[CodeCompletion] Signature help for template argument lists

[sammccall]

Summary

Provide signature while typing template arguments: Foo< ^here >
Here the parameters are e.g. "typename x", and the result type is e.g.
"struct" (class template) or "int" (variable template) or "bool (std::string)"
(function template).

Multiple overloads are possible when a template name is used for several
overloaded function templates.

Fixes https://github.com/clangd/clangd/issues/299

[sammccall]

Clean up style, remove fixed fixme, add an extra testcase.

[kadircet]

thanks this looks great! just a question around rendering of the result type chunk. feel free to leave a fixme if there's no easy way of doing that.

clang/lib/Parse/ParseTemplate.cpp

1237

s/NameHint/Template ?

clang/lib/Sema/SemaCodeComplete.cpp

3785

the rendering looks a little bit surprising to me foo<arg1,arg2> -> bool (float) we might consider printing parameters first, e.g. foo<arg1,arg2>(float) -> bool. i think this also looks more natural for the following signature help on function parameters.

[sammccall]

clang/lib/Sema/SemaCodeComplete.cpp

3785

Yeah. There are a few things going on here I wasn't sure how to square:

1. the "arrow" rendering is clangd-specific, and it reorders chunks by kind (moving the ResultType chunk to the end). So if we want a specific rendering in clangd we need to be able to infer it from the chunk sequence
2. the chunk sequence should make sense when rendered as text more directly. I think the current version fails this test: bool(float) foo<arg1, arg2>.
3. the rendering you suggest requires us to reverse the order of bool and float somehow, ideally without some fragile coupling/assumptions in clangd

I think the "natural" chunk sequence is bool foo< arg1 , arg2 > (float). The bool chunk is definitely ResultType.

If we make (float) also ResultType, we know have two RT chunks we have to stick in the signature in the right place. So maybe it's best to make it CK_Text or CK_Informative. I'll give this a spin...

[sammccall]

Change ResultType handling for template functions. Given:

template <typename T> string getName(int count) const &

We return the signature:

{ResultType=string} getName<{placeholder=typename T}>()

Clangd renders this as:

getName<$(0:typename T)>() -> string

We drop the function parameter list because

• it's potentially long and distracts from the template args we're showing
• there's no convenient way to print it!

[kadircet]

thanks! as discussed offline i think it's better to ignore function parameters for now.

clang/lib/Sema/SemaCodeComplete.cpp

3781

the comments around the suffix seems to be out-of-date (maybe mention it as a fixme?)

clang-tools-extra/clangd/ClangdLSPServer.cpp

llvm::json::Object ServerCaps{
            {"full", llvm::json::Object{{"delta", true}}},
            {"range", false},
            {"legend",
             llvm::json::Object{{"tokenTypes", semanticTokenTypes()},
                                {"tokenModifiers", semanticTokenModifiers()}}},
        }},
       {"signatureHelpProvider",
        llvm::json::Object{
-           {"triggerCharacters", {"(", ",", ")"}},
+           {"triggerCharacters", {"(", ",", ")", "<", ">"}},
        }},
       {"declarationProvider", true},
       {"definitionProvider", true},
       {"implementationProvider", true},
       {"documentHighlightProvider", true},
       {"documentLinkProvider",
        llvm::json::Object{
            {"resolveProvider", false},

clang-tools-extra/clangd/CodeComplete.cpp

 
 // Returns the index of the parameter matching argument number "Arg.
 // This is usually just "Arg", except for variadic functions/templates, where
 // "Arg" might be higher than the number of parameters. When that happens, we
 // assume the last parameter is variadic and assume all further args are
 // part of it.
 int paramIndexForArg(const CodeCompleteConsumer::OverloadCandidate &Candidate,
                      int Arg) {
-  int NumParams = 0;
+  int NumParams = Candidate.getNumParams();
   if (const auto *F = Candidate.getFunction()) {
-    NumParams = F->getNumParams();
     if (F->isVariadic())
       ++NumParams;
   } else if (auto *T = Candidate.getFunctionType()) {
     if (auto *Proto = T->getAs<FunctionProtoType>()) {
-      NumParams = Proto->getNumParams();
       if (Proto->isVariadic())
         ++NumParams;
     }
   }
   return std::min(Arg, std::max(NumParams - 1, 0));
 }
 
