[Clang Importer] Support multiple alternate decls

ImportDecl has an alternate decl quasi-escape-hatch where it can
create alternative declarations for the same original Clang node. But,
it was limited at one, which worked accidentally in the case of
VisitObjCMethodDecl's attempts at importing a factory-init-suppressed
root-class method that points to the imported-as-init Swift
declaration.

This commit changes that to a TinyPtrVector, to support the upcoming
occasional case where we have more.

Author: milseman

lib/ClangImporter/ClangImporter.cpp

@@ -2196,7 +2196,7 @@ void ClangModuleUnit::lookupObjCMethods(
       results.push_back(func);
 
     // If there is an alternate declaration, also look at it.
-    if (auto alternate = owner.Impl.getAlternateDecl(imported)) {
+    for (auto alternate : owner.Impl.getAlternateDecls(imported)) {
       if (auto func = dyn_cast<AbstractFunctionDecl>(alternate))
         results.push_back(func);
     }
@@ -2585,7 +2585,7 @@ void ClangImporter::Implementation::lookupValue(
 
     // If there is an alternate declaration and the name matches,
     // report this result.
-    if (auto alternate = getAlternateDecl(decl)) {
+    for (auto alternate : getAlternateDecls(decl)) {
       if (alternate->getFullName().matchesRef(name) &&
           alternate->getDeclContext()->isModuleScopeContext()) {
         consumer.foundDecl(alternate, DeclVisibilityKind::VisibleAtTopLevel);
@@ -2649,7 +2649,7 @@ void ClangImporter::Implementation::lookupObjCMembers(
 
     // Check for an alternate declaration; if it's name matches,
     // report it.
-    if (auto alternate = getAlternateDecl(decl)) {
+    for (auto alternate : getAlternateDecls(decl)) {
       if (alternate->getFullName().matchesRef(name)) {
         consumer.foundDecl(alternate, DeclVisibilityKind::DynamicLookup);
         matchedAny = true;

lib/ClangImporter/ImportDecl.cpp

@@ -3360,13 +3360,13 @@ namespace {
           // declaration.
           if (auto imported = VisitObjCMethodDecl(decl, dc,
                                                   /*forceClassMethod=*/true))
-            Impl.setAlternateDecl(result, cast<ValueDecl>(imported));
+            Impl.addAlternateDecl(result, cast<ValueDecl>(imported));
         } else if (auto factory = importFactoryMethodAsConstructor(
                                     result, decl, selector, dc)) {
           // We imported the factory method as an initializer, so
           // record it as an alternate declaration.
           if (*factory)
-            Impl.setAlternateDecl(result, *factory);
+            Impl.addAlternateDecl(result, *factory);
         }
 
       }
@@ -4971,7 +4971,7 @@ SwiftDeclConverter::getImplicitProperty(ImportedName importedName,
                          SourceLoc());
 
   // Make the property the alternate declaration for the getter.
-  Impl.setAlternateDecl(swiftGetter, property);
+  Impl.addAlternateDecl(swiftGetter, property);
 
   return property;
 }
@@ -5021,7 +5021,7 @@ SwiftDeclConverter::importFactoryMethodAsConstructor(
   /// Record the initializer as an alternative declaration for the
   /// member.
   if (result) {
-    Impl.setAlternateDecl(member, result);
+    Impl.addAlternateDecl(member, result);
 
     if (swift3Name)
       markAsSwift2Variant(result, *swift3Name);
@@ -5736,7 +5736,7 @@ SwiftDeclConverter::importSubscript(Decl *decl,
       TypeLoc::withoutLoc(elementContextTy), dc);
 
