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diff --git a/‎src/compiler/binder.ts
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+14-4Lines changed: 14 additions & 4 deletions b/‎src/compiler/binder.ts
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+14-4Lines changed: 14 additions & 4 deletions
diff --git a/‎src/compiler/checker.ts
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+17Lines changed: 17 additions & 0 deletions b/‎src/compiler/checker.ts
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+17Lines changed: 17 additions & 0 deletions
diff --git a/‎src/compiler/diagnosticInformationMap.generated.ts
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+1Lines changed: 1 addition & 0 deletions b/‎src/compiler/diagnosticInformationMap.generated.ts
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diff --git a/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions b/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions
diff --git a/‎src/compiler/types.ts
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+2-2Lines changed: 2 additions & 2 deletions b/‎src/compiler/types.ts
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+2-2Lines changed: 2 additions & 2 deletions
diff --git a/‎tests/baselines/reference/augmentedTypesClass2.errors.txt
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+4-4Lines changed: 4 additions & 4 deletions b/‎tests/baselines/reference/augmentedTypesClass2.errors.txt
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+4-4Lines changed: 4 additions & 4 deletions
diff --git a/‎tests/baselines/reference/augmentedTypesInterface.errors.txt
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+4-4Lines changed: 4 additions & 4 deletions b/‎tests/baselines/reference/augmentedTypesInterface.errors.txt
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+4-4Lines changed: 4 additions & 4 deletions
diff --git a/‎tests/baselines/reference/class1.errors.txt
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-11Lines changed: 0 additions & 11 deletions b/‎tests/baselines/reference/class1.errors.txt
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diff --git a/‎tests/baselines/reference/class1.js
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-10Lines changed: 0 additions & 10 deletions b/‎tests/baselines/reference/class1.js
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-10Lines changed: 0 additions & 10 deletions
diff --git a/‎tests/baselines/reference/classAbstractMergedDeclaration.errors.txt
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+54-17Lines changed: 54 additions & 17 deletions b/‎tests/baselines/reference/classAbstractMergedDeclaration.errors.txt
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+54-17Lines changed: 54 additions & 17 deletions
@@ -173,6 +173,14 @@ namespace ts {
             return node.name ? declarationNameToString(node.name) : getDeclarationName(node);
         }
 
+        /**
+         * Declares a Symbol for the node and adds it to symbols. Reports errors for conflicting identifier names.
+         * @param symbolTable - The symbol table which node will be added to.
+         * @param parent - node's parent declaration.
+         * @param node - The declaration to be added to the symbol table
+         * @param includes - The SymbolFlags that node has in addition to its declaration type (eg: export, ambient, etc.)
+         * @param excludes - The flags which node cannot be declared alongside in a symbol table. Used to report forbidden declarations. 
+         */
         function declareSymbol(symbolTable: SymbolTable, parent: Symbol, node: Declaration, includes: SymbolFlags, excludes: SymbolFlags): Symbol {
             Debug.assert(!hasDynamicName(node));
 
@@ -181,10 +189,11 @@ namespace ts {
 
             let symbol: Symbol;
             if (name !== undefined) {
+
                 // Check and see if the symbol table already has a symbol with this name.  If not,
                 // create a new symbol with this name and add it to the table.  Note that we don't
                 // give the new symbol any flags *yet*.  This ensures that it will not conflict 
-                // witht he 'excludes' flags we pass in.
+                // with the 'excludes' flags we pass in.
                 //
                 // If we do get an existing symbol, see if it conflicts with the new symbol we're
                 // creating.  For example, a 'var' symbol and a 'class' symbol will conflict within
@@ -202,10 +211,10 @@ namespace ts {
                 symbol = hasProperty(symbolTable, name)
                     ? symbolTable[name]
                     : (symbolTable[name] = createSymbol(SymbolFlags.None, name));
-                
+
                 if (name && (includes & SymbolFlags.Classifiable)) {
-                    classifiableNames[name] = name;   
-                } 
+                    classifiableNames[name] = name;
+                }
 
                 if (symbol.flags & excludes) {
                     if (node.name) {
@@ -314,6 +323,7 @@ namespace ts {
 
                 addToContainerChain(container);
             }
+
             else if (containerFlags & ContainerFlags.IsBlockScopedContainer) {
                 blockScopeContainer = node;
                 blockScopeContainer.locals = undefined;
 
@@ -12034,6 +12034,12 @@ namespace ts {
                 grammarErrorOnFirstToken(node, Diagnostics.A_class_declaration_without_the_default_modifier_must_have_a_name);
             }
             checkClassLikeDeclaration(node);
+
+            // Interfaces cannot be merged with non-ambient classes.
+            if (getSymbolOfNode(node).flags & SymbolFlags.Interface && !isInAmbientContext(node)) {
+                error(node, Diagnostics.Only_an_ambient_class_can_be_merged_with_an_interface)
+            }
+
             forEach(node.members, checkSourceElement);
         }
 
