Revert "Convert Declaration to union, not type hierarchy"

This reverts commit 573040f.

Author: sandersn

src/compiler/binder.ts

@@ -304,7 +304,7 @@ namespace ts {
         }
 
         function getDisplayName(node: Declaration): string {
-            return (node as DeclarationBase).name ? declarationNameToString((node as DeclarationBase).name) : getDeclarationName(node);
+            return (node as RealDeclaration).name ? declarationNameToString((node as RealDeclaration).name) : getDeclarationName(node);
         }
 
         /**
@@ -367,8 +367,8 @@ namespace ts {
                         symbolTable.set(name, symbol = createSymbol(SymbolFlags.None, name));
                     }
                     else {
-                        if ((node as DeclarationBase).name) {
-                            (node as DeclarationBase).name.parent = node;
+                        if ((node as RealDeclaration).name) {
+                            (node as RealDeclaration).name.parent = node;
                         }
 
                         // Report errors every position with duplicate declaration

src/compiler/checker.ts

@@ -3617,7 +3617,7 @@ namespace ts {
             });
         }
 
-        function isDeclarationVisible(node: Node): boolean {
+        function isDeclarationVisible(node: Declaration): boolean {
             if (node) {
                 const links = getNodeLinks(node);
                 if (links.isVisible === undefined) {
@@ -3631,10 +3631,10 @@ namespace ts {
             function determineIfDeclarationIsVisible() {
                 switch (node.kind) {
                     case SyntaxKind.BindingElement:
-                        return isDeclarationVisible(node.parent.parent);
+                        return isDeclarationVisible(<Declaration>node.parent.parent);
                     case SyntaxKind.VariableDeclaration:
-                        const declaration = node as VariableDeclaration;
-                        if (isBindingPattern(declaration.name) && !declaration.name.elements.length) {
+                        if (isBindingPattern(node.name) &&
+                            !(<BindingPattern>node.name).elements.length) {
                             // If the binding pattern is empty, this variable declaration is not visible
                             return false;
                         }
@@ -3657,7 +3657,7 @@ namespace ts {
                             return isGlobalSourceFile(parent);
                         }
                         // Exported members/ambient module elements (exception import declaration) are visible if parent is visible
-                        return isDeclarationVisible(parent);
+                        return isDeclarationVisible(<Declaration>parent);
 
                     case SyntaxKind.PropertyDeclaration:
                     case SyntaxKind.PropertySignature:
@@ -3687,7 +3687,7 @@ namespace ts {
                     case SyntaxKind.UnionType:
                     case SyntaxKind.IntersectionType:
                     case SyntaxKind.ParenthesizedType:
-                        return isDeclarationVisible(node.parent);
+                        return isDeclarationVisible(<Declaration>node.parent);
 
                     // Default binding, import specifier and namespace import is visible
                     // only on demand so by default it is not visible
@@ -6235,8 +6235,8 @@ namespace ts {
                     case SyntaxKind.MethodDeclaration:
                     case SyntaxKind.GetAccessor:
                     case SyntaxKind.SetAccessor:
-                        return (<DeclarationBase>node).name.kind === SyntaxKind.ComputedPropertyName
-                            && traverse((<DeclarationBase>node).name);
+                        return (<RealDeclaration>node).name.kind === SyntaxKind.ComputedPropertyName
+                            && traverse((<RealDeclaration>node).name);
 
                     default:
                         return !nodeStartsNewLexicalEnvironment(node) && !isPartOfTypeNode(node) && forEachChild(node, traverse);
@@ -21832,7 +21832,7 @@ namespace ts {
         function isTypeDeclarationName(name: Node): boolean {
             return name.kind === SyntaxKind.Identifier &&
                 isTypeDeclaration(name.parent) &&
-                (<TypeElement>name.parent).name === name;
+                (<RealDeclaration>name.parent).name === name;
         }
 
         function isTypeDeclaration(node: Node): boolean {
@@ -22460,7 +22460,7 @@ namespace ts {
 
         // Return true if the given node is a declaration of a nested block scoped entity with a name that either hides an
         // existing name or might hide a name when compiled downlevel
-        function isDeclarationWithCollidingName(node: Node): boolean {
+        function isDeclarationWithCollidingName(node: Declaration): boolean {
             node = getParseTreeNode(node, isDeclaration);
             if (node) {
                 const symbol = getSymbolOfNode(node);

src/compiler/transformers/es2015.ts

@@ -3748,7 +3748,7 @@ namespace ts {
                 case SyntaxKind.ClassDeclaration:
                 case SyntaxKind.EnumDeclaration:
                 case SyntaxKind.VariableDeclaration:
-                    return (<DeclarationBase>parent).name === node
+                    return (<RealDeclaration>parent).name === node
                         && resolver.isDeclarationWithCollidingName(<Declaration>parent);
             }