Index: COFF/Driver.cpp
===================================================================
--- COFF/Driver.cpp
+++ COFF/Driver.cpp
@@ -508,26 +508,65 @@
   return Data.str();
 }
 
-static unsigned getDefaultDebugType(const opt::InputArgList &Args) {
-  unsigned DebugTypes = static_cast<unsigned>(DebugType::CV);
+enum class DebugKind { Unknown, None, Full, FastLink, GHash, Dwarf, Symtab };
+
+static DebugKind parseDebugKind(const opt::InputArgList &Args) {
+  auto *A = Args.getLastArg(OPT_debug, OPT_debug_opt);
+  if (!A)
+    return DebugKind::None;
+  if (A->getNumValues() == 0)
+    return DebugKind::Full;
+
+  DebugKind Debug = StringSwitch<DebugKind>(A->getValue())
+                     .CaseLower("none", DebugKind::None)
+                     .CaseLower("full", DebugKind::Full)
+                     .CaseLower("fastlink", DebugKind::FastLink)
+                     // LLD extensions
+                     .CaseLower("ghash", DebugKind::GHash)
+                     .CaseLower("dwarf", DebugKind::Dwarf)
+                     .CaseLower("symtab", DebugKind::Symtab)
+                     .Default(DebugKind::Unknown);
+
+  if (Debug == DebugKind::FastLink) {
+    warn("/debug:fastlink unsupported; using /debug:full");
+    return DebugKind::Full;
+  }
+  if (Debug == DebugKind::Unknown) {
+    error("/debug: unknown option: " + Twine(A->getValue()));
+    return DebugKind::None;
+  }
+  return Debug;
+}
+
+static unsigned parseDebugTypes(const opt::InputArgList &Args) {
+  unsigned DebugTypes = static_cast<unsigned>(DebugType::None);
+
+  if (auto *A = Args.getLastArg(OPT_debugtype)) {
+    SmallVector<StringRef, 3> Types;
+    A->getSpelling().split(Types, ',', /*KeepEmpty=*/false);
+
+    for (StringRef Type : Types) {
+      unsigned V = StringSwitch<unsigned>(Type.lower())
+                       .Case("cv", static_cast<unsigned>(DebugType::CV))
+                       .Case("pdata", static_cast<unsigned>(DebugType::PData))
+                       .Case("fixup", static_cast<unsigned>(DebugType::Fixup))
+                       .Default(0);
+      if (V == 0) {
+        warn("/debugtype: unknown option: " + Twine(A->getValue()));
+        continue;
+      }
+      DebugTypes |= V;
+    }
+    return DebugTypes;
+  }
+
+  // Default debug types
+  DebugTypes = static_cast<unsigned>(DebugType::CV);
   if (Args.hasArg(OPT_driver))
     DebugTypes |= static_cast<unsigned>(DebugType::PData);
   if (Args.hasArg(OPT_profile))
     DebugTypes |= static_cast<unsigned>(DebugType::Fixup);
-  return DebugTypes;
-}
 
-static unsigned parseDebugType(StringRef Arg) {
-  SmallVector<StringRef, 3> Types;
-  Arg.split(Types, ',', /*KeepEmpty=*/false);
-
-  unsigned DebugTypes = static_cast<unsigned>(DebugType::None);
-  for (StringRef Type : Types)
-    DebugTypes |= StringSwitch<unsigned>(Type.lower())
-                      .Case("cv", static_cast<unsigned>(DebugType::CV))
-                      .Case("pdata", static_cast<unsigned>(DebugType::PData))
-                      .Case("fixup", static_cast<unsigned>(DebugType::Fixup))
-                      .Default(0);
   return DebugTypes;
 }
 
@@ -895,17 +934,18 @@
     Config->ForceMultiple = true;
 
   // Handle /debug
-  if (Args.hasArg(OPT_debug, OPT_debug_dwarf, OPT_debug_ghash)) {
+  DebugKind Debug = parseDebugKind(Args);
+
+  bool ShouldCreatePDB = false;
+  if (Debug == DebugKind::Full || Debug == DebugKind::Dwarf ||
+      Debug == DebugKind::GHash) {
     Config->Debug = true;
     Config->Incremental = true;
-    if (auto *Arg = Args.getLastArg(OPT_debugtype))
-      Config->DebugTypes = parseDebugType(Arg->getValue());
-    else
-      Config->DebugTypes = getDefaultDebugType(Args);
+    Config->DebugTypes = parseDebugTypes(Args);
+    ShouldCreatePDB = Debug != DebugKind::Dwarf;
   }
 
   // Handle /pdb
-  bool ShouldCreatePDB = Args.hasArg(OPT_debug, OPT_debug_ghash);
   if (ShouldCreatePDB) {
     if (auto *Arg = Args.getLastArg(OPT_pdb))
       Config->PDBPath = Arg->getValue();
@@ -1026,7 +1066,7 @@
     Config->Implib = Arg->getValue();
 
   // Handle /opt.
-  bool DoGC = !Args.hasArg(OPT_debug) || Args.hasArg(OPT_profile);
+  bool DoGC = Debug == DebugKind::None || Args.hasArg(OPT_profile);
   unsigned ICFLevel =
       Args.hasArg(OPT_profile) ? 0 : 1; // 0: off, 1: limited, 2: on
   unsigned TailMerge = 1;
@@ -1170,9 +1210,9 @@
   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 = Debug == DebugKind::Dwarf;
+  Config->DebugGHashes = Debug == DebugKind::GHash;
+  Config->DebugSymtab = Debug == DebugKind::Symtab;
 
   Config->MapFile = getMapFile(Args);
 
Index: COFF/Options.td
===================================================================
--- COFF/Options.td
+++ COFF/Options.td
@@ -85,7 +85,7 @@
     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>;
+def debug_opt : P<"debug", "Embed a symbol table in the image with option">;
 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">;
@@ -142,9 +142,6 @@
 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