[clangd] add inlay hints for std::forward-ed parameter packs

This adds special-case treatment for parameter packs in
make_unique-like functions to forward parameter names to inlay hints.
The parameter packs are being resolved recursively by traversing the
function body of forwarding functions looking for expressions matching
the (std::forwarded) parameters expanded from a pack.
The implementation checks whether parameters are being passed by
(rvalue) reference or value and adds reference inlay hints accordingly.
The traversal has a limited recursion stack depth, and recursive calls
like std::make_tuple are cut off to avoid hinting duplicate parameter
names.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D124690

clang-tools-extra/clangd/AST.cpp

@@ -16,19 +16,23 @@
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclarationName.h"
+#include "clang/AST/ExprCXX.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/PrettyPrinter.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/Stmt.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/TypeLoc.h"
+#include "clang/Basic/Builtins.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/Specifiers.h"
 #include "clang/Index/USRGeneration.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/raw_ostream.h"
@@ -667,5 +671,300 @@ bool isDeeplyNested(const Decl *D, unsigned MaxDepth) {
   }
   return false;
 }
+
+namespace {
+
+// returns true for `X` in `template <typename... X> void foo()`
+bool isTemplateTypeParameterPack(NamedDecl *D) {
+  if (const auto *TTPD = dyn_cast<TemplateTypeParmDecl>(D)) {
+    return TTPD->isParameterPack();
+  }
+  return false;
+}
+
+// Returns the template parameter pack type from an instantiated function
+// template, if it exists, nullptr otherwise.
+const TemplateTypeParmType *getFunctionPackType(const FunctionDecl *Callee) {
+  if (const auto *TemplateDecl = Callee->getPrimaryTemplate()) {
+    auto TemplateParams = TemplateDecl->getTemplateParameters()->asArray();
+    // find the template parameter pack from the back
+    const auto It = std::find_if(TemplateParams.rbegin(), TemplateParams.rend(),
+                                 isTemplateTypeParameterPack);
+    if (It != TemplateParams.rend()) {
+      const auto *TTPD = dyn_cast<TemplateTypeParmDecl>(*It);
+      return TTPD->getTypeForDecl()->castAs<TemplateTypeParmType>();
+    }
+  }
+  return nullptr;
+}
+
+// Returns the template parameter pack type that this parameter was expanded
+// from (if in the Args... or Args&... or Args&&... form), if this is the case,
+// nullptr otherwise.
+const TemplateTypeParmType *getUnderylingPackType(const ParmVarDecl *Param) {
+  const auto *PlainType = Param->getType().getTypePtr();
+  if (auto *RT = dyn_cast<ReferenceType>(PlainType))
+    PlainType = RT->getPointeeTypeAsWritten().getTypePtr();
+  if (const auto *SubstType = dyn_cast<SubstTemplateTypeParmType>(PlainType)) {
+    const auto *ReplacedParameter = SubstType->getReplacedParameter();
+    if (ReplacedParameter->isParameterPack()) {
+      return dyn_cast<TemplateTypeParmType>(
+          ReplacedParameter->getCanonicalTypeUnqualified()->getTypePtr());
+    }
+  }
+  return nullptr;
+}
+
+// This visitor walks over the body of an instantiated function template.
+// The template accepts a parameter pack and the visitor records whether
+// the pack parameters were forwarded to another call. For example, given:
+//
+// template <typename T, typename... Args>
+// auto make_unique(Args... args) {
+//   return unique_ptr<T>(new T(args...));
+// }
+//
+// When called as `make_unique<std::string>(2, 'x')` this yields a function
+// `make_unique<std::string, int, char>` with two parameters.
+// The visitor records that those two parameters are forwarded to the
+// `constructor std::string(int, char);`.
+//
+// This information is recorded in the `ForwardingInfo` split into fully
+// resolved parameters (passed as argument to a parameter that is not an
+// expanded template type parameter pack) and forwarding parameters (passed to a
+// parameter that is an expanded template type parameter pack).
+class ForwardingCallVisitor
+    : public RecursiveASTVisitor<ForwardingCallVisitor> {
+public:
+  ForwardingCallVisitor(ArrayRef<const ParmVarDecl *> Parameters)
+      : Parameters{Parameters}, PackType{getUnderylingPackType(