 class SignatureHelpCollector final : public CodeCompleteConsumer {
llvm::sort(ScoredSignatures, [](const ScoredSignature &L,
         using OC = CodeCompleteConsumer::OverloadCandidate;
         switch (L.Quality.Kind) {
         case OC::CK_Function:
           return true;
         case OC::CK_FunctionType:
           return R.Quality.Kind != OC::CK_Function;
         case OC::CK_FunctionTemplate:
           return false;
+        case OC::CK_Template:
+          assert(false && "Never see templates and other overloads mixed");
+          return false;
         }
         llvm_unreachable("Unknown overload candidate type.");
       }
       if (L.Signature.label.size() != R.Signature.label.size())
         return L.Signature.label.size() < R.Signature.label.size();
       return L.Signature.label < R.Signature.label;
     });
 
void ProcessOverloadCandidates(Sema &S, unsigned CurrentArg,
                                  OverloadCandidate *Candidates,
                                  unsigned NumCandidates,
                                  SourceLocation OpenParLoc) override {
     assert(CurrentArg <= (unsigned)std::numeric_limits<int>::max() &&
            "too many arguments");
 
     for (unsigned I = 0; I < NumCandidates; ++I) {
       OverloadCandidate Candidate = Candidates[I];
-      auto *Func = Candidate.getFunction();
-      if (!Func || Func->getNumParams() <= CurrentArg)
-        continue;
-      auto *PVD = Func->getParamDecl(CurrentArg);
-      if (!PVD)
+      NamedDecl *Param = nullptr;
+      if (auto *Func = Candidate.getFunction()) {
+        if (CurrentArg < Func->getNumParams())
+          Param = Func->getParamDecl(CurrentArg);
+      } else if (auto *Template = Candidate.getTemplate()) {
+        if (CurrentArg < Template->getTemplateParameters()->size())
+          Param = Template->getTemplateParameters()->getParam(CurrentArg);
+      }
+
+      if (!Param)
         continue;
-      auto *Ident = PVD->getIdentifier();
+      auto *Ident = Param->getIdentifier();
       if (!Ident)
         continue;
       auto Name = Ident->getName();
       if (!Name.empty())
         ParamNames.insert(Name.str());
     }
   }
 

clang-tools-extra/clangd/test/initialize-params.test

 # CHECK:               ]
 # CHECK-NEXT:        },
 # CHECK-NEXT:        "range": false
 # CHECK-NEXT:      },
 # CHECK-NEXT:      "signatureHelpProvider": {
 # CHECK-NEXT:        "triggerCharacters": [
 # CHECK-NEXT:          "(",
 # CHECK-NEXT:          ",",
-# CHECK-NEXT:          ")"
+# CHECK-NEXT:          ")",
+# CHECK-NEXT:          "<",
+# CHECK-NEXT:          ">"
 # CHECK-NEXT:        ]
 # CHECK-NEXT:      },
 # CHECK-NEXT:      "textDocumentSync": {
 # CHECK-NEXT:        "change": 2,
 # CHECK-NEXT:        "openClose": true,
 # CHECK-NEXT:        "save": true
 # CHECK-NEXT:      },
 # CHECK-NEXT:      "typeHierarchyProvider": true

clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp

for (auto DocumentationFormat :
        {MarkupKind::PlainText, MarkupKind::Markdown}) {
     auto Sigs = signatures(Code.code(), Code.point(), /*IndexSymbols=*/{},
                            DocumentationFormat);
     ASSERT_EQ(Sigs.signatures.size(), 1U);
     EXPECT_EQ(Sigs.signatures[0].documentation.kind, DocumentationFormat);
   }
 }
 
+TEST(SignatureHelp, TemplateArguments) {
+  std::string Top = R"cpp(
+    template <typename T, int> bool foo(char);
+    template <int I, int> bool foo(float);
+  )cpp";
+
+  auto First = signatures(Top + "bool x = foo<^");
+  EXPECT_THAT(
+      First.signatures,
+      UnorderedElementsAre(Sig("foo<[[typename T]], [[int]]>() -> bool"),
+                           Sig("foo<[[int I]], [[int]]>() -> bool")));
+  EXPECT_EQ(First.activeParameter, 0);
+
+  auto Second = signatures(Top + "bool x = foo<1, ^");
+  EXPECT_THAT(Second.signatures,
+              ElementsAre(Sig("foo<[[int I]], [[int]]>() -> bool")));
+  EXPECT_EQ(Second.activeParameter, 1);
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang

clang/include/clang/Parse/Parser.h

private:
 
   bool ParseGreaterThanInTemplateList(SourceLocation LAngleLoc,
                                       SourceLocation &RAngleLoc,
                                       bool ConsumeLastToken,
                                       bool ObjCGenericList);
   bool ParseTemplateIdAfterTemplateName(bool ConsumeLastToken,
                                         SourceLocation &LAngleLoc,
                                         TemplateArgList &TemplateArgs,
-                                        SourceLocation &RAngleLoc);
+                                        SourceLocation &RAngleLoc,
+                                        TemplateTy NameHint = nullptr);
 
