than one resource obj file not allowed, already got " +
+              toString(ResourceObjFile));
+    }
+
     ObjFile::Instances.push_back(F);
   } else if (auto *F = dyn_cast<BitcodeFile>(File)) {
     BitcodeFile::Instances.push_back(F);
Index: lld/test/CMakeLists.txt
===================================================================
--- lld/test/CMakeLists.txt
+++ lld/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/test/COFF/multiple-resource-objs.test
===================================================================
--- /dev/null
+++ lld/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/lib/Object/WindowsResource.cpp
===================================================================
--- llvm/lib/Object/WindowsResource.cpp
+++ llvm/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() {
@@ -527,7 +528,7 @@
   Header->NumberOfSections = 2;
   Header->TimeDateStamp = getTime();
   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;
   Header->Characteristics = COFF::IMAGE_FILE_32BIT_MACHINE;
Index: llvm/utils/gn/secondary/lld/test/BUILD.gn
===================================================================
--- llvm/utils/gn/secondary/lld/test/BUILD.gn
+++ llvm/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",