test for and fix microsoft#6043

Author: weswigham

src/compiler/checker.ts

@@ -14310,8 +14310,9 @@ namespace ts {
                         continue;
                     }
                     const { declarations, flags } = exports[id];
+                    let declarationLength = declarations.length;
                     // ECMA262: 15.2.1.1 It is a Syntax Error if the ExportedNames of ModuleItemList contains any duplicate entries. (TS Exceptions: namespaces, function overloads, enums, and interfaces)
-                    if (!(flags & (SymbolFlags.Namespace | SymbolFlags.Interface | SymbolFlags.Enum)) && declarations.length > 1) {
+                    if (!(flags & (SymbolFlags.Namespace | SymbolFlags.Interface | SymbolFlags.Enum)) && (flags & SymbolFlags.TypeAlias ? declarationLength-- : true) && declarationLength > 1) {
                         const exportedDeclarations: Declaration[] = filter(declarations, isNotOverload);
                         if (exportedDeclarations.length > 1) {
                             for (const declaration of exportedDeclarations) {

tests/baselines/reference/typeAliasExport.js

@@ -0,0 +1,8 @@
+//// [typeAliasExport.ts]
+declare module "a" {
+  export default 0
+  export var a;
+  export type a = typeof a;
+}
+
+//// [typeAliasExport.js]

tests/baselines/reference/typeAliasExport.symbols

@@ -0,0 +1,10 @@
+=== tests/cases/compiler/typeAliasExport.ts ===
+declare module "a" {
+  export default 0
+  export var a;
+>a : Symbol(a, Decl(typeAliasExport.ts, 2, 12), Decl(typeAliasExport.ts, 2, 15))
+
+  export type a = typeof a;
+>a : Symbol(a, Decl(typeAliasExport.ts, 2, 12), Decl(typeAliasExport.ts, 2, 15))
+>a : Symbol(a, Decl(typeAliasExport.ts, 2, 12), Decl(typeAliasExport.ts, 2, 15))
+}

tests/baselines/reference/typeAliasExport.types

@@ -0,0 +1,10 @@
+=== tests/cases/compiler/typeAliasExport.ts ===
+declare module "a" {
+  export default 0
+  export var a;
+>a : any
+
+  export type a = typeof a;
+>a : any
+>a : any
+}

tests/cases/compiler/typeAliasExport.ts

@@ -0,0 +1,5 @@
+declare module "a" {
+  export default 0
+  export var a;
+  export type a = typeof a;
+}