Add -Wno-error=unknown flag to clang-format.

[fodinabor]

Summary

Currently newer clang-format options cannot be included in .clang-format files, if not all users can be forced to use an updated version.
This patch tries to solve this by adding an option to clang-format, enabling to ignore unknown (newer) options.

[fodinabor]

Ping

[MyDeveloperDay]

This has caught me out from time to time, but the presence of such an option can lead to users mixing clang-format versions which could lead to flip/flopping of changes, so I'm not 100% sure it won't drive bad behaviour.

However breaking based on the option can be a pain if the code you are formatting isn't even using the new option

I wonder is it possible to do this without impacting the LLVM support libraries?

[fodinabor]

I see the possible issue with the possible version mismatches and that is why I'd make people opt-in for this option, to still being able to format their files (e.g. if using out-dated built-in versions like in Visual Studio - I know you can specify your own binary) but the real formatting can be done with a pre-commit script or similar, where the new options are actually supported.

Regarding not touching the LLVM support library: I'd love to find a way, but as clang-format uses the >> operator to read the YAML and this operator automatically emits the errors, I don't see any other obvious way.. (maybe there's a LLVM trick to suppress those errors that I haven't seen, yet :))

[MyDeveloperDay]

Regarding not touching the LLVM support library: I'd love to find a way, but as clang-format uses the >> operator

We need to find some reviewers who look after a wider area of LLVM, if you want to make a change out there in LLVM/support, but it above my pay grade to approve ;-) maybe one of the more experienced devs could help identify the correct person @aaron.ballman @sammccall or alternatively take a look via git log as to who has been maintaining that file @JDevlieghere, they may have an opinion about how this should be done.

clang/tools/clang-format/ClangFormat.cpp

108

feels like a mouthful is there nothing shorter we could use? -Wignore (or something)

llvm/include/llvm/Support/YAMLTraits.h

792

I'm not a massive fan of functions with out parameter names, but I see your following the local style.

1523

is this clang-formatted?

llvm/lib/Support/YAMLTraits.cpp

203

do we want to flat out ignore or just report but not fatally. (just a thought) silent failures are hard to diagnose

[fodinabor]

Thank you so far for the feedback!
maybe you can give further guidance on the comments on the comments :)

As of the git history it seems that @grimar did some work on YAML error handling..

clang/tools/clang-format/ClangFormat.cpp

108

hmm... -Wunknown
but the -W does not really make it clear that the default "errors" should now be treated as warnings instead. From compiler conventions, I'd expect the -W to enable a warning ...

and something like -Wno-error=unknown is not really shorter...

llvm/include/llvm/Support/YAMLTraits.h

1523

the patch is... the original alignment of the members is not. (also see the CurrentNode's formatting).
not sure what to do in this case, as the whole file seems rather unformatted?

llvm/lib/Support/YAMLTraits.cpp

203

true.. don't know what's the best option?

keep it as a printed out error and just don't return an error code on exit? This option would make it a clang-format only change, but feels really dirty.

Otherwise I'd have to dig my way through to llvm::yaml::Stream::printError (or maybe rather add a printWarning) to conditionally change the message type for the ignore case.

[grimar]

I am not familar with clang-format, but have a few comments inlined about the rest.
I think the new setIgnoreUnknown YAMLlib API is probably OK generally.
I'd perhaps call it differently, e.g. setAllowUnknownKeys though.

Also, I think you need to add a unit testing for the new functionality.
It seems appropriate places are: clang\unittests\Format\FormatTest.cpp (to test clang-format)
and llvm\unittests\ObjectYAML\YAMLTest.cpp (to test new YAML Input::setIgnoreUnknown API)

I'll add people who might have something useful to say too.

llvm/include/llvm/Support/YAMLTraits.h

1511

I'd add a parameter name here. Seems the style is mixed in this file,
but it is a public member, and having no names for arguments actually
reads bad I think. Not sure why it was done initially.

Probably the same applies for setIgnoreUnknown in the base class.

llvm/lib/Support/YAMLTraits.cpp

203

Yes, I think we might want to introduce a method, like Input::reportWarning
which will call Strm->printError(node, message);, but will not set a EC.
Also, it should print "warning: ..." instead of "error: ..." prefix somehow.

758

You don't actually need it for Output, right?
I think instead you can have a default implementation in the base class, which should call llvm_unreachable.

