[DataFormatters] Add formatter for C++17 std::variant

rdar://problem/43691454

Patch by Shafik Yaghmour.

Differential Revision: https://reviews.llvm.org/D51520

9 files changed, 458 insertions, 0 deletions

diff --git a/lldb/lldb.xcodeproj/project.pbxproj b/lldb/lldb.xcodeproj/project.pbxproj
index 3cbd4d99a72..2d80648a7a2 100644
--- a/lldb/lldb.xcodeproj/project.pbxproj
+++ b/lldb/lldb.xcodeproj/project.pbxproj
@@ -401,6 +401,8 @@
 		AF9FF1F71FAA79FE00474976 /* LibCxxQueue.cpp in Sources */ = {isa = PBXBuildFile; fileRef = AF9FF1F61FAA79FE00474976 /* LibCxxQueue.cpp */; };
 		AF9FF1F51FAA79A400474976 /* LibCxxTuple.cpp in Sources */ = {isa = PBXBuildFile; fileRef = AF9FF1F41FAA79A400474976 /* LibCxxTuple.cpp */; };
 		945261C41B9A11FC00BF138D /* LibCxxUnorderedMap.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 945261BA1B9A11E800BF138D /* LibCxxUnorderedMap.cpp */; };
+		E414F6F121388F6C00C50BC6 /* LibCxxVariant.cpp in Sources */ = {isa = PBXBuildFile; fileRef = E414F6F021388F6B00C50BC6 /* LibCxxVariant.cpp */; };
+		E414F6EE21388F0300C50BC6 /* LibCxxVariant.h in Headers */ = {isa = PBXBuildFile; fileRef = E414F6ED21388F0200C50BC6 /* LibCxxVariant.h */; };
 		945261C51B9A11FC00BF138D /* LibCxxVector.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 945261BB1B9A11E800BF138D /* LibCxxVector.cpp */; };
 		945261C61B9A11FC00BF138D /* LibStdcpp.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 945261BC1B9A11E800BF138D /* LibStdcpp.cpp */; };
 		4CDB8D6E1DBA91B6006C5B13 /* LibStdcppTuple.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4CDB8D681DBA91A6006C5B13 /* LibStdcppTuple.cpp */; };
@@ -2056,6 +2058,8 @@
 		AF9FF1F61FAA79FE00474976 /* LibCxxQueue.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; name = LibCxxQueue.cpp; path = Language/CPlusPlus/LibCxxQueue.cpp; sourceTree = "<group>"; };
 		AF9FF1F41FAA79A400474976 /* LibCxxTuple.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; name = LibCxxTuple.cpp; path = Language/CPlusPlus/LibCxxTuple.cpp; sourceTree = "<group>"; };
 		945261BA1B9A11E800BF138D /* LibCxxUnorderedMap.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; name = LibCxxUnorderedMap.cpp; path = Language/CPlusPlus/LibCxxUnorderedMap.cpp; sourceTree = "<group>"; };
+		E414F6F021388F6B00C50BC6 /* LibCxxVariant.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; name = LibCxxVariant.cpp; path = Language/CPlusPlus/LibCxxVariant.cpp; sourceTree = "<group>"; };
+		E414F6ED21388F0200C50BC6 /* LibCxxVariant.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = LibCxxVariant.h; path = Language/CPlusPlus/LibCxxVariant.h; sourceTree = "<group>"; };
 		945261BB1B9A11E800BF138D /* LibCxxVector.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; name = LibCxxVector.cpp; path = Language/CPlusPlus/LibCxxVector.cpp; sourceTree = "<group>"; };
 		945261BC1B9A11E800BF138D /* LibStdcpp.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; name = LibStdcpp.cpp; path = Language/CPlusPlus/LibStdcpp.cpp; sourceTree = "<group>"; };