   bool AnnotateTemplateIdToken(TemplateTy Template, TemplateNameKind TNK,
                                CXXScopeSpec &SS,
                                SourceLocation TemplateKWLoc,
                                UnqualifiedId &TemplateName,
                                bool AllowTypeAnnotation = true,
                                bool TypeConstraint = false);
   void AnnotateTemplateIdTokenAsType(CXXScopeSpec &SS,
                                      bool IsClassName = false);
-  bool ParseTemplateArgumentList(TemplateArgList &TemplateArgs);
+  bool ParseTemplateArgumentList(TemplateArgList &TemplateArgs,
+                                 TemplateTy Template, SourceLocation OpenLoc);
   ParsedTemplateArgument ParseTemplateTemplateArgument();
   ParsedTemplateArgument ParseTemplateArgument();
   Decl *ParseExplicitInstantiation(DeclaratorContext Context,
                                    SourceLocation ExternLoc,
                                    SourceLocation TemplateLoc,
                                    SourceLocation &DeclEnd,
                                    ParsedAttributes &AccessAttrs,
                                    AccessSpecifier AS = AS_none);

clang/include/clang/Sema/CodeCompleteConsumer.h

 public:
   class OverloadCandidate {
   public:
     /// Describes the type of overload candidate.
     enum CandidateKind {
       /// The candidate is a function declaration.
       CK_Function,
 
-      /// The candidate is a function template.
+      /// The candidate is a function template, arguments are being completed.
       CK_FunctionTemplate,
 
       /// The "candidate" is actually a variable, expression, or block
       /// for which we only have a function prototype.
-      CK_FunctionType
+      CK_FunctionType,
+
+      /// The candidate is a template, template arguments are being completed.
+      CK_Template,
     };
 
   private:
     /// The kind of overload candidate.
     CandidateKind Kind;
 
     union {
       /// The function overload candidate, available when
       /// Kind == CK_Function.
       FunctionDecl *Function;
 
       /// The function template overload candidate, available when
       /// Kind == CK_FunctionTemplate.
       FunctionTemplateDecl *FunctionTemplate;
 
       /// The function type that describes the entity being called,
       /// when Kind == CK_FunctionType.
       const FunctionType *Type;
+
+      /// The template overload candidate, available when
+      /// Kind == CK_Template.
+      const TemplateDecl *Template;
     };
 
   public:
     OverloadCandidate(FunctionDecl *Function)
         : Kind(CK_Function), Function(Function) {}
 
     OverloadCandidate(FunctionTemplateDecl *FunctionTemplateDecl)
         : Kind(CK_FunctionTemplate), FunctionTemplate(FunctionTemplateDecl) {}
 
     OverloadCandidate(const FunctionType *Type)
         : Kind(CK_FunctionType), Type(Type) {}
 
+    OverloadCandidate(const TemplateDecl *Template)
+        : Kind(CK_Template), Template(Template) {}
+
     /// Determine the kind of overload candidate.
     CandidateKind getKind() const { return Kind; }
 
     /// Retrieve the function overload candidate or the templated
     /// function declaration for a function template.
     FunctionDecl *getFunction() const;
 
     /// Retrieve the function template overload candidate.
     FunctionTemplateDecl *getFunctionTemplate() const {
       assert(getKind() == CK_FunctionTemplate && "Not a function template");
       return FunctionTemplate;
     }
 
     /// Retrieve the function type of the entity, regardless of how the
     /// function is stored.
     const FunctionType *getFunctionType() const;
 
+    const TemplateDecl *getTemplate() const {
+      assert(getKind() == CK_Template && "Not a template");
+      return Template;
+    }
+
+    unsigned getNumParams() const;
+
     /// Create a new code-completion string that describes the function
     /// signature of this overload candidate.
     CodeCompletionString *CreateSignatureString(unsigned CurrentArg,
                                                 Sema &S,
                                       CodeCompletionAllocator &Allocator,
                                       CodeCompletionTUInfo &CCTUInfo,
                                       bool IncludeBriefComments) const;
   };