[JakeMerdichAMD]

I can see the use of this, but I am also wary that ignoring style options will lead to people producing different results on different versions of clang-format. This is both because having set-or-unset an option will naturally lead to different code and also that newer options are a de-facto check that clang-format is at least a certain version (we have minor differences between major versions as bugs are fixed). In any case, I see this being *very* easy to misuse and the documentation should have a warning reflecting that.

As far as docs go, the bulk are in clang/docs/ClangFormat.rst and require no additional 'publish' step but definitely should be updated. The command line options are implicitly generated for the cli tool's --help flag, but the docs for them are all in that rst file and manually maintained.

clang/tools/clang-format/ClangFormat.cpp

108

I personally like -Wno-error=unknown if the behavior is to emit warnings and -Wno-unknown if the behavior is to be silent, for consistency with clang/gcc.

I would also put a note in the description saying that ignoring options can lead to dramatically different output between versions that do and don't support a given option, so it should be used with care.

llvm/lib/Support/YAMLTraits.cpp

203

Something like how clang/gcc only report unknown -Wno-foo options if there's another error is an idea, but clang-format almost never fails unless there's a bad config or input file so that's not too useful.

I'm fine with either warning or being silent. If the user has opted-into ignoring missing options, we can assume they're willing to accept the consequences of such.

[fodinabor]

Incorporating review comments:

• renaming option to -Wno-error=unknown and adding warning in description
• emit warnings instead of fully ignoring the issues

Documentation and unit tests will follow

[fodinabor]

Remove test entry form .clang-format :)

[grimar]

llvm/lib/Support/YAMLTraits.cpp

204–210

Please don't use auto when the variable type is not obvious.
Use the real type name instead.

205

The same here, but also the result type here is Twine, and you shouldn't use it like this. Documentation says:
"A Twine is not intended for use directly and should not be stored, its implementation relies on the ability to store pointers to temporary stack objects which may be deallocated at the end of a statement. Twines should only be used accepted as const references in arguments, when an API wishes to accept possibly-concatenated strings."
(https://llvm.org/doxygen/classllvm_1_1Twine.html#details)

You can probably inline it or use std::string, since this is a error path only code.

387

Why do you need both reportWarning methods? I think you can have only one for now.

[fodinabor]

Address review comments, copy the help text into docs and add some basic unit tests.

[grimar]

clang/unittests/Format/FormatTest.cpp

16079 ↗

(On Diff #291405)

It looks like it is very similar to "Test 7:" and can be a part of it?
E.g:

...
llvm::consumeError(Style7a.takeError());

// <comment>
auto Style7b = getStyle("file", "/d/.clang-format", "LLVM", "", &FS, true);
ASSERT_TRUE((bool)Style8);

llvm/unittests/ObjectYAML/YAMLTest.cpp

39 ↗

(On Diff #291405)

UnknownOption

40 ↗

(On Diff #291405)

std::string->StringRef?

41 ↗

(On Diff #291405)

Will this work if we switch the order of InvalidKey and Binary?

std::string InputYAML = "InvalidKey: InvalidValue\n"
                        "Binary: AAAA";

I expect that yes and I think it is reasonable to do it to show that we don't stop parsing an YAML
when there is an unknown key.

44 ↗

(On Diff #291405)

No full stop at the end.

57 ↗

(On Diff #291405)

Do you need BH_GT? Can it be just:

// test 2: only warn about invalid key if actively set.
llvm::yaml::Input Input2(InputYAML);
BinaryHolder BH2;
Input2.setAllowUnknownKeys(true);
Input2 >> BH2;
EXPECT_EQ(Input2.error().value(), 0);
EXPECT_EQ(BH2.Binary, yaml::BinaryRef("AAAA"));

[fodinabor]

Unit test cleanups.

[fodinabor]

All comments should be adressed now.

[grimar]

LGTM. It worth wainting for a second approvement and/or other comments to verify that people are happy with doing this for clang-format.

clang/unittests/Format/FormatTest.cpp

16062 ↗

(On Diff #291826)

nit: since you have Style7b, this perhaps should be Style7a.

[fodinabor]

Fix the nit :)
Anyone with access may commit this now (ideally ofc someone with a final opinion on the clang-format part).

[MyDeveloperDay]

IMHO I feel that given that these changes are more wide reaching than just clang-format that it might be more correct for your to request commit access yourself.

