Index: COFF/Driver.cpp
===================================================================
--- COFF/Driver.cpp
+++ COFF/Driver.cpp
@@ -497,6 +497,27 @@
   return Data.str();
 }
 
+enum class DebugGeneration {
+  Unknown,
+  None,
+  Full,
+  FastLink,
+  GHash,
+  Dwarf,
+  Symtab
+};
+
+static DebugGeneration parseDebugGeneration(StringRef Arg) {
+  return StringSwitch<DebugGeneration>(Arg.lower())
+      .Case("none", DebugGeneration::None)
+      .Case("full", DebugGeneration::Full)
+      .Case("fastlink", DebugGeneration::FastLink)
+      .Case("ghash", DebugGeneration::GHash)
+      .Case("dwarf", DebugGeneration::Dwarf)
+      .Case("symtab", DebugGeneration::Symtab)
+      .Default(DebugGeneration::Unknown);
+}
+
 static unsigned getDefaultDebugType(const opt::InputArgList &Args) {
   unsigned DebugTypes = static_cast<unsigned>(DebugType::CV);
   if (Args.hasArg(OPT_driver))
@@ -840,27 +861,44 @@
     Config->Force = true;
 
   // Handle /debug
-  if (Args.hasArg(OPT_debug, OPT_debug_dwarf, OPT_debug_ghash)) {
-    Config->Debug = true;
-    Config->Incremental = true;
-    if (auto *Arg = Args.getLastArg(OPT_debugtype))
-      Config->DebugTypes = parseDebugType(Arg->getValue());
-    else
-      Config->DebugTypes = getDefaultDebugType(Args);
-  }
-
-  // Handle /pdb
-  bool ShouldCreatePDB = Args.hasArg(OPT_debug, OPT_debug_ghash);
-  if (ShouldCreatePDB) {
-    if (auto *Arg = Args.getLastArg(OPT_pdb))
-      Config->PDBPath = Arg->getValue();
-    if (auto *Arg = Args.getLastArg(OPT_pdbaltpath))
-      Config->PDBAltPath = Arg->getValue();
-    if (Args.hasArg(OPT_natvis))
-      Config->NatvisFiles = Args.getAllArgValues(OPT_natvis);
+  DebugGeneration DebugGen = DebugGeneration::None;
+  bool ShouldCreatePDB = false;
+  if (auto *DebugArg = Args.getLastArg(OPT_debug, OPT_debug_opt)) {
+    DebugGen = DebugGeneration::Full;
+    if (DebugArg->getNumValues() != 0) {
+      DebugGen = parseDebugGeneration(DebugArg->getValue());
+      if (DebugGen == DebugGeneration::FastLink) {
+        warn("/debug:fastlink unsupported; using /debug:full");
+        DebugGen = DebugGeneration::Full;
+      } else if (DebugGen == DebugGeneration::Unknown)
+        error("/debug: unknown option: " + Twine(DebugArg->getValue()));
+    }
 
-    if (auto *Arg = Args.getLastArg(OPT_pdb_source_path))
-      Config->PDBSourcePath = Arg->getValue();
+    if (DebugGen == DebugGeneration::Full ||
+        DebugGen == DebugGeneration::Dwarf ||
+        DebugGen == DebugGeneration::GHash) {
+      Config->Debug = true;
+      Config->Incremental = true;
+      if (auto *Arg = Args.getLastArg(OPT_debugtype))
+        Config->DebugTypes = parseDebugType(Arg->getValue());
+      else
+        Config->DebugTypes = getDefaultDebugType(Args);
+
+      // Handle /pdb
+      ShouldCreatePDB = DebugGen == DebugGeneration::Full ||
+                        DebugGen == DebugGeneration::GHash;
+      if (ShouldCreatePDB) {
+        if (auto *Arg = Args.getLastArg(OPT_pdb))
+          Config->PDBPath = Arg->getValue();
+        if (auto *Arg = Args.getLastArg(OPT_pdbaltpath))
+          Config->PDBAltPath = Arg->getValue();
+        if (Args.hasArg(OPT_natvis))
+          Config->NatvisFiles = Args.getAllArgValues(OPT_natvis);
+
+        if (auto *Arg = Args.getLastArg(OPT_pdb_source_path))
+          Config->PDBSourcePath = Arg->getValue();
+      }
+    }
   }
 
   // Handle /noentry
@@ -971,7 +1009,7 @@
     Config->Implib = Arg->getValue();
 