+                                    Parameters.front())} {}
+
+  bool VisitCallExpr(CallExpr *E) {
+    auto *Callee = getCalleeDeclOrUniqueOverload(E);
+    if (Callee) {
+      handleCall(Callee, E->arguments());
+    }
+    return !Info.hasValue();
+  }
+
+  bool VisitCXXConstructExpr(CXXConstructExpr *E) {
+    auto *Callee = E->getConstructor();
+    if (Callee) {
+      handleCall(Callee, E->arguments());
+    }
+    return !Info.hasValue();
+  }
+
+  // The expanded parameter pack to be resolved
+  ArrayRef<const ParmVarDecl *> Parameters;
+  // The type of the parameter pack
+  const TemplateTypeParmType *PackType;
+
+  struct ForwardingInfo {
+    // If the parameters were resolved to another FunctionDecl, these are its
+    // first non-variadic parameters (i.e. the first entries of the parameter
+    // pack that are passed as arguments bound to a non-pack parameter.)
+    ArrayRef<const ParmVarDecl *> Head;
+    // If the parameters were resolved to another FunctionDecl, these are its
+    // variadic parameters (i.e. the entries of the parameter pack that are
+    // passed as arguments bound to a pack parameter.)
+    ArrayRef<const ParmVarDecl *> Pack;
+    // If the parameters were resolved to another FunctionDecl, these are its
+    // last non-variadic parameters (i.e. the last entries of the parameter pack
+    // that are passed as arguments bound to a non-pack parameter.)
+    ArrayRef<const ParmVarDecl *> Tail;
+    // If the parameters were resolved to another forwarding FunctionDecl, this
+    // is it.
+    Optional<FunctionDecl *> PackTarget;
+  };
+
+  // The output of this visitor
+  Optional<ForwardingInfo> Info;
+
+private:
+  // inspects the given callee with the given args to check whether it
+  // contains Parameters, and sets Info accordingly.
+  void handleCall(FunctionDecl *Callee, typename CallExpr::arg_range Args) {
+    if (std::any_of(Args.begin(), Args.end(), [](const Expr *E) {
+          return dyn_cast<PackExpansionExpr>(E) != nullptr;
+        })) {
+      return;
+    }
+    auto OptPackLocation = findPack(Args);
+    if (OptPackLocation) {
+      size_t PackLocation = OptPackLocation.getValue();
+      ArrayRef<ParmVarDecl *> MatchingParams =
+          Callee->parameters().slice(PackLocation, Parameters.size());
+      // Check whether the function has a parameter pack as the last template
+      // parameter
+      if (const auto *TTPT = getFunctionPackType(Callee)) {
+        // In this case: Separate the parameters into head, pack and tail
+        auto IsExpandedPack = [&](const ParmVarDecl *P) {
+          return getUnderylingPackType(P) == TTPT;
+        };
+        ForwardingInfo FI;
+        FI.Head = MatchingParams.take_until(IsExpandedPack);
+        FI.Pack = MatchingParams.drop_front(FI.Head.size())
+                      .take_while(IsExpandedPack);
+        FI.Tail = MatchingParams.drop_front(FI.Head.size() + FI.Pack.size());
+        FI.PackTarget = Callee;
+        Info = FI;
+        return;
+      }
+      // Default case: assume all parameters were fully resolved
+      ForwardingInfo FI;
+      FI.Head = MatchingParams;
+      Info = FI;
+    }
+  }
+
+  // Returns the beginning of the expanded pack represented by Parameters
+  // in the given arguments, if it is there.
+  llvm::Optional<size_t> findPack(typename CallExpr::arg_range Args) {
+    // find the argument directly referring to the first parameter
+    auto FirstMatch = std::find_if(Args.begin(), Args.end(), [&](Expr *Arg) {
+      const auto *RefArg = unwrapArgument(Arg);
+      if (RefArg) {
+        if (Parameters.front() == dyn_cast<ParmVarDecl>(RefArg->getDecl())) {
+          return true;
+        }
+      }
+      return false;
+    });
+    if (FirstMatch == Args.end()) {
+      return llvm::None;
+    }
+    return std::distance(Args.begin(), FirstMatch);
+  }
+
+  static FunctionDecl *getCalleeDeclOrUniqueOverload(CallExpr *E) {
+    Decl *CalleeDecl = E->getCalleeDecl();
+    auto *Callee = dyn_cast_or_null<FunctionDecl>(CalleeDecl);
+    if (!Callee) {
+      if (auto *Lookup = dyn_cast<UnresolvedLookupExpr>(E->getCallee())) {
+        Callee = resolveOverload(Lookup, E);
+      }
+    }
+    // Ignore the callee if the number of arguments is wrong (deal with va_args)
+    if (Callee->getNumParams() == E->getNumArgs())
+      return Callee;
