rwols updated this revision to Diff 116461.
rwols added a comment.

- Update doxygen comment to say "signature help" instead of "code complete",
- Refactor: put the FillDocumentation member function outside of 
`CompletionItemsCollector` so that `SignatureHelpCollector` can also use it.
- Refactor: put the FillSortText member function outside of 
`CompletionItemsCollector` because it has nothing to do with 
`CompletionItemsCollector`.
- Refactor: Make a templated function called `invokeClangAction` that will 
invoke the given CodeCompleteConsumer type. Both `clangd::codeComplete` and 
`clangd::signatureHelp` now call this function.
- Present optional/defaulted parameters too (using 
`CodeCompletionString::CK_Optional` chunks).


https://reviews.llvm.org/D38048

Files:
  clangd/ClangdLSPServer.cpp
  clangd/ClangdServer.cpp
  clangd/ClangdServer.h
  clangd/ClangdUnit.cpp
  clangd/ClangdUnit.h
  clangd/Protocol.cpp
  clangd/Protocol.h
  clangd/ProtocolHandlers.cpp
  clangd/ProtocolHandlers.h

Index: clangd/ProtocolHandlers.h
===================================================================
--- clangd/ProtocolHandlers.h
+++ clangd/ProtocolHandlers.h
@@ -46,6 +46,8 @@
                             JSONOutput &Out) = 0;
   virtual void onCompletion(TextDocumentPositionParams Params, StringRef ID,
                             JSONOutput &Out) = 0;
+  virtual void onSignatureHelp(TextDocumentPositionParams Params, StringRef ID,
+                               JSONOutput &Out) = 0;
   virtual void onGoToDefinition(TextDocumentPositionParams Params, StringRef ID,
                                 JSONOutput &Out) = 0;
 };
Index: clangd/ProtocolHandlers.cpp
===================================================================
--- clangd/ProtocolHandlers.cpp
+++ clangd/ProtocolHandlers.cpp
@@ -186,6 +186,23 @@
   ProtocolCallbacks &Callbacks;
 };
 
+struct SignatureHelpHandler : Handler {
+  SignatureHelpHandler(JSONOutput &Output, ProtocolCallbacks &Callbacks)
+      : Handler(Output), Callbacks(Callbacks) {}
+
+  void handleMethod(llvm::yaml::MappingNode *Params, StringRef ID) override {
+    auto TDPP = TextDocumentPositionParams::parse(Params);
+    if (!TDPP) {
+      Output.log("Failed to decode TextDocumentPositionParams!\n");
+      return;
+    }
+    Callbacks.onSignatureHelp(*TDPP, ID, Output);
+  }
+
+private:
+  ProtocolCallbacks &Callbacks;
+};
+
 struct GotoDefinitionHandler : Handler {
   GotoDefinitionHandler(JSONOutput &Output, ProtocolCallbacks &Callbacks)
       : Handler(Output), Callbacks(Callbacks) {}
@@ -238,6 +255,9 @@
       "textDocument/completion",
       llvm::make_unique<CompletionHandler>(Out, Callbacks));
   Dispatcher.registerHandler(
+      "textDocument/signatureHelp",
+      llvm::make_unique<SignatureHelpHandler>(Out, Callbacks));
+  Dispatcher.registerHandler(
       "textDocument/definition",
       llvm::make_unique<GotoDefinitionHandler>(Out, Callbacks));
 }
Index: clangd/Protocol.h
===================================================================
--- clangd/Protocol.h
+++ clangd/Protocol.h
@@ -400,6 +400,71 @@
   static std::string unparse(const CompletionItem &P);
 };
 
+/// Represents a parameter of a callable-signature.
+///
+/// A parameter can have a label and a doc-comment.
+struct ParameterInformation {
+
+  /// The label of this parameter. Will be shown in the UI.
+  std::string label;
+
+  /// The human-readable doc-comment of this parameter. Will be shown in the UI
+  /// but can be omitted.
+  std::string documentation;
+
+  static std::string unparse(const ParameterInformation &);
+};
+
+/// Represents the signature of something callable.
+///
+/// A signature can have a label, like a function-name, a doc-comment, and a set
+/// of parameters.
+struct SignatureInformation {
+
+  /// The label of this signature. Will be shown in the UI.
+  std::string label;
+
+  /// The human-readable doc-comment of this signature. Will be shown in the UI
+  /// but can be omitted.
+  std::string documentation;
+
+  /// The parameters of this signature.
+  std::vector<ParameterInformation> parameters;
+
+  static std::string unparse(const SignatureInformation &);
+};
+
+/// Signature help represents the signature of something callable.
+///
+/// There can be multiple signature but only one active and only one active
+/// parameter.
+struct SignatureHelp {
+
+  /// One or more signatures.
+  std::vector<SignatureInformation> signatures;
+
+  /// The active signature.
+  ///
+  /// If omitted or the value lies outside the range of `signatures` the value
+  /// defaults to zero or is ignored if `signatures.length === 0`. Whenever
+  /// possible implementors should make an active decision about the active
+  /// signature and shouldn't rely on a default value. In future version of the
+  /// protocol this property might become mandantory to better express this.
+  int activeSignature = 0;
+
+  /// The active parameter of the active signature.
+  ///
+  /// If omitted or the value lies outside the range of
+  /// `signatures[activeSignature].parameters` defaults to 0 if the active
+  /// signature has parameters. If the active signature has no parameters it is
+  /// ignored. In future version of the protocol this property might become
+  /// mandantory to better express the active parameter if the active signature
+  /// does have any.
+  int activeParameter = 0;
+
+  static std::string unparse(const SignatureHelp &);
+};
+
 } // namespace clangd
 } // namespace clang
 
Index: clangd/Protocol.cpp
===================================================================
--- clangd/Protocol.cpp
+++ clangd/Protocol.cpp
@@ -743,3 +743,58 @@
   Result.back() = '}';
   return Result;
 }
+
+std::string ParameterInformation::unparse(const ParameterInformation &PI) {
+  std::string Result = "{";
+  llvm::raw_string_ostream Os(Result);
+  assert(!PI.label.empty() && "parameter information label is required");
+  Os << R"("label":")" << llvm::yaml::escape(PI.label) << '\"';
+  if (!PI.documentation.empty())
+    Os << R"(,"documentation":")" << llvm::yaml::escape(PI.documentation)
+       << '\"';
+  Os << '}';
+  Os.flush();
+  return Result;
+}
+
+std::string SignatureInformation::unparse(const SignatureInformation &SI) {
+  std::string Result = "{";
+  llvm::raw_string_ostream Os(Result);
+  assert(!SI.label.empty() && "signature information label is required");
+  Os << R"("label":")" << llvm::yaml::escape(SI.label) << '\"';
+  if (!SI.documentation.empty())
+    Os << R"(,"documentation":")" << llvm::yaml::escape(SI.documentation)
+       << '\"';
+  Os << R"(,"parameters":[)";
+  for (const auto &Parameter : SI.parameters) {
+    Os << ParameterInformation::unparse(Parameter) << ',';
+  }
+  Os.flush();
+  if (SI.parameters.empty())
+    Result.push_back(']');
+  else
+    Result.back() = ']'; // Replace the last `,` with an `]`.
+  Result.push_back('}');
+  return Result;
+}
+
+std::string SignatureHelp::unparse(const SignatureHelp &SH) {
+  std::string Result = "{";
+  llvm::raw_string_ostream Os(Result);
+  assert(SH.activeSignature >= 0 &&
+         "Unexpected negative value for number of active signatures.");
+  assert(SH.activeParameter >= 0 &&
+         "Unexpected negative value for active parameter index");
+  Os << R"("activeSignature":)" << SH.activeSignature
+     << R"(,"activeParameter":)" << SH.activeParameter << R"(,"signatures":[)";
+  for (const auto &Signature : SH.signatures) {
+    Os << SignatureInformation::unparse(Signature) << ',';
+  }
+  Os.flush();
+  if (SH.signatures.empty())
+    Result.push_back(']');
+  else
+    Result.back() = ']'; // Replace the last `,` with an `]`.
+  Result.push_back('}');
+  return Result;
+}
Index: clangd/ClangdUnit.h
===================================================================
--- clangd/ClangdUnit.h
+++ clangd/ClangdUnit.h
@@ -256,6 +256,13 @@
              std::shared_ptr<PCHContainerOperations> PCHs,
              bool SnippetCompletions);
 
