diff --git a/clang-tools-extra/clangd/IncludeFixer.cpp b/clang-tools-extra/clangd/IncludeFixer.cpp --- a/clang-tools-extra/clangd/IncludeFixer.cpp +++ b/clang-tools-extra/clangd/IncludeFixer.cpp @@ -338,7 +338,7 @@ return TypoCorrection(); auto *Sem = SemaPtr; // Avoid capturing `this`. - Unresolved.GetScopes = [Sem, CheapUnresolved, S, LookupKind]() { + auto GetScopes = [Sem, CheapUnresolved, S, LookupKind]() { std::vector Scopes; if (CheapUnresolved.ResolvedScope) { Scopes.push_back(*CheapUnresolved.ResolvedScope); @@ -363,6 +363,9 @@ Scope.append(*CheapUnresolved.UnresolvedScope); return Scopes; }; + std::vector Scopes = GetScopes(); + Unresolved.GetScopes = [=]() { return Scopes; }; + LastUnresolvedName = std::move(Unresolved); // Never return a valid correction to try to recover. Our suggested fixes diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp --- a/clang/lib/Sema/SemaLookup.cpp +++ b/clang/lib/Sema/SemaLookup.cpp @@ -3609,328 +3609,344 @@ return nullptr; } -static void LookupVisibleDecls(DeclContext *Ctx, LookupResult &Result, - bool QualifiedNameLookup, - bool InBaseClass, - VisibleDeclConsumer &Consumer, - VisibleDeclsRecord &Visited, - bool IncludeDependentBases, - bool LoadExternal) { - if (!Ctx) - return; - - // Make sure we don't visit the same context twice. - if (Visited.visitedContext(Ctx->getPrimaryContext())) - return; - - Consumer.EnteredContext(Ctx); - - // Outside C++, lookup results for the TU live on identifiers. - if (isa(Ctx) && - !Result.getSema().getLangOpts().CPlusPlus) { - auto &S = Result.getSema(); - auto &Idents = S.Context.Idents; - - // Ensure all external identifiers are in the identifier table. - if (LoadExternal) - if (IdentifierInfoLookup *External = Idents.getExternalIdentifierLookup()) { - std::unique_ptr Iter(External->getIdentifiers()); - for (StringRef Name = Iter->Next(); !Name.empty(); Name = Iter->Next()) - Idents.get(Name); - } +struct LookupVisibleHelper { + LookupVisibleHelper(VisibleDeclConsumer &Consumer, bool IncludeDependentBases, + bool LoadExternal) + : Consumer(Consumer), IncludeDependentBases(IncludeDependentBases), + LoadExternal(LoadExternal) {} + + void lookupVisibleDecls(Sema &SemaRef, Scope *S, Sema::LookupNameKind Kind, + bool IncludeGlobalScope) { + // Determine the set of using directives available during + // unqualified name lookup. + Scope *Initial = S; + UnqualUsingDirectiveSet UDirs(SemaRef); + if (SemaRef.getLangOpts().CPlusPlus) { + // Find the first namespace or translation-unit scope. + while (S && !isNamespaceOrTranslationUnitScope(S)) + S = S->getParent(); - // Walk all lookup results in the TU for each identifier. - for (const auto &Ident : Idents) { - for (auto I = S.IdResolver.begin(Ident.getValue()), - E = S.IdResolver.end(); - I != E; ++I) { - if (S.IdResolver.isDeclInScope(*I, Ctx)) { - if (NamedDecl *ND = Result.getAcceptableDecl(*I)) { - Consumer.FoundDecl(ND, Visited.checkHidden(ND), Ctx, InBaseClass); - Visited.add(ND); - } - } - } + UDirs.visitScopeChain(Initial, S); } + UDirs.done(); - return; + // Look for visible declarations. + LookupResult Result(SemaRef, DeclarationName(), SourceLocation(), Kind); + Result.setAllowHidden(Consumer.includeHiddenDecls()); + if (!IncludeGlobalScope) + Visited.visitedContext(SemaRef.getASTContext().getTranslationUnitDecl()); + ShadowContextRAII Shadow(Visited); + lookupInScope(Initial, Result, UDirs); } - if (CXXRecordDecl *Class = dyn_cast(Ctx)) - Result.getSema().ForceDeclarationOfImplicitMembers(Class); - - // We sometimes skip loading namespace-level results (they tend to be huge). - bool Load = LoadExternal || - !