+    return nullptr;
+  }
+
+  static FunctionDecl *resolveOverload(UnresolvedLookupExpr *Lookup,
+                                       CallExpr *E) {
+    FunctionDecl *MatchingDecl = nullptr;
+    if (!Lookup->requiresADL()) {
+      // Check whether there is a single overload with this number of
+      // parameters
+      for (auto *Candidate : Lookup->decls()) {
+        if (auto *FuncCandidate = dyn_cast_or_null<FunctionDecl>(Candidate)) {
+          if (FuncCandidate->getNumParams() == E->getNumArgs()) {
+            if (MatchingDecl) {
+              // there are multiple candidates - abort
+              return nullptr;
+            }
+            MatchingDecl = FuncCandidate;
+          }
+        }
+      }
+    }
+    return MatchingDecl;
+  }
+
+  // Removes any implicit cast expressions around the given expression.
+  static const Expr *unwrapImplicitCast(const Expr *E) {
+    while (const auto *Cast = dyn_cast<ImplicitCastExpr>(E)) {
+      E = Cast->getSubExpr();
+    }
+    return E;
+  }
+
+  // Maps std::forward(E) to E, nullptr otherwise
+  static const Expr *unwrapForward(const Expr *E) {
+    if (const auto *Call = dyn_cast<CallExpr>(E)) {
+      const auto Callee = Call->getBuiltinCallee();
+      if (Callee == Builtin::BIforward) {
+        return Call->getArg(0);
+      }
+    }
+    return E;
+  }
+
+  // Maps std::forward(DeclRefExpr) to DeclRefExpr, removing any intermediate
+  // implicit casts, nullptr otherwise
+  static const DeclRefExpr *unwrapArgument(const Expr *E) {
+    E = unwrapImplicitCast(E);
+    E = unwrapForward(E);
+    E = unwrapImplicitCast(E);
+    return dyn_cast<DeclRefExpr>(E);
+  }
+};
+
+} // namespace
+
+SmallVector<const ParmVarDecl *>
+resolveForwardingParameters(const FunctionDecl *D, unsigned MaxDepth) {
+  auto Parameters = D->parameters();
+  // If the function has a template parameter pack
+  if (const auto *TTPT = getFunctionPackType(D)) {
+    // Split the parameters into head, pack and tail
+    auto IsExpandedPack = [TTPT](const ParmVarDecl *P) {
+      return getUnderylingPackType(P) == TTPT;
+    };
+    ArrayRef<const ParmVarDecl *> Head = Parameters.take_until(IsExpandedPack);
+    ArrayRef<const ParmVarDecl *> Pack =
+        Parameters.drop_front(Head.size()).take_while(IsExpandedPack);
+    ArrayRef<const ParmVarDecl *> Tail =
+        Parameters.drop_front(Head.size() + Pack.size());
+    SmallVector<const ParmVarDecl *> Result(Parameters.size());
+    // Fill in non-pack parameters
+    auto HeadIt = std::copy(Head.begin(), Head.end(), Result.begin());
+    auto TailIt = std::copy(Tail.rbegin(), Tail.rend(), Result.rbegin());
+    // Recurse on pack parameters
+    size_t Depth = 0;
+    const FunctionDecl *CurrentFunction = D;
+    llvm::SmallSet<const FunctionTemplateDecl *, 4> SeenTemplates;
+    if (const auto *Template = D->getPrimaryTemplate()) {
+      SeenTemplates.insert(Template);
+    }
+    while (!Pack.empty() && CurrentFunction && Depth < MaxDepth) {
+      // Find call expressions involving the pack
+      ForwardingCallVisitor V{Pack};
+      V.TraverseStmt(CurrentFunction->getBody());
+      if (!V.Info) {
+        break;
+      }
+      // If we found something: Fill in non-pack parameters
+      auto Info = V.Info.getValue();
+      HeadIt = std::copy(Info.Head.begin(), Info.Head.end(), HeadIt);
+      TailIt = std::copy(Info.Tail.rbegin(), Info.Tail.rend(), TailIt);
+      // Prepare next recursion level
+      Pack = Info.Pack;
+      CurrentFunction = Info.PackTarget.getValueOr(nullptr);
+      Depth++;
+      // If we are recursing into a previously encountered function: Abort
+      if (CurrentFunction) {
+        if (const auto *Template = CurrentFunction->getPrimaryTemplate()) {
+          bool NewFunction = SeenTemplates.insert(Template).second;
+          if (!NewFunction) {
+            return {Parameters.begin(), Parameters.end()};
+          }
+        }
+      }
+    }
+    // Fill in the remaining unresolved pack parameters
+    HeadIt = std::copy(Pack.begin(), Pack.end(), HeadIt);
+    assert(TailIt.base() == HeadIt);
+    return Result;
+  }
+  return {Parameters.begin(), Parameters.end()};
+}
+
+bool isExpandedFromParameterPack(const ParmVarDecl *D) {
+  return getUnderylingPackType(D) != nullptr;
+}
+
 } // namespace clangd
 } // namespace clang