+/// Get signature help at a specified \p Pos in \p FileName.
+SignatureHelp signatureHelp(PathRef FileName, tooling::CompileCommand Command,
+                            PrecompiledPreamble const *Preamble,
+                            StringRef Contents, Position Pos,
+                            IntrusiveRefCntPtr<vfs::FileSystem> VFS,
+                            std::shared_ptr<PCHContainerOperations> PCHs);
+
 /// Get definition of symbol at a specified \p Pos.
 std::vector<Location> findDefinitions(ParsedAST &AST, Position Pos);
 
Index: clangd/ClangdUnit.cpp
===================================================================
--- clangd/ClangdUnit.cpp
+++ clangd/ClangdUnit.cpp
@@ -118,6 +118,25 @@
   llvm_unreachable("Unknown diagnostic level!");
 }
 
+/// Get the optional chunk as a string. This function is possibly recursive.
+std::string getOptionalString(const CodeCompletionString &CCS) {
+  std::string Result;
+  for (const auto &Chunk : CCS) {
+    switch (Chunk.Kind) {
+    case CodeCompletionString::CK_Optional:
+      assert(Chunk.Optional &&
+             "Expected the optional code completion string to be non-null.");
+      Result += getOptionalString(*Chunk.Optional);
+      break;
+    case CodeCompletionString::CK_VerticalSpace:
+      break;
+    default:
+      Result += Chunk.Text;
+    }
+  }
+  return Result;
+}
+
 llvm::Optional<DiagWithFixIts> toClangdDiag(StoredDiagnostic D) {
   auto Location = D.getLocation();
   if (!Location.isValid() || !Location.getManager().isInMainFile(Location))
@@ -283,6 +302,43 @@
   return Result;
 }
 
+std::string getDocumentation(const CodeCompletionString &CCS) {
+  // Things like __attribute__((nonnull(1,3))) and [[noreturn]]. Present this
+  // information in the documentation field.
+  std::string Result;
+  const unsigned AnnotationCount = CCS.getAnnotationCount();
+  if (AnnotationCount > 0) {
+    Result += "Annotation";
+    if (AnnotationCount == 1) {
+      Result += ": ";
+    } else /* AnnotationCount > 1 */ {
+      Result += "s: ";
+    }
+    for (unsigned I = 0; I < AnnotationCount; ++I) {
+      Result += CCS.getAnnotation(I);
+      Result.push_back(I == AnnotationCount - 1 ? '\n' : ' ');
+    }
+  }
+  // Add brief documentation (if there is any).
+  if (CCS.getBriefComment() != nullptr) {
+    if (!Result.empty()) {
+      // This means we previously added annotations. Add an extra newline
+      // character to make the annotations stand out.
+      Result.push_back('\n');
+    }
+    Result += CCS.getBriefComment();
+  }
+  return Result;
+}
+
+void fillSortText(const CodeCompletionString &CCS, CompletionItem &Item) {
+  // Fill in the sortText of the CompletionItem.
+  assert(CCS.getPriority() < 99999 && "Expecting code completion result "
+                                      "priority to have at most 5-digits");
+  llvm::raw_string_ostream(Item.sortText)
+      << llvm::format("%05d%s", CCS.getPriority(), Item.filterText.c_str());
+}
+
 class CompletionItemsCollector : public CodeCompleteConsumer {
 
 public:
@@ -321,61 +377,23 @@
     CompletionItem Item;
     Item.insertTextFormat = InsertTextFormat::PlainText;
 
-    FillDocumentation(CCS, Item);
+    Item.documentation = getDocumentation(CCS);
 
     // Fill in the label, detail, insertText and filterText fields of the
     // CompletionItem.
     ProcessChunks(CCS, Item);
 
     // Fill in the kind field of the CompletionItem.
     Item.kind = getKind(Result.CursorKind);
 
-    FillSortText(CCS, Item);
+    fillSortText(CCS, Item);
 
     return Item;
   }
 
   virtual void ProcessChunks(const CodeCompletionString &CCS,
                              CompletionItem &Item) const = 0;
 