You need to own these changes, because if the llvm/Support changes fail the build or unit tests the person who commits it is going to have to resolve those issues.

It doesn't take long to request commit access.

[fodinabor]

Okay, I requested commit access.
So the patch is fine with you from a clang-format perspective?

[MyDeveloperDay]

it certainly LGTM, like I said I've had met this issue before and I think this does get around that situation where perhaps your not even using the option it's complaining about (especially if its a C++ option and you are using clang-format for C#)

So yes I think it's good. Thank you for the patch and getting the commit access.

When you commit don't forget to keep an eye on the build_bots at http://lab.llvm.org:8011/ this was something I didn't learn about until after my first commit... (to my cost)

[njames93]

I think the = in the option name is confusing the option parser,

➜  clang-format --Wno-error=unknown
clang-format: Unknown command line argument '--Wno-error=unknown'.  Try: 'clang-format --help'
clang-format: Did you mean '--Werror=unknown'?

Should the name be changed to Wno-error-unknown?

[fodinabor]

I can reproduce the issue, but not sure what introduced this.

In D86137#2460135, @njames93 wrote:

Should the name be changed to Wno-error-unknown?

I don't think this should be the solution, as we aimed for the common syntax using the -Wno-error= style.
I just tried to prepare a patch, changing the way, this is handled, it works already in theory, but the formatting of the --help page is still slightly off (I guess cl::values descriptions are supposed to be single line), so I guess I have to investigate my formatting options here or move the more extensive description somewhere else... if I don't find an obvious solution, I guess we can discuss that after I created the patch..

Clang-format options:

  --Werror                   - If set, changes formatting warnings to errors
  --Wno-error=<value>        - If set don't error out on the specified warning type.
    =unknown                 -   If set, unknown format options are only warned about.
This can be used to enable formatting, even if the
configuration contains unknown (newer) options.
Use with caution, as this might lead to dramatically
differing format depending on an option being
supported or not.
  --assume-filename=<string> - Override filename used to determine the language.
                               When reading from stdin, clang-format assumes this
                               filename to determine the language.
  --cursor=<uint>            - The position of the cursor when invoking
...

Btw. is there a way to add tests for command line options, so this won't happen unnoticed again?

.clang-format

 BasedOnStyle: LLVM
+testbshit: ste
 No newline at end of file

clang/include/clang/Format/Format.h

 
   static FormatStyleSet BuildStyleSetFromConfiguration(
       const FormatStyle &MainStyle,
       const std::vector<FormatStyle> &ConfigurationStyles);
 
 private:
   FormatStyleSet StyleSet;
 
-  friend std::error_code parseConfiguration(StringRef Text, FormatStyle *Style);
+  friend std::error_code parseConfiguration(StringRef Text, FormatStyle *Style,
+                                            bool AllowUnknownOptions);
 };
 
 /// Returns a format style complying with the LLVM coding standards:
 /// http://llvm.org/docs/CodingStandards.html.
 FormatStyle getLLVMStyle(
     FormatStyle::LanguageKind Language = FormatStyle::LanguageKind::LK_Cpp);
 
 /// Returns a format style complying with one of Google's style guides:

 ///
 /// Style->Language is used to get the base style, if the ``BasedOnStyle``
 /// option is present.
 ///
 /// The FormatStyleSet of Style is reset.
 ///
 /// When ``BasedOnStyle`` is not present, options not present in the YAML
 /// document, are retained in \p Style.
-std::error_code parseConfiguration(StringRef Text, FormatStyle *Style);
+///
+/// If AllowUnknownOptions is true, no errors are emitted if unknown
+/// format options are occured.
+std::error_code parseConfiguration(StringRef Text, FormatStyle *Style,
+                                   bool AllowUnknownOptions = false);
 
 /// Gets configuration in a YAML string.
 std::string configurationAsText(const FormatStyle &Style);
 
 /// Returns the replacements necessary to sort all ``#include`` blocks
 /// that are affected by ``Ranges``.
 tooling::Replacements sortIncludes(const FormatStyle &Style, StringRef Code,
                                    ArrayRef<tooling::Range> Ranges,