clang-tools-extra/clangd/AST.h

@@ -199,6 +199,18 @@ bool hasUnstableLinkage(const Decl *D);
 /// reasonable.
 bool isDeeplyNested(const Decl *D, unsigned MaxDepth = 10);
 
+/// Recursively resolves the parameters of a FunctionDecl that forwards its
+/// parameters to another function via variadic template parameters. This can
+/// for example be used to retrieve the constructor parameter ParmVarDecl for a
+/// make_unique or emplace_back call.
+llvm::SmallVector<const ParmVarDecl *>
+resolveForwardingParameters(const FunctionDecl *D, unsigned MaxDepth = 10);
+
+/// Checks whether D is instantiated from a function parameter pack
+/// whose type is a bare type parameter pack (e.g. `Args...`), or a
+/// reference to one (e.g. `Args&...` or `Args&&...`).
+bool isExpandedFromParameterPack(const ParmVarDecl *D);
+
 } // namespace clangd
 } // namespace clang
 

clang-tools-extra/clangd/InlayHints.cpp

@@ -379,7 +379,7 @@ private:
   // assume that if this location does not come from a macro definition, then
   // the entire argument list likely appears in the main file and can be hinted.
   void processCall(SourceLocation Anchor, const FunctionDecl *Callee,
-                   llvm::ArrayRef<const Expr *const> Args) {
+                   llvm::ArrayRef<const Expr *> Args) {
     if (!Cfg.InlayHints.Parameters || Args.size() == 0 || !Callee)
       return;
 
@@ -393,12 +393,10 @@ private:
       if (Ctor->isCopyOrMoveConstructor())
         return;
 
-    // Don't show hints for variadic parameters.
+    // Resolve parameter packs to their forwarded parameter
-    size_t FixedParamCount = getFixedParamCount(Callee);
+    auto ForwardedParams = resolveForwardingParameters(Callee);
-    size_t ArgCount = std::min(FixedParamCount, Args.size());
-    auto Params = Callee->parameters();
 
-    NameVec ParameterNames = chooseParameterNames(Callee, ArgCount);
+    NameVec ParameterNames = chooseParameterNames(ForwardedParams);
 
     // Exclude setters (i.e. functions with one argument whose name begins with
     // "set"), and builtins like std::move/forward/... as their parameter name
@@ -406,10 +404,18 @@ private:
     if (isSetter(Callee, ParameterNames) || isSimpleBuiltin(Callee))
       return;
 
-    for (size_t I = 0; I < ArgCount; ++I) {
+    for (size_t I = 0; I < ParameterNames.size() && I < Args.size(); ++I) {
+      // Pack expansion expressions cause the 1:1 mapping between arguments and
+      // parameters to break down, so we don't add further inlay hints if we
+      // encounter one.
+      if (isa<PackExpansionExpr>(Args[I])) {
+        break;
+      }
+
       StringRef Name = ParameterNames[I];
       bool NameHint = shouldHintName(Args[I], Name);
-      bool ReferenceHint = shouldHintReference(Params[I]);
+      bool ReferenceHint =
+          shouldHintReference(Callee->getParamDecl(I), ForwardedParams[I]);
 
       if (NameHint || ReferenceHint) {
         addInlayHint(Args[I]->getSourceRange(), HintSide::Left,
@@ -473,11 +479,36 @@ private:
     return true;
   }
 
-  bool shouldHintReference(const ParmVarDecl *Param) {
+  bool shouldHintReference(const ParmVarDecl *Param,
-    // If the parameter is a non-const reference type, print an inlay hint
+                           const ParmVarDecl *ForwardedParam) {
+    // We add a & hint only when the argument is passed as mutable reference.
+    // For parameters that are not part of an expanded pack, this is
+    // straightforward. For expanded pack parameters, it's likely that they will
+    // be forwarded to another function. In this situation, we only want to add
+    // the reference hint if the argument is actually being used via mutable
+    // reference. This means we need to check
+    // 1. whether the value category of the argument is preserved, i.e. each
+    //    pack expansion uses std::forward correctly.
+    // 2. whether the argument is ever copied/cast instead of passed
+    //    by-reference
+    // Instead of checking this explicitly, we use the following proxy:
+    // 1. the value category can only change from rvalue to lvalue during
+    //    forwarding, so checking whether both the parameter of the forwarding
+    //    function and the forwarded function are lvalue references detects such
+    //    a conversion.
+    // 2. if the argument is copied/cast somewhere in the chain of forwarding
+    //    calls, it can only be passed on to an rvalue reference or const lvalue
+    //    reference parameter. Thus if the forwarded parameter is a mutable
+    //    lvalue reference, it cannot have been copied/cast to on the way.
+    // Additionally, we should not add a reference hint if the forwarded
+    // parameter was only partially resolved, i.e. points to an expanded pack
+    // parameter, since we do not know how it will be used eventually.
     auto Type = Param->getType();
+    auto ForwardedType = ForwardedParam->getType();
     return Type->isLValueReferenceType() &&
-           !Type.getNonReferenceType().isConstQualified();
+           ForwardedType->isLValueReferenceType() &&
+           !ForwardedType.getNonReferenceType().isConstQualified() &&
+           !isExpandedFromParameterPack(ForwardedParam);
   }
 
   // Checks if "E" is spelled in the main file and preceded by a C-style comment
@@ -524,23 +555,26 @@ private:
     return {};
   }
 
