Index: clangd/ClangdLSPServer.cpp
===================================================================
--- clangd/ClangdLSPServer.cpp
+++ clangd/ClangdLSPServer.cpp
@@ -71,6 +71,8 @@
                     JSONOutput &Out) override;
   void onGoToDefinition(TextDocumentPositionParams Params, StringRef ID,
                             JSONOutput &Out) override;
+  void onSwitchSourceHeader(TextDocumentIdentifier Params, StringRef ID,
+                    JSONOutput &Out) override;                          
 
 private:
   ClangdLSPServer &LangServer;
@@ -216,6 +218,18 @@
       R"(,"result":[)" + Locations + R"(]})");
 }
 
+void ClangdLSPServer::LSPProtocolCallbacks::onSwitchSourceHeader(
+    TextDocumentIdentifier Params, StringRef ID, JSONOutput &Out) {
+
+  llvm::Optional<Path> Result = LangServer.Server.switchSourceHeader(Params.uri.file);
+  std::string test = "file://";
+  Path ResultString = "\"" + test + *Result + "\"";
+
+    Out.writeMessage(
+        R"({"jsonrpc":"2.0","id":)" + ID.str() +
+        R"(,"result":)" + ResultString + R"(})");
+}
+
 ClangdLSPServer::ClangdLSPServer(JSONOutput &Out, bool RunSynchronously)
     : Out(Out), DiagConsumer(*this),
       Server(CDB, DiagConsumer, FSProvider, RunSynchronously) {}
Index: clangd/ClangdServer.h
===================================================================
--- clangd/ClangdServer.h
+++ clangd/ClangdServer.h
@@ -182,6 +182,12 @@
   /// Get definition of symbol at a specified \p Line and \p Column in \p File.
   Tagged<std::vector<Location>> findDefinitions(PathRef File, Position Pos);
 
+  /// If an extension hasn't been found in the lowercase array, try with uppercase.
+  bool checkUpperCaseExtensions(StringRef BaseArray[], int ArraySize, StringRef PathExt);
+
+  /// Helper function that returns a path to the corresponding source file when given a header file and vice versa.
+  llvm::Optional<Path> switchSourceHeader(PathRef Path);
+
   /// Run formatting for \p Rng inside \p File.
   std::vector<tooling::Replacement> formatRange(PathRef File, Range Rng);
   /// Run formatting for the whole \p File.
Index: clangd/ClangdServer.cpp
===================================================================
--- clangd/ClangdServer.cpp
+++ clangd/ClangdServer.cpp
@@ -15,9 +15,11 @@
 #include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 #include <future>
 
+using namespace llvm;
 using namespace clang;
 using namespace clang::clangd;
 
