1 files changed, 134 insertions, 59 deletions

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 0956a1cc502..dbc322ebfdc 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -2495,35 +2495,76 @@ Decl *TemplateDeclInstantiator::VisitConstructorUsingShadowDecl(
   return nullptr;
 }
 
-Decl * TemplateDeclInstantiator
-    ::VisitUnresolvedUsingTypenameDecl(UnresolvedUsingTypenameDecl *D) {
-  NestedNameSpecifierLoc QualifierLoc
-    = SemaRef.SubstNestedNameSpecifierLoc(D->getQualifierLoc(),
-                                          TemplateArgs);
-  if (!QualifierLoc)
-    return nullptr;
+template <typename T>
+Decl *TemplateDeclInstantiator::instantiateUnresolvedUsingDecl(
+    T *D, bool InstantiatingPackElement) {
+  // If this is a pack expansion, expand it now.
+  if (D->isPackExpansion() && !InstantiatingPackElement) {
+    SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+    SemaRef.collectUnexpandedParameterPacks(D->getQualifierLoc(), Unexpanded);
+    SemaRef.collectUnexpandedParameterPacks(D->getNameInfo(), Unexpanded);
 
-  CXXScopeSpec SS;
-  SS.Adopt(QualifierLoc);
+    // Determine whether the set of unexpanded parameter packs can and should
+    // be expanded.
+    bool Expand = true;
+    bool RetainExpansion = false;
+    Optional<unsigned> NumExpansions;
+    if (SemaRef.CheckParameterPacksForExpansion(
+          D->getEllipsisLoc(), D->getSourceRange(), Unexpanded, TemplateArgs,
+            Expand, RetainExpansion, NumExpansions))
+      return nullptr;
 
-  // Since NameInfo refers to a typename, it cannot be a C++ special name.
-  // Hence, no transformation is required for it.
-  DeclarationNameInfo NameInfo(D->getDeclName(), D->getLocation());
-  NamedDecl *UD =
-    SemaRef.BuildUsingDeclaration(/*Scope*/ nullptr, D->getAccess(),
-                                  D->getUsingLoc(), SS, NameInfo, nullptr,
-                                  /*instantiation*/ true,
-                                  /*typename*/ true, D->getTypenameLoc());
-  if (UD)
-    SemaRef.Context.setInstantiatedFromUsingDecl(UD, D);
+    // This declaration cannot appear within a function template signature,
+    // so we can't have a partial argument list for a parameter pack.
+    assert(!RetainExpansion &&
+           "should never need to retain an expansion for UsingPackDecl");
 
-  return UD;
-}
+    if (!Expand) {
+      // We cannot fully expand the pack expansion now, so substitute into the
+      // pattern and create a new pack expansion.
+      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, -1);
+      return instantiateUnresolvedUsingDecl(D, true);
+    }
+
+    // Within a function, we don't have any normal way to check for conflicts
+    // between shadow declarations from different using declarations in the
+    // same pack expansion, but this is always ill-formed because all expansions
+    // must produce (conflicting) enumerators.
+    //
+    // Sadly we can't just reject this in the template definition because it
+    // could be valid if the pack is empty or has exactly one expansion.
+    if (D->getDeclContext()->isFunctionOrMethod() && *NumExpansions > 1) {
+      SemaRef.Diag(D->getEllipsisLoc(),
+                   diag::err_using_decl_redeclaration_expansion);
+      return nullptr;
+    }
+
+    // Instantiate the slices of this pack and build a UsingPackDecl.
+    SmallVector<NamedDecl*, 8> Expansions;
+    for (unsigned I = 0; I != *NumExpansions; ++I) {
+      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, I);
+      Decl *Slice = instantiateUnresolvedUsingDecl(D, true);
+      if (!Slice)
+        return nullptr;
+      // Note that we can still get unresolved using declarations here, if we
+      // had arguments for all packs but the pattern also contained other
+      // template arguments (this only happens during partial substitution, eg
+      // into the body of a generic lambda in a function template).
+      Expansions.push_back(cast<NamedDecl>(Slice));
+    }
+
+    auto *NewD = SemaRef.BuildUsingPackDecl(D, Expansions);
+    if (isDeclWithinFunction(D))
+      SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, NewD);
+    return NewD;
+  }
+
+  UnresolvedUsingTypenameDecl *TD = dyn_cast<UnresolvedUsingTypenameDecl>(D);
+  SourceLocation TypenameLoc = TD ? TD->getTypenameLoc() : SourceLocation();
 
-Decl * TemplateDeclInstantiator
-    ::VisitUnresolvedUsingValueDecl(UnresolvedUsingValueDecl *D) {
   NestedNameSpecifierLoc QualifierLoc
-      = SemaRef.SubstNestedNameSpecifierLoc(D->getQualifierLoc(), TemplateArgs);
+    = SemaRef.SubstNestedNameSpecifierLoc(D->getQualifierLoc(),
+                                          TemplateArgs);
   if (!QualifierLoc)
     return nullptr;
 
@@ -2533,17 +2574,48 @@ Decl * TemplateDeclInstantiator
   DeclarationNameInfo NameInfo
     = SemaRef.SubstDeclarationNameInfo(D->getNameInfo(), TemplateArgs);
 
-  NamedDecl *UD =
-    SemaRef.BuildUsingDeclaration(/*Scope*/ nullptr, D->getAccess(),
-                                  D->getUsingLoc(), SS, NameInfo, nullptr,
-                                  /*instantiation*/ true,
-                                  /*typename*/ false, SourceLocation());
+  // Produce a pack expansion only if we're not instantiating a particular
+  // slice of a pack expansion.
+  bool InstantiatingSlice = D->getEllipsisLoc().isValid() &&
+                            SemaRef.ArgumentPackSubstitutionIndex != -1;
+  SourceLocation EllipsisLoc =
+      InstantiatingSlice ? SourceLocation() : D->getEllipsisLoc();
+
+  NamedDecl *UD = SemaRef.BuildUsingDeclaration(
+      /*Scope*/ nullptr, D->getAccess(), D->getUsingLoc(),
+      /*HasTypename*/ TD, TypenameLoc, SS, NameInfo, EllipsisLoc, nullptr,
+      /*IsInstantiation*/ true);
   if (UD)
     SemaRef.Context.setInstantiatedFromUsingDecl(UD, D);
 
   return UD;
 }
 
+Decl *TemplateDeclInstantiator::VisitUnresolvedUsingTypenameDecl(
+    UnresolvedUsingTypenameDecl *D) {
+  return instantiateUnresolvedUsingDecl(D);
+}
+
+Decl *TemplateDeclInstantiator::VisitUnresolvedUsingValueDecl(
+    UnresolvedUsingValueDecl *D) {
+  return instantiateUnresolvedUsingDecl(D);
+}
+
+Decl *TemplateDeclInstantiator::VisitUsingPackDecl(UsingPackDecl *D) {
+  SmallVector<NamedDecl*, 8> Expansions;
+  for (auto *UD : D->expansions()) {
+    if (auto *NewUD =
+            SemaRef.FindInstantiatedDecl(D->getLocation(), UD, TemplateArgs))
+      Expansions.push_back(cast<NamedDecl>(NewUD));
+    else
+      return nullptr;
+  }
+
+  auto *NewD = SemaRef.BuildUsingPackDecl(D, Expansions);
+  if (isDeclWithinFunction(D))
+    SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, NewD);
+  return NewD;
+}
 
 Decl *TemplateDeclInstantiator::VisitClassScopeFunctionSpecializationDecl(
                                      ClassScopeFunctionSpecializationDecl *Decl) {
@@ -4513,22 +4585,36 @@ static bool isInstantiationOf(UsingShadowDecl *Pattern,
                             Pattern);
 }
 
-static bool isInstantiationOf(UsingDecl *Pattern,
-                              UsingDecl *Instance,
-                              ASTContext &C) {
-  return declaresSameEntity(C.getInstantiatedFromUsingDecl(Instance), Pattern);
-}
-
-static bool isInstantiationOf(UnresolvedUsingValueDecl *Pattern,
-                              NamedDecl *Instance,
+static bool isInstantiationOf(UsingDecl *Pattern, UsingDecl *Instance,
                               ASTContext &C) {
   return declaresSameEntity(C.getInstantiatedFromUsingDecl(Instance), Pattern);
 }
 