-  void FillDocumentation(const CodeCompletionString &CCS,
-                         CompletionItem &Item) const {
-    // Things like __attribute__((nonnull(1,3))) and [[noreturn]]. Present this
-    // information in the documentation field.
-    const unsigned AnnotationCount = CCS.getAnnotationCount();
-    if (AnnotationCount > 0) {
-      Item.documentation += "Annotation";
-      if (AnnotationCount == 1) {
-        Item.documentation += ": ";
-      } else /* AnnotationCount > 1 */ {
-        Item.documentation += "s: ";
-      }
-      for (unsigned I = 0; I < AnnotationCount; ++I) {
-        Item.documentation += CCS.getAnnotation(I);
-        Item.documentation.push_back(I == AnnotationCount - 1 ? '\n' : ' ');
-      }
-    }
-
-    // Add brief documentation (if there is any).
-    if (CCS.getBriefComment() != nullptr) {
-      if (!Item.documentation.empty()) {
-        // This means we previously added annotations. Add an extra newline
-        // character to make the annotations stand out.
-        Item.documentation.push_back('\n');
-      }
-      Item.documentation += CCS.getBriefComment();
-    }
-  }
-
-  void FillSortText(const CodeCompletionString &CCS,
-                    CompletionItem &Item) const {
-    // Fill in the sortText of the CompletionItem.
-    assert(CCS.getPriority() < 99999 && "Expecting code completion result "
-                                        "priority to have at most 5-digits");
-    llvm::raw_string_ostream(Item.sortText)
-        << llvm::format("%05d%s", CCS.getPriority(), Item.filterText.c_str());
-  }
-
   std::vector<CompletionItem> &Items;
   std::shared_ptr<clang::GlobalCodeCompletionAllocator> Allocator;
   CodeCompletionTUInfo CCTUInfo;
@@ -518,14 +536,125 @@
     }
   }
 }; // SnippetCompletionItemsCollector
-} // namespace
 