clang/include/clang/Sema/Sema.h

QualType ProduceConstructorSignatureHelp(Scope *S, QualType Type,
                                            ArrayRef<Expr *> Args,
                                            SourceLocation OpenParLoc);
   QualType ProduceCtorInitMemberSignatureHelp(Scope *S, Decl *ConstructorDecl,
                                               CXXScopeSpec SS,
                                               ParsedType TemplateTypeTy,
                                               ArrayRef<Expr *> ArgExprs,
                                               IdentifierInfo *II,
                                               SourceLocation OpenParLoc);
+  QualType ProduceTemplateArgumentSignatureHelp(
+      TemplateTy, ArrayRef<ParsedTemplateArgument>, SourceLocation LAngleLoc);
   void CodeCompleteInitializer(Scope *S, Decl *D);
   /// Trigger code completion for a record of \p BaseType. \p InitExprs are
   /// expressions in the initializer list seen so far and \p D is the current
   /// Designation being parsed.
   void CodeCompleteDesignator(const QualType BaseType,
                               llvm::ArrayRef<Expr *> InitExprs,
                               const Designation &D);
   void CodeCompleteAfterIf(Scope *S, bool IsBracedThen);

clang/lib/Parse/ParseExprCXX.cpp

bool Parser::ParseUnqualifiedIdTemplateId(
 
   default:
     return false;
   }
 
   // Parse the enclosed template argument list.
   SourceLocation LAngleLoc, RAngleLoc;
   TemplateArgList TemplateArgs;
-  if (ParseTemplateIdAfterTemplateName(true, LAngleLoc, TemplateArgs,
-                                       RAngleLoc))
+  if (ParseTemplateIdAfterTemplateName(true, LAngleLoc, TemplateArgs, RAngleLoc,
+                                       Template))
     return true;
 
   // If this is a non-template, we already issued a diagnostic.
   if (TNK == TNK_Non_template)
     return true;
 
   if (Id.getKind() == UnqualifiedIdKind::IK_Identifier ||
       Id.getKind() == UnqualifiedIdKind::IK_OperatorFunctionId ||

clang/lib/Parse/ParseTemplate.cpp

if (ConsumeLastToken) {
     PrevTokLocation = TokBeforeGreaterLoc;
     PP.EnterToken(Tok, /*IsReinject=*/true);
     Tok = Greater;
   }
 
   return false;
 }
 
-
 /// Parses a template-id that after the template name has
 /// already been parsed.
 ///
 /// This routine takes care of parsing the enclosed template argument
 /// list ('<' template-parameter-list [opt] '>') and placing the
 /// results into a form that can be transferred to semantic analysis.
 ///
 /// \param ConsumeLastToken if true, then we will consume the last
 /// token that forms the template-id. Otherwise, we will leave the
 /// last token in the stream (e.g., so that it can be replaced with an
 /// annotation token).
-bool
-Parser::ParseTemplateIdAfterTemplateName(bool ConsumeLastToken,
+///
+/// \param NameHint is not required, and merely affects code completion.
+bool Parser::ParseTemplateIdAfterTemplateName(bool ConsumeLastToken,
                                               SourceLocation &LAngleLoc,
                                               TemplateArgList &TemplateArgs,
-                                         SourceLocation &RAngleLoc) {
+                                              SourceLocation &RAngleLoc,
+                                              TemplateTy Template) {
   assert(Tok.is(tok::less) && "Must have already parsed the template-name");
 
   // Consume the '<'.
   LAngleLoc = ConsumeToken();
 
   // Parse the optional template-argument-list.
   bool Invalid = false;
   {
     GreaterThanIsOperatorScope G(GreaterThanIsOperator, false);
     if (!Tok.isOneOf(tok::greater, tok::greatergreater,
                      tok::greatergreatergreater, tok::greaterequal,
                      tok::greatergreaterequal))
-      Invalid = ParseTemplateArgumentList(TemplateArgs);
+      Invalid = ParseTemplateArgumentList(TemplateArgs, Template, LAngleLoc);
 
     if (Invalid) {
       // Try to find the closing '>'.
       if (getLangOpts().CPlusPlus11)
         SkipUntil(tok::greater, tok::greatergreater,
                   tok::greatergreatergreater, StopAtSemi | StopBeforeMatch);
       else
         SkipUntil(tok::greater, StopAtSemi | StopBeforeMatch);
bool Parser::AnnotateTemplateIdToken(TemplateTy Template, TemplateNameKind TNK,
   // Consume the template-name.
   SourceLocation TemplateNameLoc = TemplateName.getSourceRange().getBegin();
 
