diff --git a/tests/baselines/reference/augmentedTypesClass2.errors.txt b/tests/baselines/reference/augmentedTypesClass2.errors.txt
index 5881e49dae..2622429d74 100644
--- a/tests/baselines/reference/augmentedTypesClass2.errors.txt
+++ b/tests/baselines/reference/augmentedTypesClass2.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/augmentedTypesClass2.ts(4,7): error TS2300: Duplicate identifier 'c11'.
-tests/cases/compiler/augmentedTypesClass2.ts(10,11): error TS2300: Duplicate identifier 'c11'.
+tests/cases/compiler/augmentedTypesClass2.ts(4,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/augmentedTypesClass2.ts(10,11): error TS2507: An interface cannot merge with a non-ambient class.
 tests/cases/compiler/augmentedTypesClass2.ts(16,7): error TS2300: Duplicate identifier 'c33'.
 tests/cases/compiler/augmentedTypesClass2.ts(21,6): error TS2300: Duplicate identifier 'c33'.
 
@@ -10,7 +10,7 @@ tests/cases/compiler/augmentedTypesClass2.ts(21,6): error TS2300: Duplicate iden
     // class then interface
     class c11 { // error
           ~~~
-!!! error TS2300: Duplicate identifier 'c11'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
         foo() {
             return 1;
         }
@@ -18,7 +18,7 @@ tests/cases/compiler/augmentedTypesClass2.ts(21,6): error TS2300: Duplicate iden
     
     interface c11 { // error
               ~~~
-!!! error TS2300: Duplicate identifier 'c11'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
         bar(): void;
     }
     
diff --git a/tests/baselines/reference/augmentedTypesInterface.errors.txt b/tests/baselines/reference/augmentedTypesInterface.errors.txt
index a9fc28dd5a..9719539b34 100644
--- a/tests/baselines/reference/augmentedTypesInterface.errors.txt
+++ b/tests/baselines/reference/augmentedTypesInterface.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/augmentedTypesInterface.ts(12,11): error TS2300: Duplicate identifier 'i2'.
-tests/cases/compiler/augmentedTypesInterface.ts(16,7): error TS2300: Duplicate identifier 'i2'.
+tests/cases/compiler/augmentedTypesInterface.ts(12,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/augmentedTypesInterface.ts(16,7): error TS2506: A non-ambient class cannot be merged with an interface.
 tests/cases/compiler/augmentedTypesInterface.ts(23,11): error TS2300: Duplicate identifier 'i3'.
 tests/cases/compiler/augmentedTypesInterface.ts(26,6): error TS2300: Duplicate identifier 'i3'.
 
@@ -18,13 +18,13 @@ tests/cases/compiler/augmentedTypesInterface.ts(26,6): error TS2300: Duplicate i
     // interface then class
     interface i2 { // error
               ~~
-!!! error TS2300: Duplicate identifier 'i2'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
         foo(): void;
     }
     
