Index: lib/Driver/Driver.cpp
===================================================================
--- lib/Driver/Driver.cpp
+++ lib/Driver/Driver.cpp
@@ -1508,6 +1508,11 @@
   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.
+  const bool HasSpace = PassedFlags.endswith(",");
+
   // Parse PassedFlags by "," as all the command-line flags are passed to this
   // function separated by ","
   StringRef TargetFlags = PassedFlags;
@@ -1534,6 +1539,16 @@
   if (SuggestedCompletions.empty())
     SuggestedCompletions = Opts->suggestValueCompletions(Cur, "");
 
+  // If Flags were empty, it means the user typed `clang [tab]` where we should
+  // list all possible flags. If there was no value completion and the user
+  // pressed tab after a space, we should fall back to a file completion.
+  // We're printing a newline to be consistent with what we print at the end of
+  // this function.
+  if (SuggestedCompletions.empty() && HasSpace && !Flags.empty()) {
+    llvm::outs() << '\n';
+    return;
+  }
+
   // When flag ends with '=' and there was no value completion, return empty
   // string and fall back to the file autocompletion.
   if (SuggestedCompletions.empty() && !Cur.endswith("=")) {
Index: test/Driver/autocomplete.c
===================================================================
--- test/Driver/autocomplete.c
+++ test/Driver/autocomplete.c
@@ -25,6 +25,7 @@
 // STDLIBALL-NEXT: platform
 // RUN: %clang --autocomplete=-meabi,d | FileCheck %s -check-prefix=MEABI
 // MEABI: default
+// RUN: %clang --autocomplete=-meabi, | FileCheck %s -check-prefix=MEABIALL
 // RUN: %clang --autocomplete=-meabi | FileCheck %s -check-prefix=MEABIALL
 // MEABIALL: 4
 // MEABIALL-NEXT: 5
@@ -121,3 +122,8 @@
 // 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