-static bool isInstantiationOf(UnresolvedUsingTypenameDecl *Pattern,
-                              NamedDecl *Instance,
-                              ASTContext &C) {
-  return declaresSameEntity(C.getInstantiatedFromUsingDecl(Instance), Pattern);
+template<typename T>
+static bool isInstantiationOfUnresolvedUsingDecl(T *Pattern, Decl *Other,
+                                                 ASTContext &Ctx) {
+  // An unresolved using declaration can instantiate to an unresolved using
+  // declaration, or to a using declaration or a using declaration pack.
+  //
+  // Multiple declarations can claim to be instantiated from an unresolved
+  // using declaration if it's a pack expansion. We want the UsingPackDecl
+  // in that case, not the individual UsingDecls within the pack.
+  bool OtherIsPackExpansion;
+  NamedDecl *OtherFrom;
+  if (auto *OtherUUD = dyn_cast<T>(Other)) {
+    OtherIsPackExpansion = OtherUUD->isPackExpansion();
+    OtherFrom = Ctx.getInstantiatedFromUsingDecl(OtherUUD);
+  } else if (auto *OtherUPD = dyn_cast<UsingPackDecl>(Other)) {
+    OtherIsPackExpansion = true;
+    OtherFrom = OtherUPD->getInstantiatedFromUsingDecl();
+  } else if (auto *OtherUD = dyn_cast<UsingDecl>(Other)) {
+    OtherIsPackExpansion = false;
+    OtherFrom = Ctx.getInstantiatedFromUsingDecl(OtherUD);
+  } else {
+    return false;
+  }
+  return Pattern->isPackExpansion() == OtherIsPackExpansion &&
+         declaresSameEntity(OtherFrom, Pattern);
 }
 
 static bool isInstantiationOfStaticDataMember(VarDecl *Pattern,
@@ -4549,21 +4635,14 @@ static bool isInstantiationOfStaticDataMember(VarDecl *Pattern,
 // Other is the prospective instantiation
 // D is the prospective pattern
 static bool isInstantiationOf(ASTContext &Ctx, NamedDecl *D, Decl *Other) {
-  if (D->getKind() != Other->getKind()) {
-    if (auto *UUD = dyn_cast<UnresolvedUsingTypenameDecl>(D)) {
-      if (UsingDecl *UD = dyn_cast<UsingDecl>(Other)) {
-        return isInstantiationOf(UUD, UD, Ctx);
-      }
-    }
+  if (auto *UUD = dyn_cast<UnresolvedUsingTypenameDecl>(D))
+    return isInstantiationOfUnresolvedUsingDecl(UUD, Other, Ctx);
 
-    if (auto *UUD = dyn_cast<UnresolvedUsingValueDecl>(D)) {
-      if (UsingDecl *UD = dyn_cast<UsingDecl>(Other)) {
-        return isInstantiationOf(UUD, UD, Ctx);
-      }
-    }
+  if (auto *UUD = dyn_cast<UnresolvedUsingValueDecl>(D))
+    return isInstantiationOfUnresolvedUsingDecl(UUD, Other, Ctx);
 
+  if (D->getKind() != Other->getKind())
     return false;
-  }
 
   if (auto *Record = dyn_cast<CXXRecordDecl>(Other))
     return isInstantiationOf(cast<CXXRecordDecl>(D), Record);
@@ -4600,12 +4679,6 @@ static bool isInstantiationOf(ASTContext &Ctx, NamedDecl *D, Decl *Other) {
   if (auto *Using = dyn_cast<UsingDecl>(Other))
     return isInstantiationOf(cast<UsingDecl>(D), Using, Ctx);
 
-  if (auto *Using = dyn_cast<UnresolvedUsingValueDecl>(Other))
-    return isInstantiationOf(cast<UnresolvedUsingValueDecl>(D), Using, Ctx);
-
-  if (auto *Using = dyn_cast<UnresolvedUsingTypenameDecl>(Other))
-    return isInstantiationOf(cast<UnresolvedUsingTypenameDecl>(D), Using, Ctx);
-
   if (auto *Shadow = dyn_cast<UsingShadowDecl>(Other))
     return isInstantiationOf(cast<UsingShadowDecl>(D), Shadow, Ctx);
 
@@ -4846,6 +4919,8 @@ NamedDecl *Sema::FindInstantiatedDecl(SourceLocation Loc, NamedDecl *D,
     }
 
     NamedDecl *Result = nullptr;
+    // FIXME: If the name is a dependent name, this lookup won't necessarily
+    // find it. Does that ever matter?
     if (D->getDeclName()) {
       DeclContext::lookup_result Found = ParentDC->lookup(D->getDeclName());
       Result = findInstantiationOf(Context, D, Found.begin(), Found.end());