 /// \param[in] FileName Path to start search for .clang-format if ``StyleName``
 /// == "file".
 /// \param[in] FallbackStyle The name of a predefined style used to fallback to
 /// in case \p StyleName is "file" and no file can be found.
 /// \param[in] Code The actual code to be formatted. Used to determine the
 /// language if the filename isn't sufficient.
 /// \param[in] FS The underlying file system, in which the file resides. By
 /// default, the file system is the real file system.
+/// \param[in] AllowUnknownOptions If true, unknown format options only
+///             emit a warning. If false, errors are emitted on unknown format
+///             options.
 ///
 /// \returns FormatStyle as specified by ``StyleName``. If ``StyleName`` is
 /// "file" and no file is found, returns ``FallbackStyle``. If no style could be
 /// determined, returns an Error.
 llvm::Expected<FormatStyle> getStyle(StringRef StyleName, StringRef FileName,
                                      StringRef FallbackStyle,
                                      StringRef Code = "",
-                                     llvm::vfs::FileSystem *FS = nullptr);
+                                     llvm::vfs::FileSystem *FS = nullptr,
+                                     bool AllowUnknownOptions = false);
 
 // Guesses the language from the ``FileName`` and ``Code`` to be formatted.
 // Defaults to FormatStyle::LK_Cpp.
 FormatStyle::LanguageKind guessLanguage(StringRef FileName, StringRef Code);
 
 // Returns a string representation of ``Language``.
 inline StringRef getLanguageName(FormatStyle::LanguageKind Language) {
   switch (Language) {

clang/lib/Format/Format.cpp

bool getPredefinedStyle(StringRef Name, FormatStyle::LanguageKind Language,
   } else {
     return false;
   }
 
   Style->Language = Language;
   return true;
 }
 
-std::error_code parseConfiguration(StringRef Text, FormatStyle *Style) {
+std::error_code parseConfiguration(StringRef Text, FormatStyle *Style,
+                                   bool AllowUnknownOptions) {
   assert(Style);
   FormatStyle::LanguageKind Language = Style->Language;
   assert(Language != FormatStyle::LK_None);
   if (Text.trim().empty())
     return make_error_code(ParseError::Error);
   Style->StyleSet.Clear();
   std::vector<FormatStyle> Styles;
   llvm::yaml::Input Input(Text);
   // DocumentListTraits<vector<FormatStyle>> uses the context to get default
   // values for the fields, keys for which are missing from the configuration.
   // Mapping also uses the context to get the language to find the correct
   // base style.
   Input.setContext(Style);
+  Input.setAllowUnknownKeys(AllowUnknownOptions);
   Input >> Styles;
   if (Input.error())
     return Input.error();
 
   for (unsigned i = 0; i < Styles.size(); ++i) {
     // Ensures that only the first configuration can skip the Language option.
     if (Styles[i].Language == FormatStyle::LK_None && i != 0)
       return make_error_code(ParseError::Error);

 }
 
 const char *DefaultFormatStyle = "file";
 
 const char *DefaultFallbackStyle = "LLVM";
 
 llvm::Expected<FormatStyle> getStyle(StringRef StyleName, StringRef FileName,
                                      StringRef FallbackStyleName,
-                                     StringRef Code,
-                                     llvm::vfs::FileSystem *FS) {
+                                     StringRef Code, llvm::vfs::FileSystem *FS,
+                                     bool AllowUnknownOptions) {
   if (!FS) {
     FS = llvm::vfs::getRealFileSystem().get();
   }
   FormatStyle Style = getLLVMStyle(guessLanguage(FileName, Code));
 
   FormatStyle FallbackStyle = getNoStyle();
   if (!getPredefinedStyle(FallbackStyleName, Style.Language, &FallbackStyle))
     return make_string_error("Invalid fallback style \"" + FallbackStyleName);
 
   if (StyleName.startswith("{")) {
     // Parse YAML/JSON style from the command line.
-    if (std::error_code ec = parseConfiguration(StyleName, &Style))
+    if (std::error_code ec =
+            parseConfiguration(StyleName, &Style, AllowUnknownOptions))
       return make_string_error("Error parsing -style: " + ec.message());
     return Style;
   }
 
   if (!StyleName.equals_lower("file")) {
     if (!getPredefinedStyle(StyleName, Style.Language, &Style))
       return make_string_error("Invalid value for -style");
     return Style;
for (const auto &F : FilesToLookFor) {
       Status = FS->status(ConfigFile.str());
 