@@ -12306,13 +12312,24 @@ namespace ts {
                         checkIndexConstraints(type);
                     }
                 }
+
+                // Interfaces cannot merge with non-ambient classes.
+                if (symbol && symbol.declarations) {
+                    for (let declaration of symbol.declarations) {
+                        if (declaration.kind === SyntaxKind.ClassDeclaration && !isInAmbientContext(declaration)) {
+                            error(node, Diagnostics.Only_an_ambient_class_can_be_merged_with_an_interface);
+                            break;
+                        }
+                    }
+                }
             }
             forEach(getInterfaceBaseTypeNodes(node), heritageElement => {
                 if (!isSupportedExpressionWithTypeArguments(heritageElement)) {
                     error(heritageElement.expression, Diagnostics.An_interface_can_only_extend_an_identifier_Slashqualified_name_with_optional_type_arguments);
                 }
                 checkTypeReferenceNode(heritageElement);
             });
+
             forEach(node.members, checkSourceElement);
 
             if (produceDiagnostics) {
 
@@ -408,6 +408,7 @@ namespace ts {
         Non_abstract_class_0_does_not_implement_inherited_abstract_member_1_from_class_2: { code: 2515, category: DiagnosticCategory.Error, key: "Non-abstract class '{0}' does not implement inherited abstract member '{1}' from class '{2}'." },
         All_declarations_of_an_abstract_method_must_be_consecutive: { code: 2516, category: DiagnosticCategory.Error, key: "All declarations of an abstract method must be consecutive." },
         Constructor_objects_of_abstract_type_cannot_be_assigned_to_constructor_objects_of_non_abstract_type: { code: 2517, category: DiagnosticCategory.Error, key: "Constructor objects of abstract type cannot be assigned to constructor objects of non-abstract type" },
+        Only_an_ambient_class_can_be_merged_with_an_interface: { code: 2518, category: DiagnosticCategory.Error, key: "Only an ambient class can be merged with an interface." },
         Duplicate_identifier_0_Compiler_uses_declaration_1_to_support_async_functions: { code: 2520, category: DiagnosticCategory.Error, key: "Duplicate identifier '{0}'. Compiler uses declaration '{1}' to support async functions." },
         Expression_resolves_to_variable_declaration_0_that_compiler_uses_to_support_async_functions: { code: 2521, category: DiagnosticCategory.Error, key: "Expression resolves to variable declaration '{0}' that compiler uses to support async functions." },
         The_arguments_object_cannot_be_referenced_in_an_async_arrow_function_Consider_using_a_standard_async_function_expression: { code: 2522, category: DiagnosticCategory.Error, key: "The 'arguments' object cannot be referenced in an async arrow function. Consider using a standard async function expression." },
 
@@ -1621,6 +1621,10 @@
         "category": "Error",
         "code":2517
     },
+    "Only an ambient class can be merged with an interface.": {
+        "category": "Error",
+        "code": 2518
+    },
     "Duplicate identifier '{0}'. Compiler uses declaration '{1}' to support async functions.": {
         "category": "Error",
         "code": 2520
 
@@ -1597,8 +1597,8 @@ namespace ts {
         PropertyExcludes = Value,
         EnumMemberExcludes = Value,
         FunctionExcludes = Value & ~(Function | ValueModule),
-        ClassExcludes = (Value | Type) & ~ValueModule,
-        InterfaceExcludes = Type & ~Interface,
+        ClassExcludes = (Value | Type) & ~(ValueModule | Interface), // class-interface mergability done in checker.ts
+        InterfaceExcludes = Type & ~(Interface | Class),
         RegularEnumExcludes = (Value | Type) & ~(RegularEnum | ValueModule), // regular enums merge only with regular enums and modules
         ConstEnumExcludes = (Value | Type) & ~ConstEnum, // const enums merge only with const enums
         ValueModuleExcludes = Value & ~(Function | Class | RegularEnum | ValueModule),
 
@@ -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 TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/augmentedTypesClass2.ts(10,11): error TS2518: Only an ambient class can be merged with an interface.
 tests/cases/compiler/augmentedTypesClass2.ts(16,7): error TS2300: Duplicate identifier 'c33'.
 tests/cases/compiler/augmentedTypesClass2.ts(21,6): error TS2300: Duplicate identifier 'c33'.
 