 		945261BD1B9A11E800BF138D /* LibStdcpp.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; name = LibStdcpp.h; path = Language/CPlusPlus/LibStdcpp.h; sourceTree = "<group>"; };
@@ -6475,6 +6479,8 @@
 				AF9FF1F61FAA79FE00474976 /* LibCxxQueue.cpp */,
 				AF9FF1F41FAA79A400474976 /* LibCxxTuple.cpp */,
 				945261BA1B9A11E800BF138D /* LibCxxUnorderedMap.cpp */,
+				E414F6ED21388F0200C50BC6 /* LibCxxVariant.h */,
+				E414F6F021388F6B00C50BC6 /* LibCxxVariant.cpp */,
 				945261BB1B9A11E800BF138D /* LibCxxVector.cpp */,
 				945261BD1B9A11E800BF138D /* LibStdcpp.h */,
 				945261BC1B9A11E800BF138D /* LibStdcpp.cpp */,
@@ -7081,6 +7087,7 @@
 				6D0F614F1C80AB0C00A4ECEE /* JavaLanguageRuntime.h in Headers */,
 				AF27AD561D3603EA00CF2833 /* DynamicLoaderDarwin.h in Headers */,
 				AFCB2BBE1BF577F40018B553 /* PythonExceptionState.h in Headers */,
+				E414F6EE21388F0300C50BC6 /* LibCxxVariant.h in Headers */,
 				267F684B1CC02DED0086832B /* ABISysV_s390x.h in Headers */,
 				AF8AD6311BEC28A400150209 /* PlatformAppleWatchSimulator.h in Headers */,
 			);
@@ -7936,6 +7943,7 @@
 				AF0C112818580CD800C4C45B /* QueueItem.cpp in Sources */,
 				AF254E31170CCC33007AE5C9 /* PlatformDarwinKernel.cpp in Sources */,
 				2689004913353E0400698AC0 /* Scalar.cpp in Sources */,
+				E414F6F121388F6C00C50BC6 /* LibCxxVariant.cpp in Sources */,
 				263FDE601A79A01500E68013 /* FormatEntity.cpp in Sources */,
 				AF8AD62E1BEC28A400150209 /* PlatformAppleTVSimulator.cpp in Sources */,
 				2689004A13353E0400698AC0 /* SearchFilter.cpp in Sources */,
diff --git a/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/Makefile b/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/Makefile
new file mode 100644
index 00000000000..a6ea665ef63
--- /dev/null
+++ b/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/Makefile
@@ -0,0 +1,7 @@
+LEVEL = ../../../../../make
+
+CXX_SOURCES := main.cpp
+
+USE_LIBCPP := 1
+include $(LEVEL)/Makefile.rules
+CXXFLAGS += -std=c++17
diff --git a/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/TestDataFormatterLibcxxVariant.py b/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/TestDataFormatterLibcxxVariant.py
new file mode 100644
index 00000000000..9ea7ac33166
--- /dev/null
+++ b/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/TestDataFormatterLibcxxVariant.py
@@ -0,0 +1,81 @@
+"""
+Test lldb data formatter subsystem.
+"""
+
+from __future__ import print_function
+
+
+import os
+import time
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+class LibcxxOptionalDataFormatterTestCase(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    @add_test_categories(["libc++"])
+    ## We are skipping clang version less that 5.0 since this test requires -std=c++17
+    @skipIf(oslist=no_match(["macosx"]), compiler="clang", compiler_version=['<', '5.0'])
+    ## We are skipping gcc version less that 5.1 since this test requires -std=c++17
+    @skipIf(compiler="gcc", compiler_version=['<', '5.1'])
+
+    def test_with_run_command(self):
+        """Test that that file and class static variables display correctly."""