     class i2 {  // error
           ~~
-!!! error TS2300: Duplicate identifier 'i2'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
         bar() {
             return 1;
         }
diff --git a/tests/baselines/reference/class1.errors.txt b/tests/baselines/reference/class1.errors.txt
deleted file mode 100644
index b90fae9cb4..0000000000
--- a/tests/baselines/reference/class1.errors.txt
+++ /dev/null
@@ -1,11 +0,0 @@
-tests/cases/compiler/class1.ts(1,11): error TS2300: Duplicate identifier 'foo'.
-tests/cases/compiler/class1.ts(2,7): error TS2300: Duplicate identifier 'foo'.
-
-
-==== tests/cases/compiler/class1.ts (2 errors) ====
-    interface foo{ } // error
-              ~~~
-!!! error TS2300: Duplicate identifier 'foo'.
-    class foo{ } // error
-          ~~~
-!!! error TS2300: Duplicate identifier 'foo'.
\ No newline at end of file
diff --git a/tests/baselines/reference/class1.js b/tests/baselines/reference/class1.js
deleted file mode 100644
index 7a104e237b..0000000000
--- a/tests/baselines/reference/class1.js
+++ /dev/null
@@ -1,10 +0,0 @@
-//// [class1.ts]
-interface foo{ } // error
-class foo{ } // error
-
-//// [class1.js]
-var foo = (function () {
-    function foo() {
-    }
-    return foo;
-})(); // error
diff --git a/tests/baselines/reference/classAndInterface1.errors.txt b/tests/baselines/reference/classAndInterface1.errors.txt
deleted file mode 100644
index 8d31e847d7..0000000000
--- a/tests/baselines/reference/classAndInterface1.errors.txt
+++ /dev/null
@@ -1,11 +0,0 @@
-tests/cases/compiler/classAndInterface1.ts(1,8): error TS2300: Duplicate identifier 'cli'.
-tests/cases/compiler/classAndInterface1.ts(2,11): error TS2300: Duplicate identifier 'cli'.
-
-
-==== tests/cases/compiler/classAndInterface1.ts (2 errors) ====
-     class cli { } // error
-           ~~~
-!!! error TS2300: Duplicate identifier 'cli'.
-    interface cli { } // error
-              ~~~
-!!! error TS2300: Duplicate identifier 'cli'.
\ No newline at end of file
diff --git a/tests/baselines/reference/classAndInterface1.js b/tests/baselines/reference/classAndInterface1.js
deleted file mode 100644
index 5efabded83..0000000000
--- a/tests/baselines/reference/classAndInterface1.js
+++ /dev/null
@@ -1,10 +0,0 @@
-//// [classAndInterface1.ts]
- class cli { } // error
-interface cli { } // error
-
-//// [classAndInterface1.js]
-var cli = (function () {
-    function cli() {
-    }
-    return cli;
-})(); // error
diff --git a/tests/baselines/reference/classAndInterfaceMerge.d.errors.txt b/tests/baselines/reference/classAndInterfaceMerge.d.errors.txt
new file mode 100644
index 0000000000..d4afe3bc7c
--- /dev/null
+++ b/tests/baselines/reference/classAndInterfaceMerge.d.errors.txt
@@ -0,0 +1,46 @@
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts(12,15): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts(14,11): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts(16,15): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts(18,15): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts(20,18): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts(24,22): error TS2507: An interface cannot merge with a non-ambient class.
+
+
+==== tests/cases/conformance/classes/classDeclarations/classAndInterfaceMerge.d.ts (6 errors) ====
+    
+    interface C { }
+    
+    declare class C { }
+    
+    interface C { }
+    
+    interface C { }
+    
+    declare module M {
+        
+        interface C1 { }
+                  ~~
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+    
+        class C1 { }
+              ~~
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
+    
+        interface C1 { }
+                  ~~
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+    
+        interface C1 { }
+                  ~~
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+        
+        export class C2 { }
+                     ~~
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
+    }
+    
+    declare module M {
+        export interface C2 { }
+                         ~~
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/classAndInterfaceWithSameName.errors.txt b/tests/baselines/reference/classAndInterfaceWithSameName.errors.txt
index aa3e687cf4..6bf95880ce 100644
--- a/tests/baselines/reference/classAndInterfaceWithSameName.errors.txt
+++ b/tests/baselines/reference/classAndInterfaceWithSameName.errors.txt
@@ -1,27 +1,39 @@
-tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(1,7): error TS2300: Duplicate identifier 'C'.
-tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(2,11): error TS2300: Duplicate identifier 'C'.
-tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(5,11): error TS2300: Duplicate identifier 'D'.
-tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(9,15): error TS2300: Duplicate identifier 'D'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(1,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(1,11): error TS2300: Duplicate identifier 'foo'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(2,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(2,15): error TS2300: Duplicate identifier 'foo'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(5,11): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(6,9): error TS2300: Duplicate identifier 'bar'.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(9,15): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts(10,9): error TS2300: Duplicate identifier 'bar'.
 
 
-==== tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts (4 errors) ====
+==== tests/cases/conformance/classes/classDeclarations/classAndInterfaceWithSameName.ts (8 errors) ====
     class C { foo: string; }
           ~
-!!! error TS2300: Duplicate identifier 'C'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
+              ~~~
+!!! error TS2300: Duplicate identifier 'foo'.
     interface C { foo: string; } // error
               ~
-!!! error TS2300: Duplicate identifier 'C'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+                  ~~~
+!!! error TS2300: Duplicate identifier 'foo'.
     
     module M {
         class D {
               ~
-!!! error TS2300: Duplicate identifier 'D'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
             bar: string;
+            ~~~
+!!! error TS2300: Duplicate identifier 'bar'.
         }
     
