Index: clang/lib/Driver/Driver.cpp
===================================================================
--- clang/lib/Driver/Driver.cpp
+++ clang/lib/Driver/Driver.cpp
@@ -1508,6 +1508,13 @@
   unsigned short DisableFlags =
       options::NoDriverOption | options::Unsupported | options::Ignored;
 
+  // Distinguish "--autocomplete=-someflag" and "--autocomplete=-someflag,"
+  // because the latter indicates that the user put space before pushing tab
+  // which should end up in a file completion.
+  bool HasSpace = false;
+  if (PassedFlags.endswith(","))
+    HasSpace = true;
+
   // Parse PassedFlags by "," as all the command-line flags are passed to this
   // function separated by ","
   StringRef TargetFlags = PassedFlags;
@@ -1525,6 +1532,14 @@
 
   StringRef Cur;
   Cur = Flags.at(Flags.size() - 1);
+
+  // If Flags were empty, it means the user typed `clang [tab]` where we should
+  // list all possible flags.
+  if (HasSpace && !Flags.empty()) {
+    llvm::outs() << '\n';
+    return;
+  }
+
   StringRef Prev;
   if (Flags.size() >= 2) {
     Prev = Flags.at(Flags.size() - 2);
Index: clang/test/Driver/autocomplete.c
===================================================================
--- clang/test/Driver/autocomplete.c
+++ clang/test/Driver/autocomplete.c
@@ -121,3 +121,8 @@
 // MODULE_FILE_EQUAL-NOT: -fmodule-file=
 // RUN: %clang --autocomplete=-fmodule-file | FileCheck %s -check-prefix=MODULE_FILE
 // MODULE_FILE: -fmodule-file=
+
+// RUN: %clang --autocomplete=-Qunused-arguments, | FileCheck %s -check-prefix=QUNUSED_COMMA
+// QUNUSED_COMMA-NOT: -Qunused-arguments
+// RUN: %clang --autocomplete=-Qunused-arguments | FileCheck %s -check-prefix=QUNUSED
+// QUNUSED: -Qunused-arguments