Index: llvm/lib/MC/MCAsmInfoCOFF.cpp
===================================================================
--- llvm/lib/MC/MCAsmInfoCOFF.cpp
+++ llvm/lib/MC/MCAsmInfoCOFF.cpp
@@ -25,7 +25,7 @@
   COMMDirectiveAlignmentIsInBytes = false;
   LCOMMDirectiveAlignmentType = LCOMM::ByteAlignment;
   HasDotTypeDotSizeDirective = false;
-  HasSingleParameterDotFile = false;
+  HasSingleParameterDotFile = true;
   WeakRefDirective = "\t.weak\t";
   HasLinkOnceDirective = true;
 
Index: llvm/lib/MC/MCObjectStreamer.cpp
===================================================================
--- llvm/lib/MC/MCObjectStreamer.cpp
+++ llvm/lib/MC/MCObjectStreamer.cpp
@@ -704,6 +704,7 @@
 }
 
 void MCObjectStreamer::EmitFileDirective(StringRef Filename) {
+  assert(MAI->hasSingleParameterDotFile());
   getAssembler().addFileName(Filename);
 }
 
Index: llvm/lib/MC/MCParser/AsmParser.cpp
===================================================================
--- llvm/lib/MC/MCParser/AsmParser.cpp
+++ llvm/lib/MC/MCParser/AsmParser.cpp
@@ -3359,9 +3359,12 @@
     }
   }
 
-  if (FileNumber == -1)
+  if (FileNumber == -1) {
+    if (!getContext().getAsmInfo()->hasSingleParameterDotFile())
+      return Error(DirectiveLoc,
+                   "target does not support '.file' without a number");
     getStreamer().EmitFileDirective(Filename);
-  else {
+  } else {
     // In case there is a -g option as well as debug info from directive .file,
     // we turn off the -g option, directly use the existing debug info instead.
     // Also reset any implicit ".file 0" for the assembler source.
Index: llvm/test/MC/MachO/file-single.s
===================================================================
--- /dev/null
+++ llvm/test/MC/MachO/file-single.s
@@ -0,0 +1,8 @@
+// RUN: not llvm-mc -triple i386-apple-darwin9 %s -o /dev/null 2>&1 | FileCheck %s
+
+// Previously this crashed MC.
+
+// CHECK: error: target does not support '.file' without a number
+
+        .file "dir/foo"
+        nop