         interface D { // error
                   ~
-!!! error TS2300: Duplicate identifier 'D'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
             bar: string;
+            ~~~
+!!! error TS2300: Duplicate identifier 'bar'.
         }
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/clinterfaces.errors.txt b/tests/baselines/reference/clinterfaces.errors.txt
index d5eab2c799..6e172fa0d9 100644
--- a/tests/baselines/reference/clinterfaces.errors.txt
+++ b/tests/baselines/reference/clinterfaces.errors.txt
@@ -1,50 +1,50 @@
-tests/cases/compiler/clinterfaces.ts(2,11): error TS2300: Duplicate identifier 'C'.
-tests/cases/compiler/clinterfaces.ts(3,15): error TS2300: Duplicate identifier 'C'.
-tests/cases/compiler/clinterfaces.ts(4,15): error TS2300: Duplicate identifier 'D'.
-tests/cases/compiler/clinterfaces.ts(5,11): error TS2300: Duplicate identifier 'D'.
-tests/cases/compiler/clinterfaces.ts(8,11): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/clinterfaces.ts(12,7): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/clinterfaces.ts(16,7): error TS2300: Duplicate identifier 'Bar'.
-tests/cases/compiler/clinterfaces.ts(20,11): error TS2300: Duplicate identifier 'Bar'.
+tests/cases/compiler/clinterfaces.ts(2,11): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(3,15): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/clinterfaces.ts(4,15): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/clinterfaces.ts(5,11): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(8,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/clinterfaces.ts(12,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(16,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/clinterfaces.ts(20,11): error TS2507: An interface cannot merge with a non-ambient class.
 
 
 ==== tests/cases/compiler/clinterfaces.ts (8 errors) ====
     module M {
         class C { }
               ~
-!!! error TS2300: Duplicate identifier 'C'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
         interface C { }
                   ~
-!!! error TS2300: Duplicate identifier 'C'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
         interface D { }
                   ~
-!!! error TS2300: Duplicate identifier 'D'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
         class D { }
               ~
-!!! error TS2300: Duplicate identifier 'D'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
     }
     
     interface Foo<T> {
               ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
         a: string;
     }
     
     class Foo<T>{
           ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
         b: number;
     }
     
     class Bar<T>{
           ~~~
-!!! error TS2300: Duplicate identifier 'Bar'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
         b: number;
     }
     
     interface Bar<T> {
               ~~~
-!!! error TS2300: Duplicate identifier 'Bar'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
         a: string;
     }
     
diff --git a/tests/baselines/reference/clinterfaces.js b/tests/baselines/reference/clinterfaces.js
index 02ae734986..a55924227c 100644
--- a/tests/baselines/reference/clinterfaces.js
+++ b/tests/baselines/reference/clinterfaces.js
@@ -49,3 +49,4 @@ var Bar = (function () {
     }
     return Bar;
 })();
+module.exports = Foo;
diff --git a/tests/baselines/reference/declInput.errors.txt b/tests/baselines/reference/declInput.errors.txt
index 8ec341ce15..313866cf93 100644
--- a/tests/baselines/reference/declInput.errors.txt
+++ b/tests/baselines/reference/declInput.errors.txt
@@ -1,17 +1,17 @@
-tests/cases/compiler/declInput.ts(1,11): error TS2300: Duplicate identifier 'bar'.
-tests/cases/compiler/declInput.ts(5,7): error TS2300: Duplicate identifier 'bar'.
+tests/cases/compiler/declInput.ts(1,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/declInput.ts(5,7): error TS2506: A non-ambient class cannot be merged with an interface.
 
 
 ==== tests/cases/compiler/declInput.ts (2 errors) ====
     interface bar {
               ~~~
-!!! error TS2300: Duplicate identifier 'bar'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
     
     }
     
     class bar {
           ~~~
-!!! error TS2300: Duplicate identifier 'bar'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
       public f() { return ''; }
       public g() { return {a: <bar>null, b: undefined, c: void 4 }; }
       public h(x = 4, y = null, z = '') { x++; }
diff --git a/tests/baselines/reference/declaredClassMergedwithSelf.errors.txt b/tests/baselines/reference/declaredClassMergedwithSelf.errors.txt
index 8a783639c1..a46dfb0d33 100644
--- a/tests/baselines/reference/declaredClassMergedwithSelf.errors.txt
+++ b/tests/baselines/reference/declaredClassMergedwithSelf.errors.txt
@@ -1,11 +1,15 @@
-tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts(1,15): error TS2300: Duplicate identifier 'C1'.
-tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts(3,15): error TS2300: Duplicate identifier 'C1'.
-tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts(5,15): error TS2300: Duplicate identifier 'C2'.
-tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts(7,11): error TS2300: Duplicate identifier 'C2'.
-tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts(9,15): error TS2300: Duplicate identifier 'C2'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(3,15): error TS2300: Duplicate identifier 'C1'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(5,15): error TS2300: Duplicate identifier 'C1'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(7,15): error TS2300: Duplicate identifier 'C2'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(9,11): error TS2300: Duplicate identifier 'C2'.
+tests/cases/conformance/classes/classDeclarations/file1.ts(11,15): error TS2300: Duplicate identifier 'C2'.
+tests/cases/conformance/classes/classDeclarations/file2.ts(2,15): error TS2300: Duplicate identifier 'C3'.
+tests/cases/conformance/classes/classDeclarations/file3.ts(2,15): error TS2300: Duplicate identifier 'C3'.
 
 
-==== tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts (5 errors) ====
+==== tests/cases/conformance/classes/classDeclarations/file1.ts (5 errors) ====
+    
+    
     declare class C1 {}
                   ~~
 !!! error TS2300: Duplicate identifier 'C1'.
@@ -24,4 +28,16 @@ tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts
     
     declare class C2 {}
                   ~~
-!!! error TS2300: Duplicate identifier 'C2'.
\ No newline at end of file
+!!! error TS2300: Duplicate identifier 'C2'.
+    
+==== tests/cases/conformance/classes/classDeclarations/file2.ts (1 errors) ====
+    
+    declare class C3 { }
+                  ~~
+!!! error TS2300: Duplicate identifier 'C3'.
+    
+==== tests/cases/conformance/classes/classDeclarations/file3.ts (1 errors) ====
+    
+    declare class C3 { }
+                  ~~
+!!! error TS2300: Duplicate identifier 'C3'.
\ No newline at end of file
diff --git a/tests/baselines/reference/declaredClassMergedwithSelf.js b/tests/baselines/reference/declaredClassMergedwithSelf.js
index 85213668ad..a38391d90d 100644
--- a/tests/baselines/reference/declaredClassMergedwithSelf.js
+++ b/tests/baselines/reference/declaredClassMergedwithSelf.js
@@ -1,4 +1,8 @@
-//// [declaredClassMergedwithSelf.ts]
+//// [tests/cases/conformance/classes/classDeclarations/declaredClassMergedwithSelf.ts] ////
+
+//// [file1.ts]
+
+
 declare class C1 {}
 
 declare class C1 {}
@@ -7,6 +11,16 @@ declare class C2 {}
 
 interface C2 {}
 
-declare class C2 {}
+declare class C2 {}
 
-//// [declaredClassMergedwithSelf.js]
+//// [file2.ts]
+
+declare class C3 { }
+
+//// [file3.ts]
+
+declare class C3 { }
+
+//// [file1.js]
+//// [file2.js]
+//// [file3.js]
diff --git a/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt b/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
index 6c80d13b2f..77a7f4ffb3 100644
--- a/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
+++ b/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(2,22): error TS2300: Duplicate identifier 'I'.
-tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(5,18): error TS2300: Duplicate identifier 'I'.
+tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(2,22): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(5,18): error TS2506: A non-ambient class cannot be merged with an interface.
 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(9,21): error TS2300: Duplicate identifier 'f'.
 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(12,18): error TS2300: Duplicate identifier 'f'.
 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(37,12): error TS2300: Duplicate identifier 'x'.
@@ -10,12 +10,12 @@ tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(41,16): er
     module M {
         export interface I { }
                          ~
-!!! error TS2300: Duplicate identifier 'I'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
     }
     module M {
         export class I { } // error
                      ~
-!!! error TS2300: Duplicate identifier 'I'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
     }
     
     module M {
diff --git a/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt
new file mode 100644
index 0000000000..0febfd77ec
--- /dev/null
+++ b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt
@@ -0,0 +1,67 @@
+tests/cases/compiler/file1.ts(1,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/file1.ts(2,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/file1.ts(3,7): error TS2300: Duplicate identifier 'C2'.
+tests/cases/compiler/file1.ts(4,10): error TS2300: Duplicate identifier 'f'.