-  NameVec chooseParameterNames(const FunctionDecl *Callee, size_t ArgCount) {
+  NameVec chooseParameterNames(SmallVector<const ParmVarDecl *> Parameters) {
-    // The current strategy here is to use all the parameter names from the
+    NameVec ParameterNames;
-    // canonical declaration, unless they're all empty, in which case we
+    for (const auto *P : Parameters) {
-    // use all the parameter names from the definition (in present in the
+      if (isExpandedFromParameterPack(P)) {
-    // translation unit).
+        // If we haven't resolved a pack paramater (e.g. foo(Args... args)) to a
-    // We could try a bit harder, e.g.:
+        // non-pack parameter, then hinting as foo(args: 1, args: 2, args: 3) is
-    //   - try all re-declarations, not just canonical + definition
+        // unlikely to be useful.
-    //   - fall back arg-by-arg rather than wholesale
+        ParameterNames.emplace_back();
-
+      } else {
-    NameVec ParameterNames = getParameterNamesForDecl(Callee, ArgCount);
+        auto SimpleName = getSimpleName(*P);
-
+        // If the parameter is unnamed in the declaration:
-    if (llvm::all_of(ParameterNames, std::mem_fn(&StringRef::empty))) {
+        // attempt to get its name from the definition
-      if (const FunctionDecl *Def = Callee->getDefinition()) {
+        if (SimpleName.empty()) {
-        ParameterNames = getParameterNamesForDecl(Def, ArgCount);
+          if (const auto *PD = getParamDefinition(P)) {
+            SimpleName = getSimpleName(*PD);
+          }
+        }
+        ParameterNames.emplace_back(SimpleName);
       }
     }
-    assert(ParameterNames.size() == ArgCount);
 
     // Standard library functions often have parameter names that start
     // with underscores, which makes the hints noisy, so strip them out.
@@ -550,28 +584,24 @@ private:
     return ParameterNames;
   }
 
-  static void stripLeadingUnderscores(StringRef &Name) {
+  // for a ParmVarDecl from a function declaration, returns the corresponding
-    Name = Name.ltrim('_');
+  // ParmVarDecl from the definition if possible, nullptr otherwise.
+  static const ParmVarDecl *getParamDefinition(const ParmVarDecl *P) {
+    if (auto *Callee = dyn_cast<FunctionDecl>(P->getDeclContext())) {
+      if (auto *Def = Callee->getDefinition()) {
+        auto I = std::distance(
+            Callee->param_begin(),
+            std::find(Callee->param_begin(), Callee->param_end(), P));
+        if (I < Callee->getNumParams()) {
+          return Def->getParamDecl(I);
+        }
+      }
+    }
+    return nullptr;
   }
 
-  // Return the number of fixed parameters Function has, that is, not counting
+  static void stripLeadingUnderscores(StringRef &Name) {
-  // parameters that are variadic (instantiated from a parameter pack) or
+    Name = Name.ltrim('_');
-  // C-style varargs.
-  static size_t getFixedParamCount(const FunctionDecl *Function) {
-    if (FunctionTemplateDecl *Template = Function->getPrimaryTemplate()) {
-      FunctionDecl *F = Template->getTemplatedDecl();
-      size_t Result = 0;
-      for (ParmVarDecl *Parm : F->parameters()) {
-        if (Parm->isParameterPack()) {
-          break;
-        }
-        ++Result;
-      }
-      return Result;
-    }
-    // C-style varargs don't need special handling, they're already
-    // not included in getNumParams().
-    return Function->getNumParams();
   }
 
   static StringRef getSimpleName(const NamedDecl &D) {
@@ -582,17 +612,6 @@ private:
     return StringRef();
   }
 
-  NameVec getParameterNamesForDecl(const FunctionDecl *Function,
-                                   size_t ArgCount) {
-    NameVec Result;
-    for (size_t I = 0; I < ArgCount; ++I) {
-      const ParmVarDecl *Parm = Function->getParamDecl(I);
-      assert(Parm);
-      Result.emplace_back(getSimpleName(*Parm));
-    }
-    return Result;
-  }
-
   // We pass HintSide rather than SourceLocation because we want to ensure
   // it is in the same file as the common file range.
   void addInlayHint(SourceRange R, HintSide Side, InlayHintKind Kind,

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

@@ -174,6 +174,43 @@ TEST(ParameterHints, NoNameRValueReference) {
   )cpp");
 }
 
+TEST(ParameterHints, NoNameVariadicDeclaration) {
+  // No hint for anonymous variadic parameter
+  assertParameterHints(R"cpp(
+    template <typename... Args>
+    void foo(Args&& ...);
+    void bar() {
+      foo(42);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, NoNameVariadicForwarded) {
+  // No hint for anonymous variadic parameter
+  // This prototype of std::forward is sufficient for clang to recognize it
+  assertParameterHints(R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    void foo(int);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(std::forward<Args>(args)...); }
+    void baz() {
+      bar(42);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, NoNameVariadicPlain) {
+  // No hint for anonymous variadic parameter
+  assertParameterHints(R"cpp(
+    void foo(int);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(args...); }
+    void baz() {
+      bar(42);
+    }
+  )cpp");
+}
+
 TEST(ParameterHints, NameInDefinition) {
   // Parameter name picked up from definition if necessary.
   assertParameterHints(R"cpp(
@@ -186,6 +223,36 @@ TEST(ParameterHints, NameInDefinition) {
                        ExpectedHint{"param: ", "param"});
 }
 