   // Parse the enclosed template argument list.
   SourceLocation LAngleLoc, RAngleLoc;
   TemplateArgList TemplateArgs;
   bool ArgsInvalid = false;
   if (!TypeConstraint || Tok.is(tok::less)) {
-    ArgsInvalid = ParseTemplateIdAfterTemplateName(false, LAngleLoc,
-                                                   TemplateArgs, RAngleLoc);
+    ArgsInvalid = ParseTemplateIdAfterTemplateName(
+        false, LAngleLoc, TemplateArgs, RAngleLoc, Template);
     // If we couldn't recover from invalid arguments, don't form an annotation
     // token -- we don't know how much to annotate.
     // FIXME: This can lead to duplicate diagnostics if we retry parsing this
     // template-id in another context. Try to annotate anyway?
     if (RAngleLoc.isInvalid())
       return true;
   }
 

 }
 
 /// ParseTemplateArgumentList - Parse a C++ template-argument-list
 /// (C++ [temp.names]). Returns true if there was an error.
 ///
 ///       template-argument-list: [C++ 14.2]
 ///         template-argument
 ///         template-argument-list ',' template-argument
-bool
-Parser::ParseTemplateArgumentList(TemplateArgList &TemplateArgs) {
+///
+/// \param Template is only used for code completion, and may be null.
+bool Parser::ParseTemplateArgumentList(TemplateArgList &TemplateArgs,
+                                       TemplateTy Template,
+                                       SourceLocation OpenLoc) {
 
   ColonProtectionRAIIObject ColonProtection(*this, false);
 
+  auto RunSignatureHelp = [&] {
+    if (!Template)
+      return QualType();
+    CalledSignatureHelp = true;
+    return Actions.ProduceTemplateArgumentSignatureHelp(Template, TemplateArgs,
+                                                        OpenLoc);
+  };
+
   do {
+    PreferredType.enterFunctionArgument(Tok.getLocation(), RunSignatureHelp);
     ParsedTemplateArgument Arg = ParseTemplateArgument();
     SourceLocation EllipsisLoc;
     if (TryConsumeToken(tok::ellipsis, EllipsisLoc))
       Arg = Actions.ActOnPackExpansion(Arg, EllipsisLoc);
 
-    if (Arg.isInvalid())
+    if (Arg.isInvalid()) {
+      if (PP.isCodeCompletionReached() && !CalledSignatureHelp)
+        RunSignatureHelp();
       return true;
+    }
 
     // Save this template argument.
     TemplateArgs.push_back(Arg);
 
     // If the next token is a comma, consume it and keep reading
     // arguments.
   } while (TryConsumeToken(tok::comma));
 

clang/lib/Sema/CodeCompleteConsumer.cpp

CodeCompleteConsumer::OverloadCandidate::getFunctionType() const {
 
   case CK_FunctionTemplate:
     return FunctionTemplate->getTemplatedDecl()
         ->getType()
         ->getAs<FunctionType>();
 
   case CK_FunctionType:
     return Type;
+
+  case CK_Template:
+    return nullptr;
   }
 
   llvm_unreachable("Invalid CandidateKind!");
 }
 
+unsigned CodeCompleteConsumer::OverloadCandidate::getNumParams() const {
+  if (Kind == CK_Template)
+    return Template->getTemplateParameters()->size();
+  if (const auto *FPT = dyn_cast_or_null<FunctionProtoType>(getFunctionType()))
+    return FPT->getNumParams();
+  return 0;
+}
+
 //===----------------------------------------------------------------------===//
 // Code completion consumer implementation
 //===----------------------------------------------------------------------===//
 
 CodeCompleteConsumer::~CodeCompleteConsumer() = default;
 
 bool PrintingCodeCompleteConsumer::isResultFilteredOut(
     StringRef Filter, CodeCompletionResult Result) {

clang/lib/Sema/SemaCodeComplete.cpp

 #include "clang/Lex/MacroInfo.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
 #include "clang/Sema/DeclSpec.h"
 #include "clang/Sema/Designator.h"
 #include "clang/Sema/Lookup.h"
 #include "clang/Sema/Overload.h"
 #include "clang/Sema/ParsedAttr.h"
+#include "clang/Sema/ParsedTemplate.h"
 #include "clang/Sema/Scope.h"
 #include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/Sema.h"
 #include "clang/Sema/SemaInternal.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallBitVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
if (CurrentArg < NumParams)
       Opt.AddPlaceholderChunk("...");
     else
       Opt.AddCurrentParameterChunk("...");
 