+tests/cases/compiler/file1.ts(8,12): error TS2300: Duplicate identifier 'x'.
+tests/cases/compiler/file2.ts(1,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/file2.ts(2,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/file2.ts(3,10): error TS2300: Duplicate identifier 'C2'.
+tests/cases/compiler/file2.ts(4,7): error TS2300: Duplicate identifier 'f'.
+tests/cases/compiler/file2.ts(7,8): error TS2433: A namespace declaration cannot be in a different file from a class or function with which it is merged
+tests/cases/compiler/file2.ts(8,16): error TS2300: Duplicate identifier 'x'.
+
+
+==== tests/cases/compiler/file1.ts (5 errors) ====
+    interface I { }
+              ~
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+    class C1 { }
+          ~~
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
+    class C2 { }
+          ~~
+!!! error TS2300: Duplicate identifier 'C2'.
+    function f() { }
+             ~
+!!! error TS2300: Duplicate identifier 'f'.
+    var v = 3;
+    
+    class Foo {
+        static x: number;
+               ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    module N {
+        export module F {
+            var t;
+        }
+    }
+    
+==== tests/cases/compiler/file2.ts (6 errors) ====
+    class I { } // error -- cannot merge interface with non-ambient class
+          ~
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
+    interface C1 { } // error -- cannot merge interface with non-ambient class
+              ~~
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+    function C2() { } // error -- cannot merge function with non-ambient class
+             ~~
+!!! error TS2300: Duplicate identifier 'C2'.
+    class f { } // error -- cannot merge function with non-ambient class
+          ~
+!!! error TS2300: Duplicate identifier 'f'.
+    var v = 3;
+    
+    module Foo {
+           ~~~
+!!! error TS2433: A namespace declaration cannot be in a different file from a class or function with which it is merged
+        export var x: number; // error for redeclaring var in a different parent
+                   ~
+!!! error TS2300: Duplicate identifier 'x'.
+    }
+    
+    declare module N {
+        export function F(); // no error because function is ambient
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.js b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.js
new file mode 100644
index 0000000000..f6e3a5652e
--- /dev/null
+++ b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.js
@@ -0,0 +1,76 @@
+//// [tests/cases/compiler/duplicateIdentifiersAcrossFileBoundaries.ts] ////
+
+//// [file1.ts]
+interface I { }
+class C1 { }
+class C2 { }
+function f() { }
+var v = 3;
+
+class Foo {
+    static x: number;
+}
+
+module N {
+    export module F {
+        var t;
+    }
+}
+
+//// [file2.ts]
+class I { } // error -- cannot merge interface with non-ambient class
+interface C1 { } // error -- cannot merge interface with non-ambient class
+function C2() { } // error -- cannot merge function with non-ambient class
+class f { } // error -- cannot merge function with non-ambient class
+var v = 3;
+
+module Foo {
+    export var x: number; // error for redeclaring var in a different parent
+}
+
+declare module N {
+    export function F(); // no error because function is ambient
+}
+
+
+//// [file1.js]
+var C1 = (function () {
+    function C1() {
+    }
+    return C1;
+})();
+var C2 = (function () {
+    function C2() {
+    }
+    return C2;
+})();
+function f() { }
+var v = 3;
+var Foo = (function () {
+    function Foo() {
+    }
+    return Foo;
+})();
+var N;
+(function (N) {
+    var F;
+    (function (F) {
+        var t;
+    })(F = N.F || (N.F = {}));
+})(N || (N = {}));
+//// [file2.js]
+var I = (function () {
+    function I() {
+    }
+    return I;
+})(); // error -- cannot merge interface with non-ambient class
+function C2() { } // error -- cannot merge function with non-ambient class
+var f = (function () {
+    function f() {
+    }
+    return f;
+})(); // error -- cannot merge function with non-ambient class
+var v = 3;
+var Foo;
+(function (Foo) {
+})(Foo || (Foo = {}));
diff --git a/tests/baselines/reference/interfaceDeclaration2.errors.txt b/tests/baselines/reference/interfaceDeclaration2.errors.txt
index 9739d946bc..06a0e24730 100644
--- a/tests/baselines/reference/interfaceDeclaration2.errors.txt
+++ b/tests/baselines/reference/interfaceDeclaration2.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/interfaceDeclaration2.ts(4,11): error TS2300: Duplicate identifier 'I2'.
-tests/cases/compiler/interfaceDeclaration2.ts(5,7): error TS2300: Duplicate identifier 'I2'.
+tests/cases/compiler/interfaceDeclaration2.ts(4,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/interfaceDeclaration2.ts(5,7): error TS2506: A non-ambient class cannot be merged with an interface.
 