-std::vector<CompletionItem>
-clangd::codeComplete(PathRef FileName, tooling::CompileCommand Command,
-                     PrecompiledPreamble const *Preamble, StringRef Contents,
-                     Position Pos, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
-                     std::shared_ptr<PCHContainerOperations> PCHs,
-                     bool SnippetCompletions) {
+class SignatureHelpCollector final : public CodeCompleteConsumer {
+
+public:
+  SignatureHelpCollector(const CodeCompleteOptions &CodeCompleteOpts,
+                         SignatureHelp &SigHelp)
+      : CodeCompleteConsumer(CodeCompleteOpts, /*OutputIsBinary=*/false),
+        SigHelp(SigHelp),
+        Allocator(std::make_shared<clang::GlobalCodeCompletionAllocator>()),
+        CCTUInfo(Allocator) {}
+
+  void ProcessOverloadCandidates(Sema &S, unsigned CurrentArg,
+                                 OverloadCandidate *Candidates,
+                                 unsigned NumCandidates) override {
+    SigHelp.signatures.reserve(NumCandidates);
+    // FIXME(rwols): How can we determine the "active overload candidate"?
+    // Right now the overloaded candidates seem to be provided in a "best fit"
+    // order, so I'm not too worried about this.
+    SigHelp.activeSignature = 0;
+    assert(CurrentArg <= std::numeric_limits<int>::max() &&
+           "too many arguments");
+    SigHelp.activeParameter = static_cast<int>(CurrentArg);
+    for (unsigned I = 0; I < NumCandidates; ++I) {
+      const auto &Candidate = Candidates[I];
+      const auto *CCS = Candidate.CreateSignatureString(
+          CurrentArg, S, *Allocator, CCTUInfo, true);
+      assert(CCS && "Expected the CodeCompletionString to be non-null");
+      SigHelp.signatures.push_back(ProcessOverloadCandidate(Candidate, *CCS));
+    }
+  }
+
+  GlobalCodeCompletionAllocator &getAllocator() override { return *Allocator; }
+
+  CodeCompletionTUInfo &getCodeCompletionTUInfo() override { return CCTUInfo; }
+
+private:
+  SignatureInformation
+  ProcessOverloadCandidate(const OverloadCandidate &Candidate,
+                           const CodeCompletionString &CCS) const {
+    SignatureInformation Result;
+    std::string OptionalParameterLabel;
+    const char *ReturnType = nullptr;
+
+    Result.documentation = getDocumentation(CCS);
+
+    for (const auto &Chunk : CCS) {
+      switch (Chunk.Kind) {
+      case CodeCompletionString::CK_ResultType:
+        // A piece of text that describes the type of an entity or,
+        // for functions and methods, the return type.
+        assert(!ReturnType && "Unexpected CK_ResultType");
+        ReturnType = Chunk.Text;
+        break;
+      case CodeCompletionString::CK_Placeholder:
+        // A string that acts as a placeholder for, e.g., a function call
+        // argument.
+        // Intentional fallthrough here.
+      case CodeCompletionString::CK_CurrentParameter: {
+        // A piece of text that describes the parameter that corresponds to
+        // the code-completion location within a function call, message send,
+        // macro invocation, etc.
+        Result.label += Chunk.Text;
+        ParameterInformation Info;
+        Info.label = Chunk.Text;
+        Result.parameters.push_back(std::move(Info));
+        break;
+      }
+      case CodeCompletionString::CK_Optional: {
+        assert(Chunk.Optional &&
+               "Expected the optional code completion string to be non-null.");
+        ParameterInformation Info;
+        OptionalParameterLabel = getOptionalString(*Chunk.Optional);
+        Result.label += OptionalParameterLabel;
+        // Find the first non-whitespace character.
+        auto CharIndex = OptionalParameterLabel.find_first_not_of(" \t");
+        if (OptionalParameterLabel[CharIndex] == ',') {
+          // If it's a comma, then that means we have an optional parameter that
+          // is *not* the first parameter. In this case. Now find the first
+          // non-whitespace character after the first comma.
+          CharIndex =
+              OptionalParameterLabel.find_first_not_of(" \t", CharIndex + 1);
+          // Use the substring [CharIndex, size()) as the parameter label.
+          Info.label = OptionalParameterLabel.substr(CharIndex);
+        } else {
+          // Otherwise, we must have a function signature where the first
+          // parameter is optional.
+          Info.label = std::move(OptionalParameterLabel);
+        }
+        Result.parameters.push_back(std::move(Info));
+        break;
+      }
+      case CodeCompletionString::CK_VerticalSpace:
+        break;
+      default:
+        Result.label += Chunk.Text;
+        break;
+      }
+    }
+    if (ReturnType) {
+      Result.label += " -> ";
+      Result.label += ReturnType;
+    }
+    return Result;
+  }
+
+  SignatureHelp &SigHelp;
+  std::shared_ptr<clang::GlobalCodeCompletionAllocator> Allocator;
+  CodeCompletionTUInfo CCTUInfo;
+
+}; // SignatureHelpCollector
+
+template <class ReturnType, class CodeCompleteConsumerType>
+ReturnType
+invokeClangAction(PathRef FileName, tooling::CompileCommand Command,
+                  PrecompiledPreamble const *Preamble, StringRef Contents,
+                  Position Pos, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
+                  std::shared_ptr<PCHContainerOperations> PCHs,
+                  bool IncludeGlobals, bool IncludeMacros,
+                  bool IncludeCodePatterns, bool IncludeBriefComments) {
   std::vector<const char *> ArgStrs;
   for (const auto &S : Command.CommandLine)
     ArgStrs.push_back(S.c_str());
@@ -562,36 +691,64 @@
   auto &FrontendOpts = Clang->getFrontendOpts();
   FrontendOpts.SkipFunctionBodies = true;
 
-  FrontendOpts.CodeCompleteOpts.IncludeGlobals = true;
-  FrontendOpts.CodeCompleteOpts.IncludeMacros = true;
-  FrontendOpts.CodeCompleteOpts.IncludeBriefComments = true;
+  FrontendOpts.CodeCompleteOpts.IncludeGlobals = IncludeGlobals;
+  FrontendOpts.CodeCompleteOpts.IncludeMacros = IncludeMacros;
+  FrontendOpts.CodeCompleteOpts.IncludeCodePatterns = IncludeCodePatterns;
+  FrontendOpts.CodeCompleteOpts.IncludeBriefComments = IncludeBriefComments;
 
