Index: lld/trunk/COFF/Driver.cpp
===================================================================
--- lld/trunk/COFF/Driver.cpp
+++ lld/trunk/COFF/Driver.cpp
@@ -1699,6 +1699,14 @@
   Symtab->addCombinedLTOObjects();
   run();
 
+  if (Args.hasArg(OPT_include_optional)) {
+    // Handle /includeoptional
+    for (auto *Arg : Args.filtered(OPT_include_optional))
+      if (dyn_cast_or_null<Lazy>(Symtab->find(Arg->getValue())))
+        addUndefined(Arg->getValue());
+    while (run());
+  }
+
   if (Config->MinGW) {
     // Load any further object files that might be needed for doing automatic
     // imports.
Index: lld/trunk/COFF/Options.td
===================================================================
--- lld/trunk/COFF/Options.td
+++ lld/trunk/COFF/Options.td
@@ -167,6 +167,8 @@
 defm demangle : B<"demangle",
     "Demangle symbols in output (default)",
     "Do not demangle symbols in output">;
+def include_optional : Joined<["/", "-", "/?", "-?"], "includeoptional:">,
+    HelpText<"Add symbol as undefined, but allow it to remain undefined">;
 def kill_at : F<"kill-at">;
 def lldmingw : F<"lldmingw">;
 def output_def : Joined<["/", "-", "/?", "-?"], "output-def:">;
Index: lld/trunk/test/COFF/includeoptional.yaml
===================================================================
--- lld/trunk/test/COFF/includeoptional.yaml
+++ lld/trunk/test/COFF/includeoptional.yaml
@@ -0,0 +1,39 @@
+# RUN: yaml2obj < %s > %t.main.obj
+# RUN: yaml2obj < %p/Inputs/include1c.yaml > %t.lib.obj
+# RUN: rm -f %t.lib.lib
+# RUN: llvm-ar cru %t.lib.lib %t.lib.obj
+# RUN: lld-link /out:%t.exe /entry:main /includeoptional:bar /includeoptional:actuallymissing %t.main.obj %t.lib.lib /verbose >& %t.log
+# RUN: FileCheck %s < %t.log
+
+# CHECK: includeoptional.yaml.tmp.main.obj
+# CHECK: includeoptional.yaml.tmp.lib.lib
+# CHECK: includeoptional.yaml.tmp.lib.lib(includeoptional.yaml.tmp.lib.obj) for bar
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: []
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     B800000000506800000000680000000050E80000000050E800000000
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          28
+      NumberOfRelocations: 4
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+  - Name:            main
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...
Index: lld/trunk/test/COFF/includeoptional2.yaml
===================================================================
--- lld/trunk/test/COFF/includeoptional2.yaml
+++ lld/trunk/test/COFF/includeoptional2.yaml
@@ -0,0 +1,44 @@
+# RUN: yaml2obj < %s > %t.obj
+# RUN: not lld-link /out:%t.exe /entry:main /includeoptional:undeffunc %t.obj /verbose 2>&1 | FileCheck %s
+
+# CHECK: error: undefined symbol: undeffunc
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: []
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     B800000000506800000000680000000050E80000000050E800000000
+    Relocations:
+      - VirtualAddress:  17
+        SymbolName:      undeffunc
+        Type:            IMAGE_REL_AMD64_REL32
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          28
+      NumberOfRelocations: 1
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+  - Name:            main
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            undeffunc
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...