+        self.build()
+
+        (self.target, self.process, _, bkpt) = lldbutil.run_to_source_breakpoint(self, '// break here',
+                lldb.SBFileSpec("main.cpp", False))
+
+        self.runCmd( "frame variable has_variant" )
+
+        output = self.res.GetOutput()
+
+        ## The variable has_variant tells us if the test program
+        ## detected we have a sufficient libc++ version to support variant
+        ## false means we do not and therefore should skip the test
+        if output.find("(bool) has_variant = false") != -1 :
+            self.skipTest( "std::variant not supported" )
+
+        lldbutil.continue_to_breakpoint(self.process, bkpt)
+
+        self.expect("frame variable v1",
+                substrs=['v1 =  Active Type = int  {',
+                               'Value = 12',
+                               '}'])
+
+        self.expect("frame variable v1_ref",
+                substrs=['v1_ref =  Active Type = int : {',
+                               'Value = 12',
+                               '}'])
+
+        self.expect("frame variable v_v1",
+                substrs=['v_v1 =  Active Type = std::__1::variant<int, double, char>  {',
+                                 'Value =  Active Type = int  {',
+                                   'Value = 12',
+                                 '}',
+                               '}'])
+
+        lldbutil.continue_to_breakpoint(self.process, bkpt)
+
+        self.expect("frame variable v1",
+                substrs=['v1 =  Active Type = double  {',
+                               'Value = 2',
+                               '}'])
+
+        lldbutil.continue_to_breakpoint(self.process, bkpt)
+
+        self.expect("frame variable v2",
+                substrs=['v2 =  Active Type = double  {',
+                               'Value = 2',
+                               '}'])
+
+        self.expect("frame variable v3",
+                substrs=['v3 =  Active Type = char  {',
+                               'Value = \'A\'',
+                               '}'])
+
+        self.expect("frame variable v_no_value",
+                    substrs=['v_no_value =  No Value'])
diff --git a/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/main.cpp b/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/main.cpp
new file mode 100644
index 00000000000..c0bc4ae12c1
--- /dev/null
+++ b/lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/variant/main.cpp
@@ -0,0 +1,60 @@
+#include <cstdio>
+#include <string>
+#include <vector>
+
+// If we have libc++ 4.0 or greater we should have <variant>
+// According to libc++ C++1z status page https://libcxx.llvm.org/cxx1z_status.html
+#if _LIBCPP_VERSION >= 4000
+#include <variant>
+#define HAVE_VARIANT 1
+#else
+#define HAVE_VARIANT 0
+#endif
+
+struct S {
+  operator int() { throw 42; }
+} ;
+
+
+int main()
+{
+    bool has_variant = HAVE_VARIANT ;
+
+    printf( "%d\n", has_variant ) ; // break here
+
+#if HAVE_VARIANT == 1
+    std::variant<int, double, char> v1;
+    std::variant<int, double, char> &v1_ref = v1;
+    std::variant<int, double, char> v2;
+    std::variant<int, double, char> v3;
+    std::variant<std::variant<int,double,char>> v_v1 ;
+    std::variant<int, double, char> v_no_value;
+
+    v1 = 12; // v contains int
+    v_v1 = v1 ;
+    int i = std::get<int>(v1);
+    printf( "%d\n", i ); // break here
+
+    v2 = 2.0 ;
+    double d = std::get<double>(v2) ;
+    printf( "%f\n", d );
+
+    v3 = 'A' ;
+    char c = std::get<char>(v3) ;
+    printf( "%d\n", c );
+
+    // Checking v1 above and here to make sure we done maintain the incorrect
+    // state when we change its value.