       if (Status &&
           (Status->getType() == llvm::sys::fs::file_type::regular_file)) {
         llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text =
             FS->getBufferForFile(ConfigFile.str());
         if (std::error_code EC = Text.getError())
           return make_string_error(EC.message());
-        if (std::error_code ec =
-                parseConfiguration(Text.get()->getBuffer(), &Style)) {
+        if (std::error_code ec = parseConfiguration(
+                Text.get()->getBuffer(), &Style, AllowUnknownOptions)) {
           if (ec == ParseError::Unsuitable) {
             if (!UnsuitableConfigFiles.empty())
               UnsuitableConfigFiles.append(", ");
             UnsuitableConfigFiles.append(ConfigFile);
             continue;
           }
           return make_string_error("Error reading " + ConfigFile + ": " +
                                    ec.message());

clang/tools/clang-format/ClangFormat.cpp

Cursor("cursor",
            cl::init(0), cl::cat(ClangFormatCategory));
 
 static cl::opt<bool> SortIncludes(
     "sort-includes",
     cl::desc("If set, overrides the include sorting behavior determined by the "
              "SortIncludes style flag"),
     cl::cat(ClangFormatCategory));
 
+// using the full param name as Wno-error probably won't be a common use case in
+// clang-format
+static cl::opt<bool> AllowUnknownOptions(
+    "Wno-error=unknown",
+    cl::desc("If set, unknown format options are only warned about.\n"
+             "Use with caution, as this might lead to dramatically "
+             "differing format depending on an option being "
+             "supported or not."),
+    cl::init(false), cl::cat(ClangFormatCategory));
+
 static cl::opt<bool>
     Verbose("verbose", cl::desc("If set, shows the list of processed files"),
             cl::cat(ClangFormatCategory));
 
 // Use --dry-run to match other LLVM tools when you mean do it but don't
 // actually do it
 static cl::opt<bool>
     DryRun("dry-run",
if (fillRanges(Code.get(), Ranges))
     return true;
   StringRef AssumedFileName = (FileName == "-") ? AssumeFileName : FileName;
   if (AssumedFileName.empty()) {
     llvm::errs() << "error: empty filenames are not allowed\n";
     return true;
   }
 
   llvm::Expected<FormatStyle> FormatStyle =
-      getStyle(Style, AssumedFileName, FallbackStyle, Code->getBuffer());
+      getStyle(Style, AssumedFileName, FallbackStyle, Code->getBuffer(),
+               nullptr, AllowUnknownOptions.getValue());
   if (!FormatStyle) {
     llvm::errs() << llvm::toString(FormatStyle.takeError()) << "\n";
     return true;
   }
 
   if (SortIncludes.getNumOccurrences() != 0)
     FormatStyle->SortIncludes = SortIncludes;
   unsigned CursorPosition = Cursor;

llvm/include/llvm/Support/YAMLParser.h

 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_YAMLPARSER_H
 #define LLVM_SUPPORT_YAMLPARSER_H
 
 #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>
 #include <map>
 #include <memory>
 #include <string>
 #include <system_error>
 
 namespace llvm {
 
 class MemoryBufferRef;
-class SourceMgr;
 class raw_ostream;
 class Twine;
 
 namespace yaml {
 
 class Document;
 class document_iterator;
 class Node;
public:
   void skip();
   bool failed();
 
   bool validate() {
     skip();
     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;
 
   std::unique_ptr<Scanner> scanner;
   std::unique_ptr<Document> CurrentDoc;
 };
 

llvm/include/llvm/Support/YAMLTraits.h

public:
 
   virtual void scalarString(StringRef &, QuotingType) = 0;
   virtual void blockScalarString(StringRef &) = 0;
   virtual void scalarTag(std::string &) = 0;
 
   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) {
     if ( matchEnumScalar(Str, outputting() && Val == ConstVal) ) {
       Val = ConstVal;
     }
   }
 
public:
 
     std::vector<std::unique_ptr<HNode>> Entries;
   };
 
   std::unique_ptr<Input::HNode> createHNodes(Node *node);
   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.
   bool setCurrentDocument();
   bool nextDocument();
 
   /// 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;
   std::unique_ptr<HNode>              TopNode;
   std::error_code                     EC;
   BumpPtrAllocator                    StringAllocator;
   document_iterator                   DocIterator;
   std::vector<bool>                   BitValuesUsed;
   HNode *CurrentNode = nullptr;
   bool                                ScalarMatchFound = false;
+  bool AllowUnknownKeys = false;
 };
 