@@ -10,15 +10,15 @@ tests/cases/compiler/augmentedTypesClass2.ts(21,6): error TS2300: Duplicate iden
     // class then interface
     class c11 { // error
           ~~~
-!!! error TS2300: Duplicate identifier 'c11'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         foo() {
             return 1;
         }
     }
 
     interface c11 { // error
               ~~~
-!!! error TS2300: Duplicate identifier 'c11'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         bar(): void;
     }
 
 
@@ -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 TS2518: Only an ambient class can be merged with an interface.
+tests/cases/compiler/augmentedTypesInterface.ts(16,7): error TS2518: Only an ambient class can 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 TS2518: Only an ambient class can be merged with an interface.
         foo(): void;
     }
 
     class i2 {  // error
           ~~
-!!! error TS2300: Duplicate identifier 'i2'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
         bar() {
             return 1;
         }
 
@@ -1,19 +1,26 @@
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(7,16): error TS2300: Duplicate identifier 'CI'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(8,11): error TS2300: Duplicate identifier 'CI'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(10,11): error TS2300: Duplicate identifier 'IC'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(11,16): error TS2300: Duplicate identifier 'IC'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(7,16): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(8,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(10,11): error TS2518: Only an ambient class can be merged with an interface.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(11,16): error TS2518: Only an ambient class can be merged with an interface.
 tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(13,16): error TS2300: Duplicate identifier 'CC1'.
 tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(14,7): error TS2300: Duplicate identifier 'CC1'.
 tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(16,7): error TS2300: Duplicate identifier 'CC2'.
 tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(17,16): error TS2300: Duplicate identifier 'CC2'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(19,1): error TS2511: Cannot create an instance of the abstract class 'CM'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(20,1): error TS2511: Cannot create an instance of the abstract class 'MC'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(21,1): error TS2511: Cannot create an instance of the abstract class 'CI'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(22,5): error TS2304: Cannot find name 'IC'.
-tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(23,1): error TS2511: Cannot create an instance of the abstract class 'CC1'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(25,24): error TS2300: Duplicate identifier 'DCC1'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(26,15): error TS2300: Duplicate identifier 'DCC1'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(28,15): error TS2300: Duplicate identifier 'DCC2'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(29,24): error TS2300: Duplicate identifier 'DCC2'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(31,1): error TS2511: Cannot create an instance of the abstract class 'CM'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(32,1): error TS2511: Cannot create an instance of the abstract class 'MC'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(33,1): error TS2511: Cannot create an instance of the abstract class 'CI'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(34,1): error TS2511: Cannot create an instance of the abstract class 'IC'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(35,1): error TS2511: Cannot create an instance of the abstract class 'CC1'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(37,1): error TS2511: Cannot create an instance of the abstract class 'DCI'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(38,1): error TS2511: Cannot create an instance of the abstract class 'DIC'.
+tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts(39,1): error TS2511: Cannot create an instance of the abstract class 'DCC1'.
 
 
-==== tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts (13 errors) ====
+==== tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbstractMergedDeclaration.ts (20 errors) ====
     abstract class CM {}
     module CM {}
 
@@ -22,17 +29,17 @@ tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbst
 
     abstract class CI {}
                    ~~
-!!! error TS2300: Duplicate identifier 'CI'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     interface CI {}
               ~~
-!!! error TS2300: Duplicate identifier 'CI'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
 
     interface IC {}
               ~~
-!!! error TS2300: Duplicate identifier 'IC'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
     abstract class IC {}
                    ~~
-!!! error TS2300: Duplicate identifier 'IC'.
+!!! error TS2518: Only an ambient class can be merged with an interface.
 
     abstract class CC1 {}
                    ~~~
@@ -48,6 +55,26 @@ tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbst
                    ~~~
 !!! error TS2300: Duplicate identifier 'CC2'.
 
+    declare abstract class DCI {}
+    interface DCI {}
+    
+    interface DIC {}
+    declare abstract class DIC {}
+    
+    declare abstract class DCC1 {}
+                           ~~~~
+!!! error TS2300: Duplicate identifier 'DCC1'.
+    declare class DCC1 {}
+                  ~~~~
+!!! error TS2300: Duplicate identifier 'DCC1'.
+    
+    declare class DCC2 {}
+                  ~~~~
+!!! error TS2300: Duplicate identifier 'DCC2'.
+    declare abstract class DCC2 {}
+                           ~~~~
+!!! error TS2300: Duplicate identifier 'DCC2'.
+    
     new CM;
     ~~~~~~
 !!! error TS2511: Cannot create an instance of the abstract class 'CM'.
@@ -58,9 +85,19 @@ tests/cases/conformance/classes/classDeclarations/classAbstractKeyword/classAbst
     ~~~~~~
 !!! error TS2511: Cannot create an instance of the abstract class 'CI'.
     new IC;
-        ~~
-!!! error TS2304: Cannot find name 'IC'.
+    ~~~~~~
+!!! error TS2511: Cannot create an instance of the abstract class 'IC'.
     new CC1;
     ~~~~~~~
 !!! error TS2511: Cannot create an instance of the abstract class 'CC1'.
-    new CC2;
+    new CC2;
+    new DCI;
+    ~~~~~~~
+!!! error TS2511: Cannot create an instance of the abstract class 'DCI'.
+    new DIC;
+    ~~~~~~~
+!!! error TS2511: Cannot create an instance of the abstract class 'DIC'.
+    new DCC1;
+    ~~~~~~~~
+!!! error TS2511: Cannot create an instance of the abstract class 'DCC1'.
+    new DCC2;