+    v1 = 2.0;
+    d = std::get<double>(v1) ;
+    printf( "%f\n", d ); // break here
+
+     try {
+       v_no_value.emplace<0>(S());
+     } catch( ... ) {}
+
+     printf( "%zu\n", v_no_value.index() ) ;
+#endif
+
+    return 0; // break here
+}
diff --git a/lldb/source/Plugins/Language/CPlusPlus/CMakeLists.txt b/lldb/source/Plugins/Language/CPlusPlus/CMakeLists.txt
index eb0d880c641..715334412f8 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/CMakeLists.txt
+++ b/lldb/source/Plugins/Language/CPlusPlus/CMakeLists.txt
@@ -13,6 +13,7 @@ add_lldb_library(lldbPluginCPlusPlusLanguage PLUGIN
   LibCxxQueue.cpp
   LibCxxTuple.cpp
   LibCxxUnorderedMap.cpp
+  LibCxxVariant.cpp
   LibCxxVector.cpp
   LibStdcpp.cpp
   LibStdcppTuple.cpp
diff --git a/lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp b/lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp
index 7ce21146683..33bf7096d62 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp
@@ -39,6 +39,7 @@
 #include "CxxStringTypes.h"
 #include "LibCxx.h"
 #include "LibCxxAtomic.h"
+#include "LibCxxVariant.h"
 #include "LibStdcpp.h"
 
 using namespace lldb;
@@ -516,6 +517,10 @@ static void LoadLibCxxFormatters(lldb::TypeCategoryImplSP cpp_category_sp) {
                   "libc++ std::optional synthetic children",
                   ConstString("^std::__(ndk)?1::optional<.+>(( )?&)?$"),
                   stl_synth_flags, true);
+  AddCXXSynthetic(cpp_category_sp, LibcxxVariantFrontEndCreator,
+                  "libc++ std::variant synthetic children",
+                  ConstString("^std::__(ndk)?1::variant<.+>(( )?&)?$"),
+                  stl_synth_flags, true);
   AddCXXSynthetic(
       cpp_category_sp,
       lldb_private::formatters::LibcxxAtomicSyntheticFrontEndCreator,
@@ -617,6 +622,11 @@ static void LoadLibCxxFormatters(lldb::TypeCategoryImplSP cpp_category_sp) {
                 "libc++ std::optional summary provider",
                 ConstString("^std::__(ndk)?1::optional<.+>(( )?&)?$"),
                 stl_summary_flags, true);
+  AddCXXSummary(cpp_category_sp,
+                lldb_private::formatters::LibcxxVariantSummaryProvider,
+                "libc++ std::variant summary provider",
+                ConstString("^std::__(ndk)?1::variant<.+>(( )?&)?$"),
+                stl_summary_flags, true);
 
   stl_summary_flags.SetSkipPointers(true);
 
diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibCxx.h b/lldb/source/Plugins/Language/CPlusPlus/LibCxx.h
index 06f5817845b..871268720c7 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibCxx.h
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibCxx.h
@@ -142,6 +142,10 @@ SyntheticChildrenFrontEnd *
 LibcxxOptionalFrontEndCreator(CXXSyntheticChildren *,
                               lldb::ValueObjectSP valobj_sp);
 
+SyntheticChildrenFrontEnd *
+LibcxxVariantFrontEndCreator(CXXSyntheticChildren *,
+                             lldb::ValueObjectSP valobj_sp);
+
 } // namespace formatters
 } // namespace lldb_private
 
diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibCxxVariant.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibCxxVariant.cpp
new file mode 100644
index 00000000000..04d8b804434
--- /dev/null
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibCxxVariant.cpp
@@ -0,0 +1,256 @@
+//===-- LibCxxVariant.cpp --------------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "LibCxxVariant.h"
+#include "lldb/DataFormatters/FormattersHelpers.h"
+
+#include "llvm/ADT/Optional.h"
+#include "llvm/ADT/ScopeExit.h"
+
+using namespace lldb;
+using namespace lldb_private;
+
+// libc++ variant implementation contains two members that we care about both
+// are contained in the __impl member.
+// - __index which tells us which of the variadic template types is the active
+//   type for the variant
+// - __data is a variadic union which recursively contains itself as member
+//   which refers to the tailing variadic types.
+//   - __head which refers to the leading non pack type
+//     - __value refers to the actual value contained
+//   - __tail which refers to the remaining pack types
+//
+// e.g. given std::variant<int,double,char> v1
+//
+// (lldb) frame var -R v1.__impl.__data
+//(... __union<... 0, int, double, char>) v1.__impl.__data = {
+// ...
+//  __head = {
+//    __value = ...
+//  }
+//  __tail = {
+//  ...
+//    __head = {
+//      __value = ...
+//    }
+//    __tail = {
+//    ...
+//      __head = {
+//        __value = ...
+//  ...