   /// Record the subscript as an alternative declaration.
-  Impl.setAlternateDecl(associateWithSetter ? setter : getter, subscript);
+  Impl.addAlternateDecl(associateWithSetter ? setter : getter, subscript);
 
   subscript->makeComputed(SourceLoc(), getterThunk, setterThunk, nullptr,
                           SourceLoc());
@@ -5934,8 +5934,8 @@ void SwiftDeclConverter::importObjCMembers(
       continue;
 
     if (auto objcMethod = dyn_cast<clang::ObjCMethodDecl>(nd)) {
-      // If there is an alternate declaration for this member, add it.
-      if (auto alternate = Impl.getAlternateDecl(member)) {
+      // If there is are alternate declarations for this member, add it.
+      for (auto alternate : Impl.getAlternateDecls(member)) {
         if (alternate->getDeclContext() == member->getDeclContext() &&
             knownMembers.insert(alternate).second)
           members.push_back(alternate);
@@ -6063,7 +6063,7 @@ void SwiftDeclConverter::importMirroredProtocolMembers(
               Impl.importMirroredDecl(objcMethod, dc, useSwift2Name, proto)) {
         members.push_back(imported);
 
-        if (auto alternate = Impl.getAlternateDecl(imported))
+        for (auto alternate : Impl.getAlternateDecls(imported))
           if (imported->getDeclContext() == alternate->getDeclContext())
             members.push_back(alternate);
       }
@@ -6542,7 +6542,7 @@ ClangImporter::Implementation::importDeclImpl(const clang::NamedDecl *ClangDecl,
   if (Result) {
     finalizeDecl(Result);
 
-    if (auto alternate = getAlternateDecl(Result))
+    for (auto alternate : getAlternateDecls(Result))
       finalizeDecl(alternate);
   }
 
@@ -6793,7 +6793,7 @@ ClangImporter::Implementation::importMirroredDecl(const clang::NamedDecl *decl,
     updateMirroredDecl(result);
 
     // Update the alternate declaration as well.
-    if (auto alternate = getAlternateDecl(result))
+    for (auto alternate : getAlternateDecls(result))
       updateMirroredDecl(alternate);
   }
   if (result || !converter.hadForwardDeclaration())
@@ -7193,7 +7193,7 @@ ClangImporter::Implementation::loadAllMembers(Decl *D, uint64_t extra) {
       // Add the member.
       ext->addMember(member);
 
-      if (auto alternate = getAlternateDecl(member)) {
+      for (auto alternate : getAlternateDecls(member)) {
         ext->addMember(alternate);
       }
 

lib/ClangImporter/ImporterImpl.h

@@ -430,7 +430,7 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
   /// A mapping from imported declarations to their "alternate" declarations,
   /// for cases where a single Clang declaration is imported to two
   /// different Swift declarations.
-  llvm::DenseMap<Decl *, ValueDecl *> AlternateDecls;
+  llvm::DenseMap<Decl *, TinyPtrVector<ValueDecl *>> AlternateDecls;
 
 public:
   /// \brief Keep track of enum constant values that have been imported.
@@ -447,18 +447,19 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
 
   /// Retrieve the alternative declaration for the given imported
   /// Swift declaration.
-  ValueDecl *getAlternateDecl(Decl *decl) {
+  TinyPtrVector<ValueDecl *> getAlternateDecls(Decl *decl) {
     auto known = AlternateDecls.find(decl);
-    if (known == AlternateDecls.end()) return nullptr;
+    if (known == AlternateDecls.end()) return {};
     return known->second;
   }
 
-  /// Set the alternative decl
-  void setAlternateDecl(Decl *forDecl, ValueDecl *altDecl) {
-    assert((!AlternateDecls.count(forDecl) ||
-            AlternateDecls[forDecl] == altDecl) &&
-           "clobbering already-set alternative");
-    AlternateDecls[forDecl] = altDecl;
+  /// Add an alternative decl
+  void addAlternateDecl(Decl *forDecl, ValueDecl *altDecl) {
+    auto &vec = AlternateDecls[forDecl];
+    for (auto alt : vec)
+      if (alt == altDecl)
+        return;
+    vec.push_back(altDecl);
   }
 
 private: