diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index f8c8817888591..398c7c370dc6b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -26436,12 +26436,21 @@ namespace ts {
             }
 
             if (hasNonAmbientClass && !isConstructor && symbol.flags & SymbolFlags.Function) {
-                // A non-ambient class cannot be an implementation for a non-constructor function/class merge
-                // TODO: The below just replicates our older error from when classes and functions were
-                // entirely unable to merge - a more helpful message like "Class declaration cannot implement overload list"
-                // might be warranted. :shrug:
+                const relatedDiagnostics = filter(declarations, d => d.kind === SyntaxKind.ClassDeclaration)
+                    .map(d => createDiagnosticForNode(d, Diagnostics.Consider_adding_a_declare_modifier_to_this_class));
+
                 forEach(declarations, declaration => {
-                    addDuplicateDeclarationError(getNameOfDeclaration(declaration) || declaration, Diagnostics.Duplicate_identifier_0, symbolName(symbol), filter(declarations, d => d !== declaration));
+                    const diagnostic = declaration.kind === SyntaxKind.ClassDeclaration
+                            ? Diagnostics.Class_declaration_cannot_implement_overload_list_for_0
+                            : declaration.kind === SyntaxKind.FunctionDeclaration
+                                ? Diagnostics.Function_with_bodies_can_only_merge_with_classes_that_are_ambient
+                                : undefined;
+                    if (diagnostic) {
+                        addRelatedInfo(
+                            error(getNameOfDeclaration(declaration) || declaration, diagnostic, symbolName(symbol)),
+                            ...relatedDiagnostics
+                        );
+                    }
                 });
             }
 
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 86686cb9f6c1b..5c5d7415d30e2 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2693,6 +2693,14 @@
         "category": "Error",
         "code": 2773
     },
+    "Class declaration cannot implement overload list for '{0}'.": {
+        "category": "Error",
+        "code": 2774
+    },
+    "Function with bodies can only merge with classes that are ambient.": {
+        "category": "Error",
+        "code": 2775
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
@@ -4194,6 +4202,11 @@
         "code": 6502
     },
 
+    "Consider adding a 'declare' modifier to this class.": {
+        "category": "Message",
+        "code": 6503
+    },
+
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005
diff --git a/tests/baselines/reference/augmentedTypesClass2a.errors.txt b/tests/baselines/reference/augmentedTypesClass2a.errors.txt
index 6031558ac0ac4..ecd450bbb95b4 100644
--- a/tests/baselines/reference/augmentedTypesClass2a.errors.txt
+++ b/tests/baselines/reference/augmentedTypesClass2a.errors.txt
@@ -1,7 +1,7 @@
 tests/cases/compiler/augmentedTypesClass2a.ts(2,7): error TS2300: Duplicate identifier 'c2'.
-tests/cases/compiler/augmentedTypesClass2a.ts(2,7): error TS2300: Duplicate identifier 'c2'.
-tests/cases/compiler/augmentedTypesClass2a.ts(3,10): error TS2300: Duplicate identifier 'c2'.
+tests/cases/compiler/augmentedTypesClass2a.ts(2,7): error TS2774: Class declaration cannot implement overload list for 'c2'.
 tests/cases/compiler/augmentedTypesClass2a.ts(3,10): error TS2300: Duplicate identifier 'c2'.
+tests/cases/compiler/augmentedTypesClass2a.ts(3,10): error TS2775: Function with bodies can only merge with classes that are ambient.
 tests/cases/compiler/augmentedTypesClass2a.ts(4,5): error TS2300: Duplicate identifier 'c2'.
 
 
@@ -10,15 +10,15 @@ tests/cases/compiler/augmentedTypesClass2a.ts(4,5): error TS2300: Duplicate iden
     class c2 { public foo() { } } // error
           ~~
 !!! error TS2300: Duplicate identifier 'c2'.
-!!! related TS6203 tests/cases/compiler/augmentedTypesClass2a.ts:3:10: 'c2' was also declared here.
           ~~
-!!! error TS2300: Duplicate identifier 'c2'.
+!!! error TS2774: Class declaration cannot implement overload list for 'c2'.
+!!! related TS6503 tests/cases/compiler/augmentedTypesClass2a.ts:2:7: Consider adding a 'declare' modifier to this class.
     function c2() { } // error
              ~~
 !!! error TS2300: Duplicate identifier 'c2'.
-!!! related TS6203 tests/cases/compiler/augmentedTypesClass2a.ts:2:7: 'c2' was also declared here.
              ~~
-!!! error TS2300: Duplicate identifier 'c2'.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/augmentedTypesClass2a.ts:2:7: Consider adding a 'declare' modifier to this class.
     var c2 = () => { }
         ~~
 !!! error TS2300: Duplicate identifier 'c2'.
\ No newline at end of file
diff --git a/tests/baselines/reference/augmentedTypesFunction.errors.txt b/tests/baselines/reference/augmentedTypesFunction.errors.txt
index ff2ea23036a64..e553af0658c8d 100644
--- a/tests/baselines/reference/augmentedTypesFunction.errors.txt
+++ b/tests/baselines/reference/augmentedTypesFunction.errors.txt
@@ -4,10 +4,10 @@ tests/cases/compiler/augmentedTypesFunction.ts(6,10): error TS2393: Duplicate fu
 tests/cases/compiler/augmentedTypesFunction.ts(7,10): error TS2393: Duplicate function implementation.
 tests/cases/compiler/augmentedTypesFunction.ts(9,10): error TS2300: Duplicate identifier 'y2a'.
 tests/cases/compiler/augmentedTypesFunction.ts(10,5): error TS2300: Duplicate identifier 'y2a'.
-tests/cases/compiler/augmentedTypesFunction.ts(13,10): error TS2300: Duplicate identifier 'y3'.
-tests/cases/compiler/augmentedTypesFunction.ts(14,7): error TS2300: Duplicate identifier 'y3'.
-tests/cases/compiler/augmentedTypesFunction.ts(16,10): error TS2300: Duplicate identifier 'y3a'.
-tests/cases/compiler/augmentedTypesFunction.ts(17,7): error TS2300: Duplicate identifier 'y3a'.
+tests/cases/compiler/augmentedTypesFunction.ts(13,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/augmentedTypesFunction.ts(14,7): error TS2774: Class declaration cannot implement overload list for 'y3'.
+tests/cases/compiler/augmentedTypesFunction.ts(16,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/augmentedTypesFunction.ts(17,7): error TS2774: Class declaration cannot implement overload list for 'y3a'.
 tests/cases/compiler/augmentedTypesFunction.ts(20,10): error TS2567: Enum declarations can only merge with namespace or other enum declarations.
 tests/cases/compiler/augmentedTypesFunction.ts(21,6): error TS2567: Enum declarations can only merge with namespace or other enum declarations.
 
@@ -39,21 +39,21 @@ tests/cases/compiler/augmentedTypesFunction.ts(21,6): error TS2567: Enum declara
     // function then class
     function y3() { } // error
              ~~
-!!! error TS2300: Duplicate identifier 'y3'.
-!!! related TS6203 tests/cases/compiler/augmentedTypesFunction.ts:14:7: 'y3' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/augmentedTypesFunction.ts:14:7: Consider adding a 'declare' modifier to this class.
     class y3 { } // error
           ~~
-!!! error TS2300: Duplicate identifier 'y3'.
-!!! related TS6203 tests/cases/compiler/augmentedTypesFunction.ts:13:10: 'y3' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'y3'.
+!!! related TS6503 tests/cases/compiler/augmentedTypesFunction.ts:14:7: Consider adding a 'declare' modifier to this class.
     
     function y3a() { } // error
              ~~~
-!!! error TS2300: Duplicate identifier 'y3a'.
-!!! related TS6203 tests/cases/compiler/augmentedTypesFunction.ts:17:7: 'y3a' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/augmentedTypesFunction.ts:17:7: Consider adding a 'declare' modifier to this class.
     class y3a { public foo() { } } // error
           ~~~
-!!! error TS2300: Duplicate identifier 'y3a'.
-!!! related TS6203 tests/cases/compiler/augmentedTypesFunction.ts:16:10: 'y3a' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'y3a'.
+!!! related TS6503 tests/cases/compiler/augmentedTypesFunction.ts:17:7: Consider adding a 'declare' modifier to this class.
     