 ///
 /// The Output class is used to generate a yaml document from in-memory structs
 /// and vectors.
 ///
 class Output : public IO {
 public:

llvm/lib/Support/YAMLParser.cpp

 Stream::Stream(MemoryBufferRef InputBuffer, SourceMgr &SM, bool ShowColors,
                std::error_code *EC)
     : scanner(new Scanner(InputBuffer, SM, ShowColors, EC)), CurrentDoc() {}
 
 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() {
   if (CurrentDoc)
     report_fatal_error("Can only iterate over the stream once");
 
   // Skip Stream-Start.
   scanner->getNext();

llvm/lib/Support/YAMLTraits.cpp

 void *IO::getContext() const {
   return Ctxt;
 }
 
 void IO::setContext(void *Context) {
   Ctxt = Context;
 }
 
+void IO::setAllowUnknownKeys(bool Allow) {
+  llvm_unreachable("Only supported for Input");
+}
+
 //===----------------------------------------------------------------------===//
 //  Input
 //===----------------------------------------------------------------------===//
 
 Input::Input(StringRef InputContent, void *Ctxt,
              SourceMgr::DiagHandlerTy DiagHandler, void *DiagHandlerCtxt)
     : IO(Ctxt), Strm(new Stream(InputContent, SrcMgr, false, &EC)) {
   if (DiagHandler)

 void Input::endMapping() {
   if (EC)
     return;
   // CurrentNode can be null if the document is empty.
   MapHNode *MN = dyn_cast_or_null<MapHNode>(CurrentNode);
   if (!MN)
     return;
   for (const auto &NN : MN->Mapping) {
     if (!is_contained(MN->ValidKeys, NN.first())) {
-      setError(NN.second.get(), Twine("unknown key '") + NN.first() + "'");
+      auto ReportNode = NN.second.get();
+      auto ReportMessage = Twine("unknown key '") + NN.first() + "'";
+      if (!AllowUnknownKeys) {
+        setError(ReportNode, ReportMessage);
         break;
+      } else
+        reportWarning(ReportNode, ReportMessage);
     }
   }
 }
 
 void Input::beginFlowMapping() { beginMapping(); }
 
 void Input::endFlowMapping() { endMapping(); }
 
NodeKind Input::getNodeKind() {
   llvm_unreachable("Unsupported node kind");
 }
 
 void Input::setError(Node *node, const Twine &message) {
   Strm->printError(node, message);
   EC = make_error_code(errc::invalid_argument);
 }
 
+void Input::reportWarning(HNode *hnode, const Twine &message) {
+  assert(hnode && "HNode must not be NULL");
+  reportWarning(hnode->_node, message);
+}
+
+void Input::reportWarning(Node *node, const Twine &message) {
+  Strm->printError(node, message, SourceMgr::DK_Warning);
+}
+
 std::unique_ptr<Input::HNode> Input::createHNodes(Node *N) {
   SmallString<128> StringStorage;
   if (ScalarNode *SN = dyn_cast<ScalarNode>(N)) {
     StringRef KeyStr = SN->getValue(StringStorage);
     if (!StringStorage.empty()) {
       // Copy string to permanent storage
       KeyStr = StringStorage.str().copy(StringAllocator);
     }
if (ScalarNode *SN = dyn_cast<ScalarNode>(N)) {
     return nullptr;
   }
 }
 
 void Input::setError(const Twine &Message) {
   setError(CurrentNode, Message);
 }
 
+void Input::setAllowUnknownKeys(bool Allow) { AllowUnknownKeys = Allow; }
+
 bool Input::canElideEmptySequence() {
   return false;
 }
 
 //===----------------------------------------------------------------------===//
 //  Output
 //===----------------------------------------------------------------------===//
 
void Output::scalarTag(std::string &Tag) {
   newLineCheck();
   output(Tag);
   output(" ");
 }
 
 void Output::setError(const Twine &message) {
 }
 
 bool Output::canElideEmptySequence() {
   // Normally, with an optional key/value where the value is an empty sequence,
   // the whole key/value can be not written.  But, that produces wrong yaml
   // if the key/value is the only thing in the map and the map is used in
   // a sequence.  This detects if the this sequence is the first key/value
   // in map that itself is embedded in a sequence.
   if (StateStack.size() < 2)
     return true;
   if (StateStack.back() != inMapFirstKey)