+//
+// So given
+// - __index equal to 0 the active value is contained in
+//
+//     __data.__head.__value
+//
+// - __index equal to 1 the active value is contained in
+//
+//     __data.__tail.__head.__value
+//
+// - __index equal to 2 the active value is contained in
+//
+//      __data.__tail.__tail.__head.__value
+//
+
+namespace {
+// libc++ std::variant index could have one of three states
+// 1) VALID, we can obtain it and its not variant_npos
+// 2) INVALID, we can't obtain it or it is not a type we expect
+// 3) NPOS, its value is variant_npos which means the variant has no value
+enum class LibcxxVariantIndexValidity { VALID, INVALID, NPOS };
+
+LibcxxVariantIndexValidity
+LibcxxVariantGetIndexValidity(ValueObjectSP &impl_sp) {
+  ValueObjectSP index_sp(
+      impl_sp->GetChildMemberWithName(ConstString("__index"), true));
+
+  if (!index_sp)
+    return LibcxxVariantIndexValidity::INVALID;
+
+  int64_t index_value = index_sp->GetValueAsSigned(0);
+
+  if (index_value == -1)
+    return LibcxxVariantIndexValidity::NPOS;
+
+  return LibcxxVariantIndexValidity::VALID;
+}
+
+llvm::Optional<uint64_t> LibcxxVariantIndexValue(ValueObjectSP &impl_sp) {
+  ValueObjectSP index_sp(
+      impl_sp->GetChildMemberWithName(ConstString("__index"), true));
+
+  if (!index_sp)
+    return {};
+
+  return {index_sp->GetValueAsUnsigned(0)};
+}
+
+ValueObjectSP LibcxxVariantGetNthHead(ValueObjectSP &impl_sp, uint64_t index) {
+  ValueObjectSP data_sp(
+      impl_sp->GetChildMemberWithName(ConstString("__data"), true));
+
+  if (!data_sp)
+    return ValueObjectSP{};
+
+  ValueObjectSP current_level = data_sp;
+  for (uint64_t n = index; n != 0; --n) {
+    ValueObjectSP tail_sp(
+        current_level->GetChildMemberWithName(ConstString("__tail"), true));
+
+    if (!tail_sp)
+      return ValueObjectSP{};
+
+    current_level = tail_sp;
+  }
+
+  return current_level->GetChildMemberWithName(ConstString("__head"), true);
+};
+} // namespace
+
+namespace lldb_private {
+namespace formatters {
+bool LibcxxVariantSummaryProvider(ValueObject &valobj, Stream &stream,
+                                  const TypeSummaryOptions &options) {
+  ValueObjectSP valobj_sp(valobj.GetNonSyntheticValue());
+  if (!valobj_sp)
+    return false;
+
+  ValueObjectSP impl_sp(
+      valobj_sp->GetChildMemberWithName(ConstString("__impl"), true));
+
+  if (!impl_sp)
+    return false;
+
+  LibcxxVariantIndexValidity validity = LibcxxVariantGetIndexValidity(impl_sp);
+
+  if (validity == LibcxxVariantIndexValidity::INVALID)
+    return false;
+
+  if (validity == LibcxxVariantIndexValidity::NPOS) {
+    stream.Printf(" No Value");
+    return true;
+  }
+
+  auto optional_index_value = LibcxxVariantIndexValue(impl_sp);
+
+  if (!optional_index_value)
+    return false;
+
+  uint64_t index_value = *optional_index_value;
+
+  ValueObjectSP nth_head = LibcxxVariantGetNthHead(impl_sp, index_value);
+
+  if (!nth_head)
+    return false;
+
+  CompilerType head_type = nth_head->GetCompilerType();
+
+  if (!head_type)
+    return false;
+
+  CompilerType template_type = head_type.GetTypeTemplateArgument(1);
+
+  if (!template_type)
+    return false;
+
+  stream.Printf(" Active Type = %s ", template_type.GetTypeName().GetCString());
+
+  return true;
+}
+} // namespace formatters
+} // namespace lldb_private
+
+namespace {