     // function then enum
     function y4() { } // error
diff --git a/tests/baselines/reference/callOverloads1.errors.txt b/tests/baselines/reference/callOverloads1.errors.txt
index 95b96aa7ec133..3a2cc4c02f23c 100644
--- a/tests/baselines/reference/callOverloads1.errors.txt
+++ b/tests/baselines/reference/callOverloads1.errors.txt
@@ -1,13 +1,13 @@
-tests/cases/compiler/callOverloads1.ts(1,7): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/callOverloads1.ts(9,10): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/callOverloads1.ts(1,7): error TS2774: Class declaration cannot implement overload list for 'Foo'.
 tests/cases/compiler/callOverloads1.ts(9,10): error TS2391: Function implementation is missing or not immediately following the declaration.
+tests/cases/compiler/callOverloads1.ts(9,10): error TS2775: Function with bodies can only merge with classes that are ambient.
 
 
 ==== tests/cases/compiler/callOverloads1.ts (3 errors) ====
     class Foo { // error
           ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads1.ts:9:10: 'Foo' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'Foo'.
+!!! related TS6503 tests/cases/compiler/callOverloads1.ts:1:7: Consider adding a 'declare' modifier to this class.
         bar1() { /*WScript.Echo("bar1");*/ }
     
         constructor(x: any) {
@@ -17,10 +17,10 @@ tests/cases/compiler/callOverloads1.ts(9,10): error TS2391: Function implementat
     
     function Foo(); // error
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads1.ts:1:7: 'Foo' was also declared here.
-             ~~~
 !!! error TS2391: Function implementation is missing or not immediately following the declaration.
+             ~~~
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/callOverloads1.ts:1:7: Consider adding a 'declare' modifier to this class.
     function F1(s:string);
     function F1(a:any) { return a;}
     
diff --git a/tests/baselines/reference/callOverloads2.errors.txt b/tests/baselines/reference/callOverloads2.errors.txt
index 8377e5fa4daad..e60da10230b70 100644
--- a/tests/baselines/reference/callOverloads2.errors.txt
+++ b/tests/baselines/reference/callOverloads2.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/compiler/callOverloads2.ts(1,7): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/callOverloads2.ts(9,10): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/callOverloads2.ts(1,7): error TS2774: Class declaration cannot implement overload list for 'Foo'.
+tests/cases/compiler/callOverloads2.ts(9,10): error TS2775: Function with bodies can only merge with classes that are ambient.
 tests/cases/compiler/callOverloads2.ts(11,10): error TS2389: Function implementation name must be 'Foo'.
 tests/cases/compiler/callOverloads2.ts(11,10): error TS2393: Duplicate function implementation.
 tests/cases/compiler/callOverloads2.ts(12,10): error TS2393: Duplicate function implementation.
@@ -9,8 +9,8 @@ tests/cases/compiler/callOverloads2.ts(14,10): error TS2391: Function implementa
 ==== tests/cases/compiler/callOverloads2.ts (6 errors) ====
     class Foo { // error
           ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads2.ts:9:10: 'Foo' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'Foo'.
+!!! related TS6503 tests/cases/compiler/callOverloads2.ts:1:7: Consider adding a 'declare' modifier to this class.
         bar1() { /*WScript.Echo("bar1");*/ }
     
         constructor(x: any) {
@@ -20,8 +20,8 @@ tests/cases/compiler/callOverloads2.ts(14,10): error TS2391: Function implementa
     
     function Foo(); // error
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads2.ts:1:7: 'Foo' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/callOverloads2.ts:1:7: Consider adding a 'declare' modifier to this class.
     
     function F1(s:string) {return s;} // error
              ~~
diff --git a/tests/baselines/reference/callOverloads3.errors.txt b/tests/baselines/reference/callOverloads3.errors.txt
index 7c064ba1775c5..cc71492ea9d9e 100644
--- a/tests/baselines/reference/callOverloads3.errors.txt
+++ b/tests/baselines/reference/callOverloads3.errors.txt
@@ -1,27 +1,24 @@
-tests/cases/compiler/callOverloads3.ts(1,10): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/callOverloads3.ts(2,10): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/callOverloads3.ts(1,10): error TS2775: Function with bodies can only merge with classes that are ambient.
 tests/cases/compiler/callOverloads3.ts(2,10): error TS2391: Function implementation is missing or not immediately following the declaration.
-tests/cases/compiler/callOverloads3.ts(3,7): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/callOverloads3.ts(2,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/callOverloads3.ts(3,7): error TS2774: Class declaration cannot implement overload list for 'Foo'.
 