   // Handle /opt.
-  bool DoGC = !Args.hasArg(OPT_debug) || Args.hasArg(OPT_profile);
+  bool DoGC = DebugGen == DebugGeneration::None || Args.hasArg(OPT_profile);
   unsigned ICFLevel =
       Args.hasArg(OPT_profile) ? 0 : 1; // 0: off, 1: limited, 2: on
   unsigned TailMerge = 1;
@@ -1109,9 +1147,9 @@
   Config->NxCompat = Args.hasFlag(OPT_nxcompat, OPT_nxcompat_no, true);
   Config->TerminalServerAware =
       !Config->DLL && Args.hasFlag(OPT_tsaware, OPT_tsaware_no, true);
-  Config->DebugDwarf = Args.hasArg(OPT_debug_dwarf);
-  Config->DebugGHashes = Args.hasArg(OPT_debug_ghash);
-  Config->DebugSymtab = Args.hasArg(OPT_debug_symtab);
+  Config->DebugDwarf = DebugGen == DebugGeneration::Dwarf;
+  Config->DebugGHashes = DebugGen == DebugGeneration::GHash;
+  Config->DebugSymtab = DebugGen == DebugGeneration::Symtab;
 
   Config->MapFile = getMapFile(Args);
 
Index: COFF/Options.td
===================================================================
--- COFF/Options.td
+++ COFF/Options.td
@@ -9,6 +9,12 @@
 class P<string name, string help> :
       Joined<["/", "-", "-?"], name#":">, HelpText<help>;
 
+// Flag that may take optional argument after ":".
+multiclass POpt<string name, string help> {
+  def NAME: F<name>;
+  def NAME # _opt: P<name, help>;
+}
+
 // Boolean flag which can be suffixed by ":no". Using it unsuffixed turns the
 // flag on and using it suffixed by ":no" turns it off.
 multiclass B<string name, string help_on, string help_off> {
@@ -84,8 +90,7 @@
 def deffile : Joined<["/", "-"], "def:">,
     HelpText<"Use module-definition file">;
 
-def debug : F<"debug">, HelpText<"Embed a symbol table in the image">;
-def debug_full : F<"debug:full">, Alias<debug>;
+defm debug : POpt<"debug", "Embed a symbol table in the image">;
 def debugtype : P<"debugtype", "Debug Info Options">;
 def dll : F<"dll">, HelpText<"Create a DLL">;
 def driver : P<"driver", "Generate a Windows NT Kernel Mode Driver">;
@@ -139,9 +144,6 @@
 def help_q : Flag<["/?", "-?"], "">, Alias<help>;
 
 // LLD extensions
-def debug_ghash : F<"debug:ghash">;
-def debug_dwarf : F<"debug:dwarf">;
-def debug_symtab : F<"debug:symtab">;
 def export_all_symbols : F<"export-all-symbols">;
 def kill_at : F<"kill-at">;
 def lldmingw : F<"lldmingw">;
Index: test/COFF/debug-fastlink.test
===================================================================
--- test/COFF/debug-fastlink.test
+++ test/COFF/debug-fastlink.test
@@ -0,0 +1,12 @@
+# RUN: yaml2obj < %p/Inputs/pdb1.yaml > %t1.obj
+# RUN: yaml2obj < %p/Inputs/pdb2.yaml > %t2.obj
+
+; If /DEBUG:FASTLINK is specified, /DEBUG:FULL is used instead
+# RUN: rm -f %t.pdb
+# RUN: lld-link /DEBUG /pdb:%t.pdb /DEBUG:FASTLINK /entry:main /nodefaultlib %t1.obj %t2.obj \
+# RUN: 2>&1 | FileCheck %s
+
+# CHECK: /debug:fastlink unsupported; using /debug:full
+
+# RUN: ls %t.pdb
+
Index: test/COFF/invalid-debug.test
===================================================================
--- test/COFF/invalid-debug.test
+++ test/COFF/invalid-debug.test
@@ -0,0 +1,6 @@
+# RUN: yaml2obj < %p/Inputs/pdb1.yaml > %t1.obj
+# RUN: yaml2obj < %p/Inputs/pdb2.yaml > %t2.obj
+# RUN: not lld-link /debug /debug:invalid /pdb:%t.pdb /dll /out:%t.dll /entry:main /nodefaultlib \
+# RUN:   %t1.obj %t2.obj 2>&1 | FileCheck %s
+
+# CHECK: /debug: unknown option: invalid
Index: test/COFF/pdb-options.test
===================================================================
--- test/COFF/pdb-options.test
+++ test/COFF/pdb-options.test
@@ -6,6 +6,11 @@
 # RUN: lld-link /pdb:%t.pdb /entry:main /nodefaultlib %t1.obj %t2.obj
 # RUN: not ls %t.pdb
 
+; If /DEBUG:NONE is specified after /DEBUG, /pdb is ignored.
+# RUN: rm -f %t.pdb
+# RUN: lld-link /DEBUG /pdb:%t.pdb /DEBUG:NONE /entry:main /nodefaultlib %t1.obj %t2.obj
+# RUN: not ls %t.pdb
+
 ; If /DEBUG and /pdb are specified, it uses the specified name.
 # RUN: lld-link /DEBUG /pdb:%t.pdb /entry:main /nodefaultlib %t1.obj %t2.obj
 # RUN: ls %t.pdb