diff --git a/include/swift/AST/DiagnosticsParse.def b/include/swift/AST/DiagnosticsParse.def
index 3e6a8d8f99e..b420d438b4d 100644
--- a/include/swift/AST/DiagnosticsParse.def
+++ b/include/swift/AST/DiagnosticsParse.def
@@ -1585,8 +1585,8 @@ ERROR(availability_query_repeated_platform, none,
 //------------------------------------------------------------------------------
 // syntax parsing diagnostics
 //------------------------------------------------------------------------------
-WARNING(unknown_syntax_entity, PointsToFirstBadToken,
-        "unknown %0 syntax exists in the source", (StringRef))
+ERROR(unknown_syntax_entity, PointsToFirstBadToken,
+      "unknown %0 syntax exists in the source", (StringRef))
 
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
diff --git a/include/swift/Option/FrontendOptions.td b/include/swift/Option/FrontendOptions.td
index 7308af13e61..c09c6a3b7e1 100644
--- a/include/swift/Option/FrontendOptions.td
+++ b/include/swift/Option/FrontendOptions.td
@@ -81,6 +81,9 @@ def verify_generic_signatures : Separate<["-"], "verify-generic-signatures">,
   MetaVarName<"<module-name>">,
   HelpText<"Verify the generic signatures in the given module">;
 
+def verify_syntax_tree : Flag<["-"], "verify-syntax-tree">,
+  HelpText<"Verify that no unknown nodes exist in the libSyntax tree">;
+
 def show_diagnostics_after_fatal : Flag<["-"], "show-diagnostics-after-fatal">,
   HelpText<"Keep emitting subsequent diagnostics after a fatal error">;
 
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 8af50c86407..21ef308c12d 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -214,6 +214,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.NamedLazyMemberLoading &= !Args.hasArg(OPT_disable_named_lazy_member_loading);
   Opts.DebugGenericSignatures |= Args.hasArg(OPT_debug_generic_signatures);
 
+  if (Args.hasArg(OPT_verify_syntax_tree)) {
+    Opts.BuildSyntaxTree = true;
+    Opts.VerifySyntaxTree = true;
+  }
+
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);
   if (Opts.DebuggerSupport)
     Opts.EnableDollarIdentifiers = true;
diff --git a/test/Syntax/syntax_diagnostics.swift b/test/Syntax/syntax_diagnostics.swift
index d17ab38b363..e97652902e6 100644
--- a/test/Syntax/syntax_diagnostics.swift
+++ b/test/Syntax/syntax_diagnostics.swift
@@ -1,3 +1,3 @@
 // RUN: %target-swift-frontend -emit-syntax -primary-file %s -verify
 
-typealias Inner: Foo // expected-warning{{unknown declaration syntax exists in the source}} expected-error{{expected '=' in type alias declaration}}
+typealias Inner: Foo // expected-error{{unknown declaration syntax exists in the source}} expected-error{{expected '=' in type alias declaration}}