   FrontendOpts.CodeCompletionAt.FileName = FileName;
   FrontendOpts.CodeCompletionAt.Line = Pos.line + 1;
   FrontendOpts.CodeCompletionAt.Column = Pos.character + 1;
 
-  std::vector<CompletionItem> Items;
-  if (SnippetCompletions) {
-    FrontendOpts.CodeCompleteOpts.IncludeCodePatterns = true;
-    Clang->setCodeCompletionConsumer(new SnippetCompletionItemsCollector(
-        FrontendOpts.CodeCompleteOpts, Items));
-  } else {
-    FrontendOpts.CodeCompleteOpts.IncludeCodePatterns = false;
-    Clang->setCodeCompletionConsumer(new PlainTextCompletionItemsCollector(
-        FrontendOpts.CodeCompleteOpts, Items));
-  }
+  ReturnType Results;
+  Clang->setCodeCompletionConsumer(
+      new CodeCompleteConsumerType(FrontendOpts.CodeCompleteOpts, Results));
 
   SyntaxOnlyAction Action;
   if (!Action.BeginSourceFile(*Clang, Clang->getFrontendOpts().Inputs[0])) {
     // FIXME(ibiryukov): log errors
-    return Items;
+    return Results;
   }
   if (!Action.Execute()) {
     // FIXME(ibiryukov): log errors
   }
   Action.EndSourceFile();
 
-  return Items;
+  return Results;
+}
+
+} // namespace
+
+std::vector<CompletionItem>
+clangd::codeComplete(PathRef FileName, tooling::CompileCommand Command,
+                     PrecompiledPreamble const *Preamble, StringRef Contents,
+                     Position Pos, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
+                     std::shared_ptr<PCHContainerOperations> PCHs,
+                     bool SnippetCompletions) {
+  if (SnippetCompletions) {
+    return invokeClangAction<std::vector<CompletionItem>,
+                             SnippetCompletionItemsCollector>(
+        FileName, Command, Preamble, Contents, Pos, std::move(VFS),
+        std::move(PCHs), /*IncludeGlobals=*/true, /*IncludeMacros=*/true,
+        /*IncludeCodePatterns=*/true, /*IncludeBriefComments=*/true);
+  } else {
+    return invokeClangAction<std::vector<CompletionItem>,
+                             PlainTextCompletionItemsCollector>(
+        FileName, Command, Preamble, Contents, Pos, std::move(VFS),
+        std::move(PCHs), /*IncludeGlobals=*/true, /*IncludeMacros=*/true,
+        /*IncludeCodePatterns=*/false, /*IncludeBriefComments=*/true);
+  }
+}
+
+SignatureHelp
+clangd::signatureHelp(PathRef FileName, tooling::CompileCommand Command,
+                      PrecompiledPreamble const *Preamble, StringRef Contents,
+                      Position Pos, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
+                      std::shared_ptr<PCHContainerOperations> PCHs) {
+  return invokeClangAction<SignatureHelp, SignatureHelpCollector>(
+      FileName, Command, Preamble, Contents, Pos, std::move(VFS),
+      std::move(PCHs), /*IncludeGlobals=*/false, /*IncludeMacros=*/false,
+      /*IncludeCodePatterns=*/false, /*IncludeBriefComments=*/true);
 }
 
 void clangd::dumpAST(ParsedAST &AST, llvm::raw_ostream &OS) {
Index: clangd/ClangdServer.h
===================================================================
--- clangd/ClangdServer.h
+++ clangd/ClangdServer.h
@@ -238,6 +238,19 @@
   codeComplete(PathRef File, Position Pos,
                llvm::Optional<StringRef> OverridenContents = llvm::None,
                IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS = nullptr);
+
+  /// Provide signature help for \p File at \p Pos. If \p OverridenContents is
+  /// not None, they will used only for signature help, i.e. no diagnostics
+  /// update will be scheduled and a draft for \p File will not be updated. If
+  /// \p OverridenContents is None, contents of the current draft for \p File
+  /// will be used. If \p UsedFS is non-null, it will be overwritten by
+  /// vfs::FileSystem used for signature help. This method should only be called
+  /// for currently tracked files.
+  Tagged<SignatureHelp>
+  signatureHelp(PathRef File, Position Pos,
+                llvm::Optional<StringRef> OverridenContents = llvm::None,
+                IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS = nullptr);
+
   /// Get definition of symbol at a specified \p Line and \p Column in \p File.
   Tagged<std::vector<Location>> findDefinitions(PathRef File, Position Pos);
 