 
 ==== tests/cases/compiler/interfaceDeclaration2.ts (2 errors) ====
@@ -8,10 +8,10 @@ tests/cases/compiler/interfaceDeclaration2.ts(5,7): error TS2300: Duplicate iden
     
     interface I2 { }
               ~~
-!!! error TS2300: Duplicate identifier 'I2'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
     class I2 { }
           ~~
-!!! error TS2300: Duplicate identifier 'I2'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
     
     interface I3 { }
     function I3() { }
diff --git a/tests/baselines/reference/mergedClassInterface.errors.txt b/tests/baselines/reference/mergedClassInterface.errors.txt
new file mode 100644
index 0000000000..42f0a14b8e
--- /dev/null
+++ b/tests/baselines/reference/mergedClassInterface.errors.txt
@@ -0,0 +1,67 @@
+tests/cases/conformance/classes/classDeclarations/file1.ts(11,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/file1.ts(13,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/conformance/classes/classDeclarations/file1.ts(15,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/conformance/classes/classDeclarations/file1.ts(17,7): error TS2506: A non-ambient class cannot be merged with an interface.
+
+
+==== tests/cases/conformance/classes/classDeclarations/file1.ts (4 errors) ====
+    
+    
+    declare class C1 { }
+    
+    interface C1 { }
+    
+    interface C2 { }
+    
+    declare class C2 { }
+    
+    class C3 { } // error -- cannot merge non-ambient class and interface
+          ~~
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
+    
+    interface C3 { } // error -- cannot merge non-ambient class and interface
+              ~~
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+    
+    interface C4 { } // error -- cannot merge non-ambient class and interface
+              ~~
+!!! error TS2507: An interface cannot merge with a non-ambient class.
+    
+    class C4 { } // error -- cannot merge non-ambient class and interface
+          ~~
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
+    
+    interface C5 {
+        x1: number;
+    }
+    
+    declare class C5 {
+        x2: number;
+    }
+    
+    interface C5 {
+        x3: number;
+    }
+    
+    interface C5 {
+        x4: number;
+    }
+    
+    // checks if properties actually were merged
+    var c5 : C5;
+    c5.x1;
+    c5.x2;
+    c5.x3;
+    c5.x4;
+    
+==== tests/cases/conformance/classes/classDeclarations/file2.ts (0 errors) ====
+    
+    declare class C6 { }
+    
+    interface C7 { }
+    
+==== tests/cases/conformance/classes/classDeclarations/file3.ts (0 errors) ====
+    
+    interface C6 { }
+    
+    declare class C7 { }
\ No newline at end of file
diff --git a/tests/baselines/reference/mergedClassInterface.js b/tests/baselines/reference/mergedClassInterface.js
index 70c8b7d2b2..7176255655 100644
--- a/tests/baselines/reference/mergedClassInterface.js
+++ b/tests/baselines/reference/mergedClassInterface.js
@@ -1,14 +1,117 @@
-//// [mergedClassInterface.ts]
-declare class C1 {}
-
-interface C1 {}
-
-interface C2 {}
-
-declare class C2 {}
-
-interface C2 {}
-
-interface C2 {}
+//// [tests/cases/conformance/classes/classDeclarations/mergedClassInterface.ts] ////
 