(isa(Ctx) || isa(Ctx)); - // Enumerate all of the results in this context. - for (DeclContextLookupResult R : - Load ? Ctx->lookups() - : Ctx->noload_lookups(/*PreserveInternalState=*/false)) { - for (auto *D : R) { - if (auto *ND = Result.getAcceptableDecl(D)) { - Consumer.FoundDecl(ND, Visited.checkHidden(ND), Ctx, InBaseClass); - Visited.add(ND); - } - } - } + void lookupVisibleDecls(Sema &SemaRef, DeclContext *Ctx, + Sema::LookupNameKind Kind, bool IncludeGlobalScope) { + LookupResult Result(SemaRef, DeclarationName(), SourceLocation(), Kind); + Result.setAllowHidden(Consumer.includeHiddenDecls()); + if (!IncludeGlobalScope) + Visited.visitedContext(SemaRef.getASTContext().getTranslationUnitDecl()); - // Traverse using directives for qualified name lookup. - if (QualifiedNameLookup) { ShadowContextRAII Shadow(Visited); - for (auto I : Ctx->using_directives()) { - if (!Result.getSema().isVisible(I)) - continue; - LookupVisibleDecls(I->getNominatedNamespace(), Result, - QualifiedNameLookup, InBaseClass, Consumer, Visited, - IncludeDependentBases, LoadExternal); - } + lookupInDeclContext(Ctx, Result, /*QualifiedNameLookup=*/true, + /*InBaseClass=*/false); } - // Traverse the contexts of inherited C++ classes. - if (CXXRecordDecl *Record = dyn_cast(Ctx)) { - if (!Record->hasDefinition()) +private: + void lookupInDeclContext(DeclContext *Ctx, LookupResult &Result, + bool QualifiedNameLookup, bool InBaseClass) { + if (!Ctx) return; - for (const auto &B : Record->bases()) { - QualType BaseType = B.getType(); + // Make sure we don't visit the same context twice. + if (Visited.visitedContext(Ctx->getPrimaryContext())) + return; - RecordDecl *RD; - if (BaseType->isDependentType()) { - if (!IncludeDependentBases) { - // Don't look into dependent bases, because name lookup can't look - // there anyway. - continue; + Consumer.EnteredContext(Ctx); + + // Outside C++, lookup results for the TU live on identifiers. + if (isa(Ctx) && + !Result.getSema().getLangOpts().CPlusPlus) { + auto &S = Result.getSema(); + auto &Idents = S.Context.Idents; + + // Ensure all external identifiers are in the identifier table. + if (LoadExternal) + if (IdentifierInfoLookup *External = + Idents.getExternalIdentifierLookup()) { + std::unique_ptr Iter(External->getIdentifiers()); + for (StringRef Name = Iter->Next(); !Name.empty(); + Name = Iter->Next()) + Idents.get(Name); + } + + // Walk all lookup results in the TU for each identifier. + for (const auto &Ident : Idents) { + for (auto I = S.IdResolver.begin(Ident.getValue()), + E = S.IdResolver.end(); + I != E; ++I) { + if (S.IdResolver.isDeclInScope(*I, Ctx)) { + if (NamedDecl *ND = Result.getAcceptableDecl(*I)) { + Consumer.FoundDecl(ND, Visited.checkHidden(ND), Ctx, InBaseClass); + Visited.add(ND); + } + } } - const auto *TST = BaseType->getAs(); - if (!TST) - continue; - TemplateName TN = TST->getTemplateName(); - const auto *TD = - dyn_cast_or_null(TN.getAsTemplateDecl()); - if (!TD) - continue; - RD = TD->getTemplatedDecl(); - } else { - const auto *Record = BaseType->getAs(); - if (!Record) - continue; - RD = Record->getDecl(); } - // FIXME: It would be nice to be able to determine whether referencing - // a particular member would be ambiguous. For example, given - // - // struct A { int member; }; - // struct B { int member; }; - // struct C : A, B { }; - // - // void f(C *c) { c->### } - // - // accessing 'member' would result in an ambiguity. However, we - // could be smart enough to qualify the member with the base - // class, e.g., - // - // c->B::member - // - // or - // - // c->A::member - - // Find results in this base class (and its bases). - ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(RD, Result, QualifiedNameLookup, /*InBaseClass=*/true, - Consumer, Visited, IncludeDependentBases, - LoadExternal); + return; } - } - // Traverse the contexts of Objective-C classes. - if (ObjCInterfaceDecl *IFace = dyn_cast(Ctx)) { - // Traverse categories. - for (auto *Cat : IFace->visible_categories()) { - ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(Cat, Result, QualifiedNameLookup, false, Consumer, - Visited, IncludeDependentBases, LoadExternal); + if (CXXRecordDecl *Class = dyn_cast(Ctx)) + Result.getSema().ForceDeclarationOfImplicitMembers(Class); + + // We sometimes skip loading namespace-level results (they tend to be huge). + bool Load = LoadExternal || + !