     Result.AddOptionalChunk(Opt.TakeString());
   }
 }
 
+static std::string
+formatTemplateParameterPlaceholder(const NamedDecl *Param, bool &Optional,
+                                   const PrintingPolicy &Policy) {
+  if (const auto *Type = dyn_cast<TemplateTypeParmDecl>(Param)) {
+    Optional = Type->hasDefaultArgument();
+  } else if (const auto *NonType = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
+    Optional = NonType->hasDefaultArgument();
+  } else if (const auto *Template = dyn_cast<TemplateTemplateParmDecl>(Param)) {
+    Optional = Template->hasDefaultArgument();
+  }
+  std::string Result;
+  llvm::raw_string_ostream OS(Result);
+  Param->print(OS, Policy);
+  return Result;
+}
+
+// A string describing the result of a template.
+//
+// If the template is for a function `bool foo<class>(int)`, then returns
+// `bool` and sets Suffix to `(int)`.
+static std::string templateResultType(const TemplateDecl *TD,
+                                      const PrintingPolicy &Policy,
+                                      std::string &Suffix) {
+  if (const auto *CTD = dyn_cast<ClassTemplateDecl>(TD))
+    return CTD->getTemplatedDecl()->getKindName().str();
+  if (const auto *VTD = dyn_cast<VarTemplateDecl>(TD))
+    return VTD->getTemplatedDecl()->getType().getAsString(Policy);
+  if (const auto *FTD = dyn_cast<FunctionTemplateDecl>(TD)) {
+    // For now, indicate this is a function, but don't attempt to list args.
+    // It's fiddly and potentially bulky/distracting.
+    Suffix = "()";
+    return FTD->getTemplatedDecl()->getReturnType().getAsString(Policy);
+  }
+  if (isa<TypeAliasTemplateDecl>(TD))
+    return "type";
+  if (isa<TemplateTemplateParmDecl>(TD))
+    return "class";
+  if (isa<ConceptDecl>(TD))
+    return "concept";
+  return "";
+}
+
+static CodeCompletionString *createTemplateSignatureString(
+    const TemplateDecl *TD, CodeCompletionBuilder &Builder, unsigned CurrentArg,
+    const PrintingPolicy &Policy) {
+  llvm::ArrayRef<NamedDecl *> Params = TD->getTemplateParameters()->asArray();
+  CodeCompletionBuilder OptionalBuilder(Builder.getAllocator(),
+                                        Builder.getCodeCompletionTUInfo());
+  std::string ArgsSuffix;
+  std::string ResultType = templateResultType(TD, Policy, ArgsSuffix);
+  if (!ResultType.empty())
+    Builder.AddResultTypeChunk(Builder.getAllocator().CopyString(ResultType));
+  Builder.AddTextChunk(
+      Builder.getAllocator().CopyString(TD->getNameAsString()));
+  Builder.AddChunk(CodeCompletionString::CK_LeftAngle);
+  // Initially we're writing into the main string. Once we see an optional arg
+  // (with default), we're writing into the nested optional chunk.
+  CodeCompletionBuilder *Current = &Builder;
+  for (unsigned I = 0; I < Params.size(); ++I) {
+    bool Optional = false;
+    std::string Placeholder =
+        formatTemplateParameterPlaceholder(Params[I], Optional, Policy);
+    if (Optional)
+      Current = &OptionalBuilder;
+    if (I > 0)
+      Current->AddChunk(CodeCompletionString::CK_Comma);
+    Current->AddChunk(I == CurrentArg
+                          ? CodeCompletionString::CK_CurrentParameter
+                          : CodeCompletionString::CK_Placeholder,
+                      Current->getAllocator().CopyString(Placeholder));
+  }
+  // Add the optional chunk to the main string if we ever used it.
+  if (Current == &OptionalBuilder)
+    Builder.AddOptionalChunk(OptionalBuilder.TakeString());
+  Builder.AddChunk(CodeCompletionString::CK_RightAngle);
+  if (!ArgsSuffix.empty())
+    Builder.AddInformativeChunk(Builder.getAllocator().CopyString(ArgsSuffix));
+  return Builder.TakeString();
+}
+
 CodeCompletionString *
 CodeCompleteConsumer::OverloadCandidate::CreateSignatureString(
     unsigned CurrentArg, Sema &S, CodeCompletionAllocator &Allocator,
     CodeCompletionTUInfo &CCTUInfo, bool IncludeBriefComments) const {
   PrintingPolicy Policy = getCompletionPrintingPolicy(S);
   // Show signatures of constructors as they are declared:
   //   vector(int n) rather than vector<string>(int n)
   // This is less noisy without being less clear, and avoids tricky cases.
   Policy.SuppressTemplateArgsInCXXConstructors = true;
 
