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 //===- ASTImporterLookupTable.h - ASTImporter specific lookup--*- C++ -*---===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file defines the ASTImporterLookupTable class which implements a
 //  lookup procedure for the import mechanism.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_AST_ASTIMPORTERLOOKUPTABLE_H
 #define LLVM_CLANG_AST_ASTIMPORTERLOOKUPTABLE_H
 
 #include "clang/AST/DeclBase.h" // lookup_result
 #include "clang/AST/DeclarationName.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SetVector.h"
 
 namespace clang {
 
 class NamedDecl;
 class DeclContext;
 
 // There are certain cases when normal C/C++ lookup (localUncachedLookup)
 // does not find AST nodes. E.g.:
 // Example 1:
 //   template <class T>
 //   struct X {
 //     friend void foo(); // this is never found in the DC of the TU.
 //   };
 // Example 2:
 //   // The fwd decl to Foo is not found in the lookupPtr of the DC of the
 //   // translation unit decl.
 //   // Here we could find the node by doing a traverse throught the list of
 //   // the Decls in the DC, but that would not scale.
 //   struct A { struct Foo *p; };
 // This is a severe problem because the importer decides if it has to create a
 // new Decl or not based on the lookup results.
 // To overcome these cases we need an importer specific lookup table which
 // holds every node and we are not interested in any C/C++ specific visibility
 // considerations. Simply, we must know if there is an existing Decl in a
 // given DC. Once we found it then we can handle any visibility related tasks.
 class ASTImporterLookupTable {
 
   // We store a list of declarations for each name.
   // And we collect these lists for each DeclContext.
   // We could have a flat map with (DeclContext, Name) tuple as key, but a two
   // level map seems easier to handle.
   using DeclList = llvm::SmallSetVector<NamedDecl *, 2>;
   using NameMap = llvm::SmallDenseMap<DeclarationName, DeclList, 4>;
   using DCMap = llvm::DenseMap<DeclContext *, NameMap>;
 
   void add(DeclContext *DC, NamedDecl *ND);
   void remove(DeclContext *DC, NamedDecl *ND);
 
   DCMap LookupTable;
 
 public:
   ASTImporterLookupTable(TranslationUnitDecl &TU);
   void add(NamedDecl *ND);
   void remove(NamedDecl *ND);
   // Sometimes a declaration is created first with a temporarily value of decl
   // context (often the translation unit) and later moved to the final context.
   // This happens for declarations that are created before the final declaration
   // context. In such cases the lookup table needs to be updated.
   // (The declaration is in these cases not added to the temporary decl context,
   // only its parent is set.)
   // FIXME: It would be better to not add the declaration to the temporary
   // context at all in the lookup table, but this requires big change in
   // ASTImporter.
   // The function should be called when the old context is definitely different
   // from the new.
   void update(NamedDecl *ND, DeclContext *OldDC);
   // Same as 'update' but allow if 'ND' is not in the table or the old context
   // is the same as the new.
   // FIXME: The old redeclaration context is not handled.
   void updateForced(NamedDecl *ND, DeclContext *OldDC);
   using LookupResult = DeclList;
   LookupResult lookup(DeclContext *DC, DeclarationName Name) const;
   // Check if the `ND` is within the lookup table (with its current name) in
   // context `DC`. This is intended for debug purposes when the DeclContext of a
   // NamedDecl is changed.
   bool contains(DeclContext *DC, NamedDecl *ND) const;
   void dump(DeclContext *DC) const;
   void dump() const;
 };
 
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_ASTIMPORTERLOOKUPTABLE_H

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