-//// [mergedClassInterface.js]
+//// [file1.ts]
+
+
+declare class C1 { }
+
+interface C1 { }
+
+interface C2 { }
+
+declare class C2 { }
+
+class C3 { } // error -- cannot merge non-ambient class and interface
+
+interface C3 { } // error -- cannot merge non-ambient class and interface
+
+interface C4 { } // error -- cannot merge non-ambient class and interface
+
+class C4 { } // error -- cannot merge non-ambient class and interface
+
+interface C5 {
+    x1: number;
+}
+
+declare class C5 {
+    x2: number;
+}
+
+interface C5 {
+    x3: number;
+}
+
+interface C5 {
+    x4: number;
+}
+
+// checks if properties actually were merged
+var c5 : C5;
+c5.x1;
+c5.x2;
+c5.x3;
+c5.x4;
+
+//// [file2.ts]
+
+declare class C6 { }
+
+interface C7 { }
+
+//// [file3.ts]
+
+interface C6 { }
+
+declare class C7 { }
+
+//// [file1.js]
+var C3 = (function () {
+    function C3() {
+    }
+    return C3;
+})(); // error -- cannot merge non-ambient class and interface
+var C4 = (function () {
+    function C4() {
+    }
+    return C4;
+})(); // error -- cannot merge non-ambient class and interface
+// checks if properties actually were merged
+var c5;
+c5.x1;
+c5.x2;
+c5.x3;
+c5.x4;
+//// [file2.js]
+//// [file3.js]
+
+
+//// [file1.d.ts]
+declare class C1 {
+}
+interface C1 {
+}
+interface C2 {
+}
+declare class C2 {
+}
+declare class C3 {
+}
+interface C3 {
+}
+interface C4 {
+}
+declare class C4 {
+}
+interface C5 {
+    x1: number;
+}
+declare class C5 {
+    x2: number;
+}
+interface C5 {
+    x3: number;
+}
+interface C5 {
+    x4: number;
+}
+declare var c5: C5;
+//// [file2.d.ts]
+declare class C6 {
+}
+interface C7 {
+}
+//// [file3.d.ts]
+interface C6 {
+}
+declare class C7 {
+}
diff --git a/tests/baselines/reference/mergedClassInterface.symbols b/tests/baselines/reference/mergedClassInterface.symbols
deleted file mode 100644
index dc7ba89155..0000000000
--- a/tests/baselines/reference/mergedClassInterface.symbols
+++ /dev/null
@@ -1,19 +0,0 @@
-=== tests/cases/conformance/classes/classDeclarations/mergedClassInterface.ts ===
-declare class C1 {}
->C1 : Symbol(C1, Decl(mergedClassInterface.ts, 0, 0), Decl(mergedClassInterface.ts, 0, 19))
-
-interface C1 {}
->C1 : Symbol(C1, Decl(mergedClassInterface.ts, 0, 0), Decl(mergedClassInterface.ts, 0, 19))
-
-interface C2 {}
->C2 : Symbol(C2, Decl(mergedClassInterface.ts, 2, 15), Decl(mergedClassInterface.ts, 4, 15), Decl(mergedClassInterface.ts, 6, 19), Decl(mergedClassInterface.ts, 8, 15))
-
-declare class C2 {}
->C2 : Symbol(C2, Decl(mergedClassInterface.ts, 2, 15), Decl(mergedClassInterface.ts, 4, 15), Decl(mergedClassInterface.ts, 6, 19), Decl(mergedClassInterface.ts, 8, 15))
-
-interface C2 {}
->C2 : Symbol(C2, Decl(mergedClassInterface.ts, 2, 15), Decl(mergedClassInterface.ts, 4, 15), Decl(mergedClassInterface.ts, 6, 19), Decl(mergedClassInterface.ts, 8, 15))
-
-interface C2 {}
->C2 : Symbol(C2, Decl(mergedClassInterface.ts, 2, 15), Decl(mergedClassInterface.ts, 4, 15), Decl(mergedClassInterface.ts, 6, 19), Decl(mergedClassInterface.ts, 8, 15))
-
diff --git a/tests/baselines/reference/mergedClassInterface.types b/tests/baselines/reference/mergedClassInterface.types
deleted file mode 100644
index a31f147b73..0000000000
--- a/tests/baselines/reference/mergedClassInterface.types
+++ /dev/null
@@ -1,19 +0,0 @@
-=== tests/cases/conformance/classes/classDeclarations/mergedClassInterface.ts ===
-declare class C1 {}
->C1 : C1
-
-interface C1 {}
->C1 : C1
-
-interface C2 {}
->C2 : C2
-
-declare class C2 {}
->C2 : C2
-
-interface C2 {}
->C2 : C2
-
-interface C2 {}
->C2 : C2
-
diff --git a/tests/baselines/reference/nameCollisions.errors.txt b/tests/baselines/reference/nameCollisions.errors.txt
index 5eb7ea2cf8..d570d9e127 100644
--- a/tests/baselines/reference/nameCollisions.errors.txt
+++ b/tests/baselines/reference/nameCollisions.errors.txt
@@ -11,10 +11,10 @@ tests/cases/compiler/nameCollisions.ts(33,11): error TS2300: Duplicate identifie
 tests/cases/compiler/nameCollisions.ts(34,14): error TS2300: Duplicate identifier 'C'.
 tests/cases/compiler/nameCollisions.ts(36,14): error TS2300: Duplicate identifier 'C2'.
 tests/cases/compiler/nameCollisions.ts(37,11): error TS2300: Duplicate identifier 'C2'.
-tests/cases/compiler/nameCollisions.ts(42,11): error TS2300: Duplicate identifier 'cli'.
-tests/cases/compiler/nameCollisions.ts(43,15): error TS2300: Duplicate identifier 'cli'.
-tests/cases/compiler/nameCollisions.ts(45,15): error TS2300: Duplicate identifier 'cli2'.
-tests/cases/compiler/nameCollisions.ts(46,11): error TS2300: Duplicate identifier 'cli2'.
+tests/cases/compiler/nameCollisions.ts(42,11): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/nameCollisions.ts(43,15): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/nameCollisions.ts(45,15): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/nameCollisions.ts(46,11): error TS2506: A non-ambient class cannot be merged with an interface.
 