   // FIXME: Set priority, availability appropriately.
   CodeCompletionBuilder Result(Allocator, CCTUInfo, 1,
                                CXAvailability_Available);
+
+  if (getKind() == CK_Template)
+    return createTemplateSignatureString(getTemplate(), Result, CurrentArg,
+                                         Policy);
+
   FunctionDecl *FDecl = getFunction();
   const FunctionProtoType *Proto =
       dyn_cast<FunctionProtoType>(getFunctionType());
   if (!FDecl && !Proto) {
     // Function without a prototype. Just give the return type and a
     // highlighted ellipsis.
     const FunctionType *FT = getFunctionType();
     Result.AddResultTypeChunk(Result.getAllocator().CopyString(

 static QualType getParamType(Sema &SemaRef,
                              ArrayRef<ResultCandidate> Candidates, unsigned N) {
 
   // Given the overloads 'Candidates' for a function call matching all arguments
   // up to N, return the type of the Nth parameter if it is the same for all
   // overload candidates.
   QualType ParamType;
   for (auto &Candidate : Candidates) {
+    // FIXME: handle non-type-template-parameters by merging with D116326
     if (const auto *FType = Candidate.getFunctionType())
       if (const auto *Proto = dyn_cast<FunctionProtoType>(FType))
         if (N < Proto->getNumParams()) {
           if (ParamType.isNull())
             ParamType = Proto->getParamType(N);
           else if (!SemaRef.Context.hasSameUnqualifiedType(
                        ParamType.getNonReferenceType(),
                        Proto->getParamType(N).getNonReferenceType()))
             // Otherwise return a default-constructed QualType.
             return QualType();
         }
   }
 
   return ParamType;
 }
 
 static QualType
-ProduceSignatureHelp(Sema &SemaRef, Scope *S,
-                     MutableArrayRef<ResultCandidate> Candidates,
+ProduceSignatureHelp(Sema &SemaRef, MutableArrayRef<ResultCandidate> Candidates,
                      unsigned CurrentArg, SourceLocation OpenParLoc) {
   if (Candidates.empty())
     return QualType();
   if (SemaRef.getPreprocessor().isCodeCompletionReached())
     SemaRef.CodeCompleter->ProcessOverloadCandidates(
         SemaRef, CurrentArg, Candidates.data(), Candidates.size(), OpenParLoc);
   return getParamType(SemaRef, Candidates, CurrentArg);
 }
if (FD) { // We check whether it's a resolved function declaration.
           Results.push_back(ResultCandidate(FP));
       } else if (auto FT = T->getAs<FunctionType>())
         // No prototype and declaration, it may be a K & R style function.
         Results.push_back(ResultCandidate(FT));
     }
   }
   mergeCandidatesWithResults(*this, Results, CandidateSet, Loc, Args.size());
   QualType ParamType =
-      ProduceSignatureHelp(*this, S, Results, Args.size(), OpenParLoc);
+      ProduceSignatureHelp(*this, Results, Args.size(), OpenParLoc);
   return !CandidateSet.empty() ? ParamType : QualType();
 }
 
 QualType Sema::ProduceConstructorSignatureHelp(Scope *S, QualType Type,
                                                SourceLocation Loc,
                                                ArrayRef<Expr *> Args,
                                                SourceLocation OpenParLoc) {
   if (!CodeCompleter)
if (auto *FD = dyn_cast<FunctionDecl>(C)) {
           /*ExplicitTemplateArgs=*/nullptr, Args, CandidateSet,
           /*SuppressUserConversions=*/false,
           /*PartialOverloading=*/true);
     }
   }
 
   SmallVector<ResultCandidate, 8> Results;
   mergeCandidatesWithResults(*this, Results, CandidateSet, Loc, Args.size());
-  return ProduceSignatureHelp(*this, S, Results, Args.size(), OpenParLoc);
+  return ProduceSignatureHelp(*this, Results, Args.size(), OpenParLoc);
 }
 
 QualType Sema::ProduceCtorInitMemberSignatureHelp(
     Scope *S, Decl *ConstructorDecl, CXXScopeSpec SS, ParsedType TemplateTypeTy,
     ArrayRef<Expr *> ArgExprs, IdentifierInfo *II, SourceLocation OpenParLoc) {
   if (!CodeCompleter)
     return QualType();
 
   CXXConstructorDecl *Constructor =
       dyn_cast<CXXConstructorDecl>(ConstructorDecl);
   if (!Constructor)
     return QualType();
   // FIXME: Add support for Base class constructors as well.
   if (ValueDecl *MemberDecl = tryLookupCtorInitMemberDecl(
           Constructor->getParent(), SS, TemplateTypeTy, II))
     return ProduceConstructorSignatureHelp(getCurScope(), MemberDecl->getType(),
                                            MemberDecl->getLocation(), ArgExprs,
                                            OpenParLoc);
   return QualType();
 }
 
+static bool argMatchesTemplateParams(const ParsedTemplateArgument &Arg,
+                                     unsigned Index,
+                                     const TemplateParameterList &Params) {
+  const NamedDecl *Param;
+  if (Index < Params.size())
+    Param = Params.getParam(Index);
+  else if (Params.hasParameterPack())
+    Param = Params.asArray().back();
+  else
+    return false; // too many args
+
+  switch (Arg.getKind()) {
+  case ParsedTemplateArgument::Type:
+    return llvm::isa<TemplateTypeParmDecl>(Param); // constraints not checked
+  case ParsedTemplateArgument::NonType:
+    return llvm::isa<NonTypeTemplateParmDecl>(Param); // type not checked
+  case ParsedTemplateArgument::Template:
+    return llvm::isa<TemplateTemplateParmDecl>(Param); // signature not checked
+  }
+}
+
+QualType Sema::ProduceTemplateArgumentSignatureHelp(
+    TemplateTy ParsedTemplate, ArrayRef<ParsedTemplateArgument> Args,
+    SourceLocation LAngleLoc) {
+  if (!CodeCompleter || !ParsedTemplate)
+    return QualType();
+
+  SmallVector<ResultCandidate, 8> Results;
+  auto Consider = [&](const TemplateDecl *TD) {
+    // Only add if the existing args are compatible with the template.
+    bool Matches = true;
+    for (unsigned I = 0; I < Args.size(); ++I) {
+      if (!argMatchesTemplateParams(Args[I], I, *TD->getTemplateParameters())) {
+        Matches = false;
+        break;
+      }
+    }
+    if (Matches)
+      Results.emplace_back(TD);
+  };
+
+  TemplateName Template = ParsedTemplate.get();
+  if (const auto *TD = Template.getAsTemplateDecl()) {
+    Consider(TD);
+  } else if (const auto *OTS = Template.getAsOverloadedTemplate()) {
+    for (const NamedDecl *ND : *OTS)
+      if (const auto *TD = llvm::dyn_cast<TemplateDecl>(ND))
+        Consider(TD);
+  }
+  return ProduceSignatureHelp(*this, Results, Args.size(), LAngleLoc);
+}
+
 static QualType getDesignatedType(QualType BaseType, const Designation &Desig) {
   for (unsigned I = 0; I < Desig.getNumDesignators(); ++I) {
     if (BaseType.isNull())
       break;
     QualType NextType;
     const auto &D = Desig.getDesignator(I);
     if (D.isArrayDesignator() || D.isArrayRangeDesignator()) {
       if (BaseType->isArrayType())

clang/test/CodeCompletion/template-signature.cpp

+template <int, char y> float overloaded(int);
+template <class, int x> bool overloaded(char);
+
+auto m = overloaded<1, 2>(0);
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:4:21 %s -o - | FileCheck -check-prefix=CHECK-CC1 %s
+// CHECK-CC1: OPENING_PAREN_LOC: {{.*}}4:20
+// CHECK-CC1-DAG: OVERLOAD: [#float#]overloaded<<#int#>, char y>[#()#]
+// CHECK-CC1-DAG: OVERLOAD: [#bool#]overloaded<<#class#>, int x>[#()#]
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:4:24 %s -o - | FileCheck -check-prefix=CHECK-CC2 %s
+// CHECK-CC2-NOT: OVERLOAD: {{.*}}int x
+// CHECK-CC2: OVERLOAD: [#float#]overloaded<int, <#char y#>>[#()#]
+
+template <class T, T... args> int n = 0;
+int val = n<int, 1, 2, 3>;
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:14:18 %s -o - | FileCheck -check-prefix=CHECK-CC3 %s
+// CHECK-CC3: OVERLOAD: [#int#]n<class T, <#T ...args#>>
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:14:24 %s -o - | FileCheck -check-prefix=CHECK-CC4 %s
+// CHECK-CC4: OVERLOAD: [#int#]n<class T, T ...args>
+
+template <typename> struct Vector {};
+template <typename Element, template <typename E> class Container = Vector>
+struct Collection { Container<Element> container; };
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:22:31 %s -o - | FileCheck -check-prefix=CHECK-CC5 %s
+// CHECK-CC5: OVERLOAD: [#class#]Container<<#typename E#>>
+Collection<int, Vector> collection;
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:25:12 %s -o - | FileCheck -check-prefix=CHECK-CC6 %s
+// CHECK-CC6: OVERLOAD: [#struct#]Collection<<#typename Element#>>
+