diff --git a/include/llvm/Support/YAMLParser.h b/include/llvm/Support/YAMLParser.h
index 44daf785090..7baf69eebac 100644
--- a/include/llvm/Support/YAMLParser.h
+++ b/include/llvm/Support/YAMLParser.h
@@ -40,6 +40,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/SMLoc.h"
+#include "llvm/Support/SourceMgr.h"
 #include <cassert>
 #include <cstddef>
 #include <iterator>
@@ -51,7 +52,6 @@
 namespace llvm {
 
 class MemoryBufferRef;
-class SourceMgr;
 class raw_ostream;
 class Twine;
 
@@ -100,7 +100,8 @@ public:
     return !failed();
   }
 
-  void printError(Node *N, const Twine &Msg);
+  void printError(Node *N, const Twine &Msg,
+                  SourceMgr::DiagKind Kind = SourceMgr::DK_Error);
 
 private:
   friend class Document;
diff --git a/include/llvm/Support/YAMLTraits.h b/include/llvm/Support/YAMLTraits.h
index acb1d61cf56..f32a6ca92d2 100644
--- a/include/llvm/Support/YAMLTraits.h
+++ b/include/llvm/Support/YAMLTraits.h
@@ -789,6 +789,7 @@ public:
   virtual NodeKind getNodeKind() = 0;
 
   virtual void setError(const Twine &) = 0;
+  virtual void setAllowUnknownKeys(bool Allow);
 
   template <typename T>
   void enumCase(T &Val, const char* Str, const T ConstVal) {
@@ -1495,6 +1496,9 @@ private:
   void setError(HNode *hnode, const Twine &message);
   void setError(Node *node, const Twine &message);
 
+  void reportWarning(HNode *hnode, const Twine &message);
+  void reportWarning(Node *hnode, const Twine &message);
+
 public:
   // These are only used by operator>>. They could be private
   // if those templated things could be made friends.
@@ -1504,6 +1508,8 @@ public:
   /// Returns the current node that's being parsed by the YAML Parser.
   const Node *getCurrentNode() const;
 
+  void setAllowUnknownKeys(bool Allow) override;
+
 private:
   SourceMgr                           SrcMgr; // must be before Strm
   std::unique_ptr<llvm::yaml::Stream> Strm;
@@ -1514,6 +1520,7 @@ private:
   std::vector<bool>                   BitValuesUsed;
   HNode *CurrentNode = nullptr;
   bool                                ScalarMatchFound = false;
+  bool AllowUnknownKeys = false;
 };
 
 ///
diff --git a/lib/Support/YAMLParser.cpp b/lib/Support/YAMLParser.cpp
index ca8ffdc47af..b9bbdc33a43 100644
--- a/lib/Support/YAMLParser.cpp
+++ b/lib/Support/YAMLParser.cpp
@@ -1775,12 +1775,9 @@ Stream::~Stream() = default;
 
 bool Stream::failed() { return scanner->failed(); }
 
-void Stream::printError(Node *N, const Twine &Msg) {
+void Stream::printError(Node *N, const Twine &Msg, SourceMgr::DiagKind Kind) {
   SMRange Range = N ? N->getSourceRange() : SMRange();
-  scanner->printError( Range.Start
-                     , SourceMgr::DK_Error
-                     , Msg
-                     , Range);
+  scanner->printError(Range.Start, Kind, Msg, Range);
 }
 
 document_iterator Stream::begin() {
diff --git a/lib/Support/YAMLTraits.cpp b/lib/Support/YAMLTraits.cpp
index 9ac7c65e19f..8a5bb653d7a 100644
--- a/lib/Support/YAMLTraits.cpp
+++ b/lib/Support/YAMLTraits.cpp
@@ -48,6 +48,10 @@ void IO::setContext(void *Context) {
   Ctxt = Context;
 }
 
+void IO::setAllowUnknownKeys(bool Allow) {
+  llvm_unreachable("Only supported for Input");
+}
+
 //===----------------------------------------------------------------------===//
 //  Input
 //===----------------------------------------------------------------------===//
@@ -197,8 +201,12 @@ void Input::endMapping() {
     return;
   for (const auto &NN : MN->Mapping) {
     if (!is_contained(MN->ValidKeys, NN.first())) {
-      setError(NN.second.get(), Twine("unknown key '") + NN.first() + "'");
-      break;
+      HNode *ReportNode = NN.second.get();
+      if (!AllowUnknownKeys) {
+        setError(ReportNode, Twine("unknown key '") + NN.first() + "'");
+        break;
+      } else
+        reportWarning(ReportNode, Twine("unknown key '") + NN.first() + "'");
     }
   }
 }
@@ -370,6 +378,11 @@ void Input::setError(Node *node, const Twine &message) {
   EC = make_error_code(errc::invalid_argument);
 }
 
+void Input::reportWarning(HNode *hnode, const Twine &message) {
+  assert(hnode && "HNode must not be NULL");
+  Strm->printError(hnode->_node, message, SourceMgr::DK_Warning);
+}
+
 std::unique_ptr<Input::HNode> Input::createHNodes(Node *N) {
   SmallString<128> StringStorage;
   if (ScalarNode *SN = dyn_cast<ScalarNode>(N)) {
@@ -428,6 +441,8 @@ void Input::setError(const Twine &Message) {
   setError(CurrentNode, Message);
 }
 
+void Input::setAllowUnknownKeys(bool Allow) { AllowUnknownKeys = Allow; }
+
 bool Input::canElideEmptySequence() {
   return false;
 }
diff --git a/unittests/ObjectYAML/YAMLTest.cpp b/unittests/ObjectYAML/YAMLTest.cpp
index 3d283a03b22..53582a98721 100644
--- a/unittests/ObjectYAML/YAMLTest.cpp
+++ b/unittests/ObjectYAML/YAMLTest.cpp
@@ -35,3 +35,21 @@ TEST(ObjectYAML, BinaryRef) {
   YOut << BH;
   EXPECT_NE(OS.str().find("''"), StringRef::npos);
 }
+
+TEST(ObjectYAML, UnknownOption) {
+  StringRef InputYAML = "InvalidKey: InvalidValue\n"
+                        "Binary: AAAA\n";
+  BinaryHolder BH;
+  yaml::Input Input(InputYAML);
+  // test 1: default in trying to parse invalid key is an error case.
+  Input >> BH;
+  EXPECT_EQ(Input.error().value(), 22);
+
+  // test 2: only warn about invalid key if actively set.
+  yaml::Input Input2(InputYAML);
+  BinaryHolder BH2;
+  Input2.setAllowUnknownKeys(true);
+  Input2 >> BH2;
+  EXPECT_EQ(BH2.Binary, yaml::BinaryRef("AAAA"));
+  EXPECT_EQ(Input2.error().value(), 0);
+}