(isa(Ctx) || isa(Ctx)); + // Enumerate all of the results in this context. + for (DeclContextLookupResult R : + Load ? Ctx->lookups() + : Ctx->noload_lookups(/*PreserveInternalState=*/false)) { + for (auto *D : R) { + if (auto *ND = Result.getAcceptableDecl(D)) { + Consumer.FoundDecl(ND, Visited.checkHidden(ND), Ctx, InBaseClass); + Visited.add(ND); + } + } } - // Traverse protocols. - for (auto *I : IFace->all_referenced_protocols()) { + // Traverse using directives for qualified name lookup. + if (QualifiedNameLookup) { ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(I, Result, QualifiedNameLookup, false, Consumer, - Visited, IncludeDependentBases, LoadExternal); + for (auto I : Ctx->using_directives()) { + if (!Result.getSema().isVisible(I)) + continue; + lookupInDeclContext(I->getNominatedNamespace(), Result, + QualifiedNameLookup, InBaseClass); + } } - // Traverse the superclass. - if (IFace->getSuperClass()) { - ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(IFace->getSuperClass(), Result, QualifiedNameLookup, - true, Consumer, Visited, IncludeDependentBases, - LoadExternal); - } + // Traverse the contexts of inherited C++ classes. + if (CXXRecordDecl *Record = dyn_cast(Ctx)) { + if (!Record->hasDefinition()) + return; - // If there is an implementation, traverse it. We do this to find - // synthesized ivars. - if (IFace->getImplementation()) { - ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(IFace->getImplementation(), Result, - QualifiedNameLookup, InBaseClass, Consumer, Visited, - IncludeDependentBases, LoadExternal); - } - } else if (ObjCProtocolDecl *Protocol = dyn_cast(Ctx)) { - for (auto *I : Protocol->protocols()) { - ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(I, Result, QualifiedNameLookup, false, Consumer, - Visited, IncludeDependentBases, LoadExternal); - } - } else if (ObjCCategoryDecl *Category = dyn_cast(Ctx)) { - for (auto *I : Category->protocols()) { - ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(I, Result, QualifiedNameLookup, false, Consumer, - Visited, IncludeDependentBases, LoadExternal); + for (const auto &B : Record->bases()) { + QualType BaseType = B.getType(); + + RecordDecl *RD; + if (BaseType->isDependentType()) { + if (!IncludeDependentBases) { + // Don't look into dependent bases, because name lookup can't look + // there anyway. + continue; + } + const auto *TST = BaseType->getAs(); + if (!TST) + continue; + TemplateName TN = TST->getTemplateName(); + const auto *TD = + dyn_cast_or_null(TN.getAsTemplateDecl()); + if (!TD) + continue; + RD = TD->getTemplatedDecl(); + } else { + const auto *Record = BaseType->getAs(); + if (!Record) + continue; + RD = Record->getDecl(); + } + + // FIXME: It would be nice to be able to determine whether referencing + // a particular member would be ambiguous. For example, given + // + // struct A { int member; }; + // struct B { int member; }; + // struct C : A, B { }; + // + // void f(C *c) { c->### } + // + // accessing 'member' would result in an ambiguity. However, we + // could be smart enough to qualify the member with the base + // class, e.g., + // + // c->B::member + // + // or + // + // c->A::member + + // Find results in this base class (and its bases). + ShadowContextRAII Shadow(Visited); + lookupInDeclContext(RD, Result, QualifiedNameLookup, + /*InBaseClass=*/true); + } } - // If there is an implementation, traverse it. - if (Category->getImplementation()) { - ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(Category->getImplementation(), Result, - QualifiedNameLookup, true, Consumer, Visited, - IncludeDependentBases, LoadExternal); + // Traverse the contexts of Objective-C classes. + if (ObjCInterfaceDecl *IFace = dyn_cast(Ctx)) { + // Traverse categories. + for (auto *Cat : IFace->visible_categories()) { + ShadowContextRAII Shadow(Visited); + lookupInDeclContext(Cat, Result, QualifiedNameLookup, + /*InBaseClass=*/false); + } + + // Traverse protocols. + for (auto *I : IFace->all_referenced_protocols()) { + ShadowContextRAII Shadow(Visited); + lookupInDeclContext(I, Result, QualifiedNameLookup, + /*InBaseClass=*/false); + } + + // Traverse the superclass. + if (IFace->getSuperClass()) { + ShadowContextRAII Shadow(Visited); + lookupInDeclContext(IFace->getSuperClass(), Result, QualifiedNameLookup, + /*InBaseClass=*/true); + } + + // If there is an implementation, traverse it. We do this to find + // synthesized ivars. + if (IFace->getImplementation()) { + ShadowContextRAII Shadow(Visited); + lookupInDeclContext(IFace->getImplementation(), Result, + QualifiedNameLookup, InBaseClass); + } + } else if (ObjCProtocolDecl *Protocol = dyn_cast(Ctx)) { + for (auto *I : Protocol->protocols()) { + ShadowContextRAII Shadow(Visited); + lookupInDeclContext(I, Result, QualifiedNameLookup, + /*InBaseClass=*/false); + } + } else if (ObjCCategoryDecl *Category = dyn_cast(Ctx)) { + for (auto *I : Category->protocols()) { + ShadowContextRAII Shadow(Visited); + lookupInDeclContext(I, Result, QualifiedNameLookup, + /*InBaseClass=*/false); + } + + // If there is an implementation, traverse it. + if (Category->getImplementation()) { + ShadowContextRAII Shadow(Visited); + lookupInDeclContext(Category->getImplementation(), Result, + QualifiedNameLookup, /*InBaseClass=*/true); + } } } -} -static void LookupVisibleDecls(Scope *S, LookupResult &Result, - UnqualUsingDirectiveSet &UDirs, - VisibleDeclConsumer &Consumer, - VisibleDeclsRecord &Visited, - bool LoadExternal) { - if (!S) - return; + void lookupInScope(Scope *S, LookupResult &Result, + UnqualUsingDirectiveSet &UDirs) { + // No clients run in this mode and it's not supported. Please add tests and + // remove the assertion if you start relying on it. + assert(!IncludeDependentBases && "Unsupported flag for lookupInScope"); - if (!S->getEntity() || - (!S->getParent() && - !Visited.alreadyVisitedContext(S->getEntity())) || - (S->getEntity())->isFunctionOrMethod()) { - FindLocalExternScope FindLocals(Result); - // Walk through the declarations in this Scope. The consumer might add new - // decls to the scope as part of deserialization, so make a copy first. - SmallVector ScopeDecls(S->decls().begin(), S->decls().end()); - for (Decl *D : ScopeDecls) { - if (NamedDecl *ND = dyn_cast(D)) - if ((ND = Result.getAcceptableDecl(ND))) { - Consumer.FoundDecl(ND, Visited.checkHidden(ND), nullptr, false); - Visited.add(ND); - } + if (!S) + return; + + if (!S->getEntity() || + (!S->getParent() && !Visited.alreadyVisitedContext(S->getEntity())) || + (S->getEntity())->isFunctionOrMethod()) { + FindLocalExternScope FindLocals(Result); + // Walk through the declarations in this Scope. The consumer might add new + // decls to the scope as part of deserialization, so make a copy first. + SmallVector ScopeDecls(S->decls().begin(), S->decls().end()); + for (Decl *D : ScopeDecls) { + if (NamedDecl *ND = dyn_cast(D)) + if ((ND = Result.getAcceptableDecl(ND))) { + Consumer.FoundDecl(ND, Visited.checkHidden(ND), nullptr, false); + Visited.add(ND); + } + } } - } - // FIXME: C++ [temp.local]p8 - DeclContext *Entity = nullptr; - if (S->getEntity()) { - // Look into this scope's declaration context, along with any of its - // parent lookup contexts (e.g., enclosing classes), up to the point - // where we hit the context stored in the next outer scope. - Entity = S->getEntity(); - DeclContext *OuterCtx = findOuterContext(S).first; // FIXME - - for (DeclContext *Ctx = Entity; Ctx && !Ctx->Equals(OuterCtx); - Ctx = Ctx->getLookupParent()) { - if (ObjCMethodDecl *Method = dyn_cast(Ctx)) { - if (Method->isInstanceMethod()) { - // For instance methods, look for ivars in the method's interface. - LookupResult IvarResult(Result.getSema(), Result.getLookupName(), - Result.getNameLoc(), Sema::LookupMemberName); - if (ObjCInterfaceDecl *IFace = Method->getClassInterface()) { - LookupVisibleDecls(IFace, IvarResult, /*QualifiedNameLookup=*/false, - /*InBaseClass=*/false, Consumer, Visited, - /*IncludeDependentBases=*/false, LoadExternal); + // FIXME: C++ [temp.local]p8 + DeclContext *Entity = nullptr; + if (S->getEntity()) { + // Look into this scope's declaration context, along with any of its + // parent lookup contexts (e.g., enclosing classes), up to the point + // where we hit the context stored in the next outer scope. + Entity = S->getEntity(); + DeclContext *OuterCtx = findOuterContext(S).first; // FIXME + + for (DeclContext *Ctx = Entity; Ctx && !Ctx->Equals(OuterCtx); + Ctx = Ctx->getLookupParent()) { + if (ObjCMethodDecl *Method = dyn_cast(Ctx)) { + if (Method->isInstanceMethod()) { + // For instance methods, look for ivars in the method's interface. + LookupResult IvarResult(Result.getSema(), Result.getLookupName(), + Result.getNameLoc(), + Sema::LookupMemberName); + if (ObjCInterfaceDecl *IFace = Method->getClassInterface()) { + lookupInDeclContext(IFace, IvarResult, + /*QualifiedNameLookup=*/false, + /*InBaseClass=*/false); + } } + + // We've already performed all of the name lookup that we need + // to for Objective-C methods; the next context will be the + // outer scope. + break; } - // We've already performed all of the name lookup that we need - // to for Objective-C methods; the next context will be the - // outer scope. - break; - } + if (Ctx->isFunctionOrMethod()) + continue; - if (Ctx->isFunctionOrMethod()) - continue; + lookupInDeclContext(Ctx, Result, /*QualifiedNameLookup=*/false, + /*InBaseClass=*/false); + } + } else if (!S->getParent()) { + // Look into the translation unit scope. We walk through the translation + // unit's declaration context, because the Scope itself won't have all of + // the declarations if we loaded a precompiled header. + // FIXME: We would like the translation unit's Scope object to point to + // the translation unit, so we don't need this special "if" branch. + // However, doing so would force the normal C++ name-lookup code to look + // into the translation unit decl when the IdentifierInfo chains would + // suffice. Once we fix that problem (which is part of a more general + // "don't look in DeclContexts unless we have to" optimization), we can + // eliminate this. + Entity = Result.getSema().Context.getTranslationUnitDecl(); + lookupInDeclContext(Entity, Result, /*QualifiedNameLookup=*/false, + /*InBaseClass=*/false); + } - LookupVisibleDecls(Ctx, Result, /*QualifiedNameLookup=*/false, - /*InBaseClass=*/false, Consumer, Visited, - /*IncludeDependentBases=*/false, LoadExternal); + if (Entity) { + // Lookup visible declarations in any namespaces found by using + // directives. + for (const UnqualUsingEntry &UUE : UDirs.getNamespacesFor(Entity)) + lookupInDeclContext( + const_cast(UUE.getNominatedNamespace()), Result, + /*QualifiedNameLookup=*/false, + /*InBaseClass=*/false); } - } else if (!S->getParent()) { - // Look into the translation unit scope. We walk through the translation - // unit's declaration context, because the Scope itself won't have all of - // the declarations if we loaded a precompiled header. - // FIXME: We would like the translation unit's Scope object to point to the - // translation unit, so we don't need this special "if" branch. However, - // doing so would force the normal C++ name-lookup code to look into the - // translation unit decl when the IdentifierInfo chains would suffice. - // Once we fix that problem (which is part of a more general "don't look - // in DeclContexts unless we have to" optimization), we can eliminate this. - Entity = Result.getSema().Context.getTranslationUnitDecl(); - LookupVisibleDecls(Entity, Result, /*QualifiedNameLookup=*/false, - /*InBaseClass=*/false, Consumer, Visited, - /*IncludeDependentBases=*/false, LoadExternal); - } - if (Entity) { - // Lookup visible declarations in any namespaces found by using - // directives. - for (const UnqualUsingEntry &UUE : UDirs.getNamespacesFor(Entity)) - LookupVisibleDecls(const_cast(UUE.getNominatedNamespace()), - Result, /*QualifiedNameLookup=*/false, - /*InBaseClass=*/false, Consumer, Visited, - /*IncludeDependentBases=*/false, LoadExternal); + // Lookup names in the parent scope. + ShadowContextRAII Shadow(Visited); + lookupInScope(S->getParent(), Result, UDirs); } - // Lookup names in the parent scope. - ShadowContextRAII Shadow(Visited); - LookupVisibleDecls(S->getParent(), Result, UDirs, Consumer, Visited, - LoadExternal); -} +private: + VisibleDeclsRecord Visited; + VisibleDeclConsumer &Consumer; + bool IncludeDependentBases; + bool LoadExternal; +}; void Sema::LookupVisibleDecls(Scope *S, LookupNameKind Kind, VisibleDeclConsumer &Consumer, bool IncludeGlobalScope, bool LoadExternal) { - // Determine the set of using directives available during - // unqualified name lookup. - Scope *Initial = S; - UnqualUsingDirectiveSet UDirs(*this); - if (getLangOpts().CPlusPlus) { - // Find the first namespace or translation-unit scope. - while (S && !isNamespaceOrTranslationUnitScope(S)) - S = S->getParent(); - - UDirs.visitScopeChain(Initial, S); - } - UDirs.done(); - - // Look for visible declarations. - LookupResult Result(*this, DeclarationName(), SourceLocation(), Kind); - Result.setAllowHidden(Consumer.includeHiddenDecls()); - VisibleDeclsRecord Visited; - if (!IncludeGlobalScope) - Visited.visitedContext(Context.getTranslationUnitDecl()); - ShadowContextRAII Shadow(Visited); - ::LookupVisibleDecls(Initial, Result, UDirs, Consumer, Visited, LoadExternal); + LookupVisibleHelper H(Consumer, /*IncludeDependentBases=*/false, + LoadExternal); + H.lookupVisibleDecls(*this, S, Kind, IncludeGlobalScope); } void Sema::LookupVisibleDecls(DeclContext *Ctx, LookupNameKind Kind, VisibleDeclConsumer &Consumer, bool IncludeGlobalScope, bool IncludeDependentBases, bool LoadExternal) { - LookupResult Result(*this, DeclarationName(), SourceLocation(), Kind); - Result.setAllowHidden(Consumer.includeHiddenDecls()); - VisibleDeclsRecord Visited; - if (!IncludeGlobalScope) - Visited.visitedContext(Context.getTranslationUnitDecl()); - ShadowContextRAII Shadow(Visited); - ::LookupVisibleDecls(Ctx, Result, /*QualifiedNameLookup=*/true, - /*InBaseClass=*/false, Consumer, Visited, - IncludeDependentBases, LoadExternal); + LookupVisibleHelper H(Consumer, IncludeDependentBases, LoadExternal); + H.lookupVisibleDecls(*this, Ctx, Kind, IncludeGlobalScope); } /// LookupOrCreateLabel - Do a name lookup of a label with the specified name.