+TEST(ParameterHints, NamePartiallyInDefinition) {
+  // Parameter name picked up from definition if necessary.
+  assertParameterHints(R"cpp(
+    void foo(int, int b);
+    void bar() {
+      foo($param1[[42]], $param2[[42]]);
+    }
+    void foo(int a, int) {};
+  )cpp",
+                       ExpectedHint{"a: ", "param1"},
+                       ExpectedHint{"b: ", "param2"});
+}
+
+TEST(ParameterHints, NameInDefinitionVariadic) {
+  // Parameter name picked up from definition in a resolved forwarded parameter.
+  assertParameterHints(R"cpp(
+    void foo(int, int);
+    template <typename... Args>
+    void bar(Args... args) {
+      foo(args...);
+    }
+    void baz() {
+      bar($param1[[42]], $param2[[42]]);
+    }
+    void foo(int a, int b) {};
+  )cpp",
+                       ExpectedHint{"a: ", "param1"},
+                       ExpectedHint{"b: ", "param2"});
+}
+
 TEST(ParameterHints, NameMismatch) {
   // Prefer name from declaration.
   assertParameterHints(R"cpp(
@@ -258,6 +325,455 @@ TEST(ParameterHints, NameRValueReference) {
                        ExpectedHint{"param: ", "param"});
 }
 