Index: clangd/ClangdServer.cpp
===================================================================
--- clangd/ClangdServer.cpp
+++ clangd/ClangdServer.cpp
@@ -214,6 +214,35 @@
   return make_tagged(std::move(Result), TaggedFS.Tag);
 }
 
+Tagged<SignatureHelp>
+ClangdServer::signatureHelp(PathRef File, Position Pos,
+                            llvm::Optional<StringRef> OverridenContents,
+                            IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS) {
+  std::string DraftStorage;
+  if (!OverridenContents) {
+    auto FileContents = DraftMgr.getDraft(File);
+    assert(FileContents.Draft &&
+           "signatureHelp is called for non-added document");
+
+    DraftStorage = std::move(*FileContents.Draft);
+    OverridenContents = DraftStorage;
+  }
+
+  auto TaggedFS = FSProvider.getTaggedFileSystem(File);
+  if (UsedFS)
+    *UsedFS = TaggedFS.Value;
+
+  std::shared_ptr<CppFile> Resources = Units.getFile(File);
+  assert(Resources && "Calling signatureHelp on non-added file");
+
+  auto Preamble = Resources->getPossiblyStalePreamble();
+  auto Result =
+      clangd::signatureHelp(File, Resources->getCompileCommand(),
+                            Preamble ? &Preamble->Preamble : nullptr,
+                            *OverridenContents, Pos, TaggedFS.Value, PCHs);
+  return make_tagged(std::move(Result), TaggedFS.Tag);
+}
+
 std::vector<tooling::Replacement> ClangdServer::formatRange(PathRef File,
                                                             Range Rng) {
   std::string Code = getDocument(File);
Index: clangd/ClangdLSPServer.cpp
===================================================================
--- clangd/ClangdLSPServer.cpp
+++ clangd/ClangdLSPServer.cpp
@@ -69,6 +69,8 @@
                     JSONOutput &Out) override;
   void onCompletion(TextDocumentPositionParams Params, StringRef ID,
                     JSONOutput &Out) override;
+  void onSignatureHelp(TextDocumentPositionParams Params, StringRef ID,
+                       JSONOutput &Out) override;
   void onGoToDefinition(TextDocumentPositionParams Params, StringRef ID,
                         JSONOutput &Out) override;
 
@@ -87,6 +89,7 @@
           "documentOnTypeFormattingProvider": {"firstTriggerCharacter":"}","moreTriggerCharacter":[]},
           "codeActionProvider": true,
           "completionProvider": {"resolveProvider": false, "triggerCharacters": [".",">",":"]},
+          "signatureHelpProvider": {"triggerCharacters": ["(", ","]},
           "definitionProvider": true
         }}})");
 }
@@ -199,6 +202,18 @@
       R"(,"result":[)" + Completions + R"(]})");
 }
 
+void ClangdLSPServer::LSPProtocolCallbacks::onSignatureHelp(
+    TextDocumentPositionParams Params, StringRef ID, JSONOutput &Out) {
+  const auto SigHelp = SignatureHelp::unparse(
+      LangServer.Server
+          .signatureHelp(
+              Params.textDocument.uri.file,
+              Position{Params.position.line, Params.position.character})
+          .Value);
+  Out.writeMessage(R"({"jsonrpc":"2.0","id":)" + ID.str() + R"(,"result":)" +
+                   SigHelp + "}");
+}
+
 void ClangdLSPServer::LSPProtocolCallbacks::onGoToDefinition(
     TextDocumentPositionParams Params, StringRef ID, JSONOutput &Out) {
 
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