+class VariantFrontEnd : public SyntheticChildrenFrontEnd {
+public:
+  VariantFrontEnd(ValueObject &valobj) : SyntheticChildrenFrontEnd(valobj) {
+    Update();
+  }
+
+  size_t GetIndexOfChildWithName(const ConstString &name) override {
+    return formatters::ExtractIndexFromString(name.GetCString());
+  }
+
+  bool MightHaveChildren() override { return true; }
+  bool Update() override;
+  size_t CalculateNumChildren() override { return m_size; }
+  ValueObjectSP GetChildAtIndex(size_t idx) override;
+
+private:
+  size_t m_size = 0;
+  ValueObjectSP m_base_sp;
+};
+} // namespace
+
+bool VariantFrontEnd::Update() {
+  m_size = 0;
+  ValueObjectSP impl_sp(
+      m_backend.GetChildMemberWithName(ConstString("__impl"), true));
+  if (!impl_sp)
+    return false;
+
+  LibcxxVariantIndexValidity validity = LibcxxVariantGetIndexValidity(impl_sp);
+
+  if (validity == LibcxxVariantIndexValidity::INVALID)
+    return false;
+
+  if (validity == LibcxxVariantIndexValidity::NPOS)
+    return true;
+
+  m_size = 1;
+
+  return false;
+}
+
+ValueObjectSP VariantFrontEnd::GetChildAtIndex(size_t idx) {
+  if (idx >= m_size)
+    return ValueObjectSP();
+
+  ValueObjectSP impl_sp(
+      m_backend.GetChildMemberWithName(ConstString("__impl"), true));
+
+  auto optional_index_value = LibcxxVariantIndexValue(impl_sp);
+
+  if (!optional_index_value)
+    return ValueObjectSP();
+
+  uint64_t index_value = *optional_index_value;
+
+  ValueObjectSP nth_head = LibcxxVariantGetNthHead(impl_sp, index_value);
+
+  if (!nth_head)
+    return ValueObjectSP();
+
+  CompilerType head_type = nth_head->GetCompilerType();
+
+  if (!head_type)
+    return ValueObjectSP();
+
+  CompilerType template_type = head_type.GetTypeTemplateArgument(1);
+
+  if (!template_type)
+    return ValueObjectSP();
+
+  ValueObjectSP head_value(
+      nth_head->GetChildMemberWithName(ConstString("__value"), true));
+
+  if (!head_value)
+    return ValueObjectSP();
+
+  return head_value->Clone(ConstString(ConstString("Value").AsCString()));
+}
+
+SyntheticChildrenFrontEnd *
+formatters::LibcxxVariantFrontEndCreator(CXXSyntheticChildren *,
+                                         lldb::ValueObjectSP valobj_sp) {
+  if (valobj_sp)
+    return new VariantFrontEnd(*valobj_sp);
+  return nullptr;
+}
diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibCxxVariant.h b/lldb/source/Plugins/Language/CPlusPlus/LibCxxVariant.h
new file mode 100644
index 00000000000..04834581963
--- /dev/null
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibCxxVariant.h
@@ -0,0 +1,31 @@
+//===-- LibCxxVariant.h -------------------------------------------*- C++
+//-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef liblldb_LibCxxVariant_h_
+#define liblldb_LibCxxVariant_h_
+
+#include "lldb/Core/ValueObject.h"
+#include "lldb/DataFormatters/TypeSummary.h"
+#include "lldb/DataFormatters/TypeSynthetic.h"
+#include "lldb/Utility/Stream.h"
+
+namespace lldb_private {
+namespace formatters {
+bool LibcxxVariantSummaryProvider(
+    ValueObject &valobj, Stream &stream,
+    const TypeSummaryOptions &options); // libc++ std::variant<>
+
+SyntheticChildrenFrontEnd *LibcxxVariantFrontEndCreator(CXXSyntheticChildren *,
+                                                        lldb::ValueObjectSP);
+
+} // namespace formatters
+} // namespace lldb_private
+
+#endif // liblldb_LibCxxVariant_h_