+TEST(ParameterHints, VariadicForwardedConstructor) {
+  // Name hint for variadic parameter using std::forward in a constructor call
+  // This prototype of std::forward is sufficient for clang to recognize it
+  assertParameterHints(R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    struct S { S(int a); };
+    template <typename T, typename... Args>
+    T bar(Args&&... args) { return T{std::forward<Args>(args)...}; }
+    void baz() {
+      int b;
+      bar<S>($param[[b]]);
+    }
+  )cpp",
+                       ExpectedHint{"a: ", "param"});
+}
+
+TEST(ParameterHints, VariadicPlainConstructor) {
+  // Name hint for variadic parameter in a constructor call
+  assertParameterHints(R"cpp(
+    struct S { S(int a); };
+    template <typename T, typename... Args>
+    T bar(Args&&... args) { return T{args...}; }
+    void baz() {
+      int b;
+      bar<S>($param[[b]]);
+    }
+  )cpp",
+                       ExpectedHint{"a: ", "param"});
+}
+
+TEST(ParameterHints, VariadicForwardedNewConstructor) {
+  // Name hint for variadic parameter using std::forward in a new expression
+  // This prototype of std::forward is sufficient for clang to recognize it
+  assertParameterHints(R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    struct S { S(int a); };
+    template <typename T, typename... Args>
+    T* bar(Args&&... args) { return new T{std::forward<Args>(args)...}; }
+    void baz() {
+      int b;
+      bar<S>($param[[b]]);
+    }
+  )cpp",
+                       ExpectedHint{"a: ", "param"});
+}
+
+TEST(ParameterHints, VariadicPlainNewConstructor) {
+  // Name hint for variadic parameter in a new expression
+  assertParameterHints(R"cpp(
+    struct S { S(int a); };
+    template <typename T, typename... Args>
+    T* bar(Args&&... args) { return new T{args...}; }
+    void baz() {
+      int b;
+      bar<S>($param[[b]]);
+    }
+  )cpp",
+                       ExpectedHint{"a: ", "param"});
+}
+
+TEST(ParameterHints, VariadicForwarded) {
+  // Name for variadic parameter using std::forward
+  // This prototype of std::forward is sufficient for clang to recognize it
+  assertParameterHints(R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    void foo(int a);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(std::forward<Args>(args)...); }
+    void baz() {
+      int b;
+      bar($param[[b]]);
+    }
+  )cpp",
+                       ExpectedHint{"a: ", "param"});
+}
+
+TEST(ParameterHints, VariadicPlain) {
+  // Name hint for variadic parameter
+  assertParameterHints(R"cpp(
+    void foo(int a);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(args...); }
+    void baz() {
+      bar($param[[42]]);
+    }
+  )cpp",
+                       ExpectedHint{"a: ", "param"});
+}
+
+TEST(ParameterHints, VariadicPlainWithPackFirst) {
+  // Name hint for variadic parameter when the parameter pack is not the last
+  // template parameter
+  assertParameterHints(R"cpp(
+    void foo(int a);
+    template <typename... Args, typename Arg>
+    void bar(Arg, Args&&... args) { return foo(args...); }
+    void baz() {
+      bar(1, $param[[42]]);
+    }
+  )cpp",
+                       ExpectedHint{"a: ", "param"});
+}
+
+TEST(ParameterHints, VariadicSplitTwolevel) {
+  // Name for variadic parameter that involves both head and tail parameters to
+  // deal with.
+  // This prototype of std::forward is sufficient for clang to recognize it
+  assertParameterHints(R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    void baz(int, int b, double);
+    template <typename... Args>
+    void foo(int a, Args&&... args) {
+      return baz(1, std::forward<Args>(args)..., 1.0);
+    }
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(std::forward<Args>(args)...); }
+    void bazz() {
+      bar($param1[[32]], $param2[[42]]);
+    }
+  )cpp",
+                       ExpectedHint{"a: ", "param1"},
+                       ExpectedHint{"b: ", "param2"});
+}
+
+TEST(ParameterHints, VariadicNameFromSpecialization) {
+  // We don't try to resolve forwarding parameters if the function call uses a
+  // specialization.
+  assertParameterHints(R"cpp(
+    void foo(int a);
+    template <typename... Args>
+    void bar(Args... args) {
+      foo(args...);
+    }
+    template <>
+    void bar<int>(int b);
+    void baz() {
+      bar($param[[42]]);
+    }
+  )cpp",
+                       ExpectedHint{"b: ", "param"});
+}
+
+TEST(ParameterHints, VariadicNameFromSpecializationRecursive) {
+  // We don't try to resolve forwarding parameters inside a forwarding function
+  // call if that function call uses a specialization.
+  assertParameterHints(R"cpp(
+    void foo2(int a);
+    template <typename... Args>
+    void foo(Args... args) {
+      foo2(args...);
+    }
+    template <typename... Args>
+    void bar(Args... args) {
+      foo(args...);
+    }
+    template <>
+    void foo<int>(int b);
+    void baz() {
+      bar($param[[42]]);
+    }
+  )cpp",
+                       ExpectedHint{"b: ", "param"});
+}
+
+TEST(ParameterHints, VariadicOverloaded) {
+  // Name for variadic parameter for an overloaded function with unique number
+  // of parameters.
+  // This prototype of std::forward is sufficient for clang to recognize it
+  assertParameterHints(
+      R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    void baz(int b, int c);
+    void baz(int bb, int cc, int dd);
+    template <typename... Args>
+    void foo(int a, Args&&... args) {
+      return baz(std::forward<Args>(args)...);
+    }
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(std::forward<Args>(args)...); }
+    void bazz() {
+      bar($param1[[32]], $param2[[42]], $param3[[52]]);
+      bar($param4[[1]], $param5[[2]], $param6[[3]], $param7[[4]]);
+    }
+  )cpp",
+      ExpectedHint{"a: ", "param1"}, ExpectedHint{"b: ", "param2"},
+      ExpectedHint{"c: ", "param3"}, ExpectedHint{"a: ", "param4"},
+      ExpectedHint{"bb: ", "param5"}, ExpectedHint{"cc: ", "param6"},
+      ExpectedHint{"dd: ", "param7"});
+}
+
+TEST(ParameterHints, VariadicRecursive) {
+  // make_tuple-like recursive variadic call
+  assertParameterHints(
+      R"cpp(
+    void foo();
+
+    template <typename Head, typename... Tail>
+    void foo(Head head, Tail... tail) {
+      foo(tail...);
+    }
+
+    template <typename... Args>
+    void bar(Args... args) {
+      foo(args...);