@@ -286,3 +288,84 @@
       });
   return make_tagged(std::move(Result), TaggedFS.Tag);
 }
+
+
+bool ClangdServer::checkUpperCaseExtensions(StringRef BaseArray[], int ArraySize, StringRef PathExt)
+{
+  std::string UpperExtensions[ArraySize];
+  for(int i = 0; i < ArraySize; i++)
+  {
+    UpperExtensions[i] = BaseArray[i];
+  }
+  // Uppercase the whole array
+  for (std::string & s : UpperExtensions)
+  std::transform(s.begin(), s.end(), s.begin(),
+  [](unsigned char c) { return std::toupper(c); }); 
+
+  bool isUpperCase = false;
+  // Iterator such as std::begin won't work here so we use a standard loop
+  for(int i = 0; i < ArraySize; i++)
+  {
+    if (UpperExtensions[i] == PathExt.str())
+    {
+      isUpperCase = true;
+    }
+  }
+  return isUpperCase;
+}
+
+llvm::Optional<Path> ClangdServer::switchSourceHeader(PathRef Path) {
+
+  StringRef SourceExtensions[] = {".cpp", ".c", ".cc", ".cxx", ".c++", ".m", ".mm"};
+  StringRef HeaderExtensions[] = {".h", ".hh", ".hpp", ".hxx", ".inc"};
+
+  StringRef PathExt = llvm::sys::path::extension(Path);
+  
+  // Lookup in a list of known extensions.
+  auto SourceIter = std::find(std::begin(SourceExtensions), std::end(SourceExtensions), PathExt);
+  bool IsSource = SourceIter != std::end(SourceExtensions);
+
+  if (!IsSource)
+  {
+    IsSource = checkUpperCaseExtensions(SourceExtensions, llvm::array_lengthof(SourceExtensions), PathExt);
+  }
+  
+  auto HeaderIter = std::find(std::begin(HeaderExtensions), std::end(HeaderExtensions), PathExt);
+  bool IsHeader = HeaderIter != std::end(HeaderExtensions);
+
+  if (!IsHeader)
+  {
+    IsHeader = checkUpperCaseExtensions(HeaderExtensions, llvm::array_lengthof(HeaderExtensions), PathExt);
+  }
+
+  // We can only switch between extensions known extensions.
+  if (!IsSource && !IsHeader)
+    return llvm::None;
+
+  // Array to lookup extensions for the switch. An opposite of where original extension was found.
+  ArrayRef<StringRef> NewExts;
+  if (IsSource)
+    NewExts = HeaderExtensions;
+  else
+    NewExts = SourceExtensions;
+
+  // Storage for the new path.
+  SmallString<128> NewPath = StringRef(Path);
+
+  // Instance of vfs::FileSystem, used for file existence checks.
+  auto FS = FSProvider.getTaggedFileSystem(Path).Value;
+
+  // Loop through switched extension candidates.
+  for (StringRef NewExt : NewExts) { 
+    llvm::sys::path::replace_extension(NewPath, NewExt);
+    if (FS->exists(NewPath))
+      return NewPath.str().str(); // First str() to convert from SmallString to StringRef, second to convert from StringRef to std::string
+
+    // Also check NewExt in upper-case, just in case.
+    llvm::sys::path::replace_extension(NewPath, NewExt.upper());
+    if (FS->exists(NewPath))
+      return NewPath.str().str();
+  }
+
+  return llvm::None;
+}
Index: clangd/ProtocolHandlers.h
===================================================================
--- clangd/ProtocolHandlers.h
+++ clangd/ProtocolHandlers.h
@@ -48,6 +48,8 @@
                             JSONOutput &Out) = 0;
   virtual void onGoToDefinition(TextDocumentPositionParams Params, StringRef ID,
                             JSONOutput &Out) = 0;
+  virtual void onSwitchSourceHeader(TextDocumentIdentifier Params, StringRef ID,
+                            JSONOutput &Out) = 0;
 };
 
 void regiterCallbackHandlers(JSONRPCDispatcher &Dispatcher, JSONOutput &Out,
Index: clangd/ProtocolHandlers.cpp
===================================================================
--- clangd/ProtocolHandlers.cpp
+++ clangd/ProtocolHandlers.cpp
@@ -204,6 +204,23 @@
   ProtocolCallbacks &Callbacks;
 };
 
+struct SwitchSourceHeaderHandler : Handler {
+  SwitchSourceHeaderHandler(JSONOutput &Output, ProtocolCallbacks &Callbacks)
+      : Handler(Output), Callbacks(Callbacks) {}
+
+  void handleMethod(llvm::yaml::MappingNode *Params, StringRef ID) override {
+    auto TDPP = TextDocumentIdentifier::parse(Params);
+    if (!TDPP) {
+      return;
+    }
+
+    Callbacks.onSwitchSourceHeader(*TDPP, ID, Output);
+  }
+
+private:
+  ProtocolCallbacks &Callbacks;
+};
+
 } // namespace
 
 void clangd::regiterCallbackHandlers(JSONRPCDispatcher &Dispatcher,
@@ -239,4 +256,7 @@
       llvm::make_unique<CompletionHandler>(Out, Callbacks));
   Dispatcher.registerHandler("textDocument/definition",
       llvm::make_unique<GotoDefinitionHandler>(Out, Callbacks));
+  Dispatcher.registerHandler(
+      "textDocument/switchSourceHeader",
+      llvm::make_unique<SwitchSourceHeaderHandler>(Out, Callbacks));
 }