 
 ==== tests/cases/compiler/nameCollisions.ts (17 errors) ====
@@ -87,15 +87,15 @@ tests/cases/compiler/nameCollisions.ts(46,11): error TS2300: Duplicate identifie
     
         class cli { }
               ~~~
-!!! error TS2300: Duplicate identifier 'cli'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
         interface cli { } // error
                   ~~~
-!!! error TS2300: Duplicate identifier 'cli'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
     
         interface cli2 { }
                   ~~~~
-!!! error TS2300: Duplicate identifier 'cli2'.
+!!! error TS2507: An interface cannot merge with a non-ambient class.
         class cli2 { } // error
               ~~~~
-!!! error TS2300: Duplicate identifier 'cli2'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/templateStringsArrayTypeDefinedInES5Mode.errors.txt b/tests/baselines/reference/templateStringsArrayTypeDefinedInES5Mode.errors.txt
index 7c796d4739..9d6da3ec4b 100644
--- a/tests/baselines/reference/templateStringsArrayTypeDefinedInES5Mode.errors.txt
+++ b/tests/baselines/reference/templateStringsArrayTypeDefinedInES5Mode.errors.txt
@@ -1,14 +1,14 @@
-lib.d.ts(521,11): error TS2300: Duplicate identifier 'TemplateStringsArray'.
-tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(2,7): error TS2300: Duplicate identifier 'TemplateStringsArray'.
-tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(8,3): error TS2345: Argument of type '{ [x: number]: undefined; }' is not assignable to parameter of type 'TemplateStringsArray'.
-  Property 'raw' is missing in type '{ [x: number]: undefined; }'.
+lib.d.ts(521,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(2,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(8,3): error TS2345: Argument of type '{}' is not assignable to parameter of type 'TemplateStringsArray'.
+  Property 'raw' is missing in type '{}'.
 
 
 ==== tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts (2 errors) ====
     
     class TemplateStringsArray {
           ~~~~~~~~~~~~~~~~~~~~
-!!! error TS2300: Duplicate identifier 'TemplateStringsArray'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
     }
     
     function f(x: TemplateStringsArray, y: number, z: number) {
@@ -16,7 +16,7 @@ tests/cases/compiler/templateStringsArrayTypeDefinedInES5Mode.ts(8,3): error TS2
     
     f({}, 10, 10);
       ~~
-!!! error TS2345: Argument of type '{ [x: number]: undefined; }' is not assignable to parameter of type 'TemplateStringsArray'.
-!!! error TS2345:   Property 'raw' is missing in type '{ [x: number]: undefined; }'.
+!!! error TS2345: Argument of type '{}' is not assignable to parameter of type 'TemplateStringsArray'.
+!!! error TS2345:   Property 'raw' is missing in type '{}'.
     
     f `abcdef${ 1234 }${ 5678 }ghijkl`;
\ No newline at end of file
diff --git a/tests/baselines/reference/templateStringsArrayTypeRedefinedInES6Mode.errors.txt b/tests/baselines/reference/templateStringsArrayTypeRedefinedInES6Mode.errors.txt
index 4b9cb63d9a..ffb619cedc 100644
--- a/tests/baselines/reference/templateStringsArrayTypeRedefinedInES6Mode.errors.txt
+++ b/tests/baselines/reference/templateStringsArrayTypeRedefinedInES6Mode.errors.txt
@@ -1,14 +1,14 @@
-lib.d.ts(521,11): error TS2300: Duplicate identifier 'TemplateStringsArray'.
-tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(2,7): error TS2300: Duplicate identifier 'TemplateStringsArray'.
-tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(8,3): error TS2345: Argument of type '{ [x: number]: undefined; }' is not assignable to parameter of type 'TemplateStringsArray'.
-  Property 'raw' is missing in type '{ [x: number]: undefined; }'.
+lib.d.ts(521,11): error TS2507: An interface cannot merge with a non-ambient class.
+tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(2,7): error TS2506: A non-ambient class cannot be merged with an interface.
+tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(8,3): error TS2345: Argument of type '{}' is not assignable to parameter of type 'TemplateStringsArray'.
+  Property 'raw' is missing in type '{}'.
 
 
 ==== tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts (2 errors) ====
     
     class TemplateStringsArray {
           ~~~~~~~~~~~~~~~~~~~~
-!!! error TS2300: Duplicate identifier 'TemplateStringsArray'.
+!!! error TS2506: A non-ambient class cannot be merged with an interface.
     }
     
     function f(x: TemplateStringsArray, y: number, z: number) {
@@ -16,7 +16,7 @@ tests/cases/compiler/templateStringsArrayTypeRedefinedInES6Mode.ts(8,3): error T
     
     f({}, 10, 10);
       ~~
-!!! error TS2345: Argument of type '{ [x: number]: undefined; }' is not assignable to parameter of type 'TemplateStringsArray'.
-!!! error TS2345:   Property 'raw' is missing in type '{ [x: number]: undefined; }'.
+!!! error TS2345: Argument of type '{}' is not assignable to parameter of type 'TemplateStringsArray'.
+!!! error TS2345:   Property 'raw' is missing in type '{}'.
     
     f `abcdef${ 1234 }${ 5678 }ghijkl`;
\ No newline at end of file