+    }
+
+    int main() {
+      bar(1, 2, 3);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, VariadicVarargs) {
+  // variadic call involving varargs (to make sure we don't crash)
+  assertParameterHints(R"cpp(
+    void foo(int fixed, ...);
+    template <typename... Args>
+    void bar(Args&&... args) {
+      foo(args...);
+    }
+
+    void baz() {
+      bar($fixed[[41]], 42, 43);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, VariadicTwolevelUnresolved) {
+  // the same setting as VariadicVarargs, only with parameter pack
+  assertParameterHints(R"cpp(
+    template <typename... Args>
+    void foo(int fixed, Args&& ... args);
+    template <typename... Args>
+    void bar(Args&&... args) {
+      foo(args...);
+    }
+
+    void baz() {
+      bar($fixed[[41]], 42, 43);
+    }
+  )cpp",
+                       ExpectedHint{"fixed: ", "fixed"});
+}
+
+TEST(ParameterHints, VariadicTwoCalls) {
+  // only the first call using the parameter pack should be picked up
+  assertParameterHints(
+      R"cpp(
+    void f1(int a, int b);
+    void f2(int c, int d);
+
+    bool cond;
+
+    template <typename... Args>
+    void foo(Args... args) {
+      if (cond) {
+        f1(args...);
+      } else {
+        f2(args...);
+      }
+    }
+
+    int main() {
+      foo($param1[[1]], $param2[[2]]);
+    }
+  )cpp",
+      ExpectedHint{"a: ", "param1"}, ExpectedHint{"b: ", "param2"});
+}
+
+TEST(ParameterHints, VariadicInfinite) {
+  // infinite recursion should not break clangd
+  assertParameterHints(
+      R"cpp(
+    template <typename... Args>
+    void foo(Args...);
+
+    template <typename... Args>
+    void bar(Args... args) {
+      foo(args...);
+    }
+
+    template <typename... Args>
+    void foo(Args... args) {
+      bar(args...);
+    }
+
+    int main() {
+      foo(1, 2);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, VariadicDuplicatePack) {
+  // edge cases with multiple adjacent packs should work
+  assertParameterHints(
+      R"cpp(
+    void foo(int a, int b, int c, int);
+
+    template <typename... Args>
+    void bar(int, Args... args, int d) {
+      foo(args..., d);
+    }
+
+    template <typename... Args>
+    void baz(Args... args, Args... args2) {
+      bar<Args..., int>(1, args..., args2...);
+    }
+
+    int main() {
+      baz<int, int>($p1[[1]], $p2[[2]], $p3[[3]], $p4[[4]]);
+    }
+  )cpp",
+      ExpectedHint{"a: ", "p1"}, ExpectedHint{"b: ", "p2"},
+      ExpectedHint{"c: ", "p3"}, ExpectedHint{"d: ", "p4"});
+}
+
+TEST(ParameterHints, VariadicEmplace) {
+  // emplace-like calls should forward constructor parameters
+  // This prototype of std::forward is sufficient for clang to recognize it
+  assertParameterHints(
+      R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    using size_t = decltype(sizeof(0));
+    void *operator new(size_t, void *);
+    struct S {
+      S(int A);
+      S(int B, int C);
+    };
+    struct alloc {
+      template <typename T>
+      T* allocate();
+      template <typename T, typename... Args>
+      void construct(T* ptr, Args&&... args) {
+        ::new ((void*)ptr) T{std::forward<Args>(args)...};
+      }
+    };
+    template <typename T>
+    struct container {
+      template <typename... Args>
+      void emplace(Args&&... args) {
+        alloc a;
+        auto ptr = a.template allocate<T>();
+        a.construct(ptr, std::forward<Args>(args)...);
+      }
+    };
+    void foo() {
+      container<S> c;
+      c.emplace($param1[[1]]);
+      c.emplace($param2[[2]], $param3[[3]]);
+    }
+  )cpp",
+      ExpectedHint{"A: ", "param1"}, ExpectedHint{"B: ", "param2"},
+      ExpectedHint{"C: ", "param3"});
+}
+
+TEST(ParameterHints, VariadicReferenceHint) {
+  assertParameterHints(R"cpp(
+    void foo(int&);
+    template <typename... Args>
+    void bar(Args... args) { return foo(args...); }
+    void baz() {
+      int a;
+      bar(a);
+      bar(1);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, VariadicReferenceHintForwardingRef) {
+  assertParameterHints(R"cpp(
+    void foo(int&);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(args...); }
+    void baz() {
+      int a;
+      bar($param[[a]]);
+      bar(1);
+    }
+  )cpp",
+                       ExpectedHint{"&: ", "param"});
+}
+
+TEST(ParameterHints, VariadicReferenceHintForwardingRefStdForward) {
+  assertParameterHints(R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    void foo(int&);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(std::forward<Args>(args)...); }
+    void baz() {
+      int a;
+      bar($param[[a]]);
+    }
+  )cpp",
+                       ExpectedHint{"&: ", "param"});
+}
+
+TEST(ParameterHints, VariadicNoReferenceHintForwardingRefStdForward) {
+  assertParameterHints(R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    void foo(int);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(std::forward<Args>(args)...); }
+    void baz() {
+      int a;
+      bar(a);
+      bar(1);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, VariadicNoReferenceHintUnresolvedForward) {
+  assertParameterHints(R"cpp(
+    template <typename... Args>
+    void foo(Args&&... args);
+    void bar() {
+      int a;
+      foo(a);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, MatchingNameVariadicForwarded) {
+  // No name hint for variadic parameter with matching name
+  // This prototype of std::forward is sufficient for clang to recognize it
+  assertParameterHints(R"cpp(
+    namespace std { template <typename T> T&& forward(T&); }
+    void foo(int a);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(std::forward<Args>(args)...); }
+    void baz() {
+      int a;
+      bar(a);
+    }
+  )cpp");
+}
+
+TEST(ParameterHints, MatchingNameVariadicPlain) {
+  // No name hint for variadic parameter with matching name
+  assertParameterHints(R"cpp(
+    void foo(int a);
+    template <typename... Args>
+    void bar(Args&&... args) { return foo(args...); }
+    void baz() {
+      int a;
+      bar(a);
+    }
+  )cpp");
+}
+
 TEST(ParameterHints, Operator) {
   // No hint for operator call with operator syntax.
   assertParameterHints(R"cpp(
@@ -470,7 +986,7 @@ TEST(ParameterHints, VarargsFunction) {
   assertParameterHints(R"cpp(
     void foo(int fixed, ...);
 
-    void bar() { 
+    void bar() {
       foo($fixed[[41]], 42, 43);
     }
   )cpp",