 
 ==== tests/cases/compiler/callOverloads3.ts (4 errors) ====
     function Foo():Foo; // error
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads3.ts:2:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads3.ts:3:7: and here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/callOverloads3.ts:3:7: Consider adding a 'declare' modifier to this class.
     function Foo(s:string):Foo; // error
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads3.ts:1:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads3.ts:3:7: and here.
-             ~~~
 !!! error TS2391: Function implementation is missing or not immediately following the declaration.
+             ~~~
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/callOverloads3.ts:3:7: Consider adding a 'declare' modifier to this class.
     class Foo { // error
           ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads3.ts:1:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads3.ts:2:10: and here.
+!!! error TS2774: Class declaration cannot implement overload list for 'Foo'.
+!!! related TS6503 tests/cases/compiler/callOverloads3.ts:3:7: Consider adding a 'declare' modifier to this class.
         bar1() { /*WScript.Echo("bar1");*/ }
         constructor(x: any) {
             // WScript.Echo("Constructor function has executed");
diff --git a/tests/baselines/reference/callOverloads4.errors.txt b/tests/baselines/reference/callOverloads4.errors.txt
index cf86024f99517..d1659cacb89d9 100644
--- a/tests/baselines/reference/callOverloads4.errors.txt
+++ b/tests/baselines/reference/callOverloads4.errors.txt
@@ -1,27 +1,24 @@
-tests/cases/compiler/callOverloads4.ts(1,10): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/callOverloads4.ts(2,10): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/callOverloads4.ts(1,10): error TS2775: Function with bodies can only merge with classes that are ambient.
 tests/cases/compiler/callOverloads4.ts(2,10): error TS2391: Function implementation is missing or not immediately following the declaration.
-tests/cases/compiler/callOverloads4.ts(3,7): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/callOverloads4.ts(2,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/callOverloads4.ts(3,7): error TS2774: Class declaration cannot implement overload list for 'Foo'.
 
 
 ==== tests/cases/compiler/callOverloads4.ts (4 errors) ====
     function Foo():Foo; // error
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads4.ts:2:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads4.ts:3:7: and here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/callOverloads4.ts:3:7: Consider adding a 'declare' modifier to this class.
     function Foo(s:string):Foo; // error
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads4.ts:1:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads4.ts:3:7: and here.
-             ~~~
 !!! error TS2391: Function implementation is missing or not immediately following the declaration.
+             ~~~
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/callOverloads4.ts:3:7: Consider adding a 'declare' modifier to this class.
     class Foo { // error
           ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads4.ts:1:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads4.ts:2:10: and here.
+!!! error TS2774: Class declaration cannot implement overload list for 'Foo'.
+!!! related TS6503 tests/cases/compiler/callOverloads4.ts:3:7: Consider adding a 'declare' modifier to this class.
         bar1() { /*WScript.Echo("bar1");*/ }
         constructor(s: string);
         constructor(x: any) {
diff --git a/tests/baselines/reference/callOverloads5.errors.txt b/tests/baselines/reference/callOverloads5.errors.txt
index bbf4b4bad8b22..a63bfa98b4070 100644
--- a/tests/baselines/reference/callOverloads5.errors.txt
+++ b/tests/baselines/reference/callOverloads5.errors.txt
@@ -1,27 +1,24 @@
-tests/cases/compiler/callOverloads5.ts(1,10): error TS2300: Duplicate identifier 'Foo'.
-tests/cases/compiler/callOverloads5.ts(2,10): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/callOverloads5.ts(1,10): error TS2775: Function with bodies can only merge with classes that are ambient.
 tests/cases/compiler/callOverloads5.ts(2,10): error TS2391: Function implementation is missing or not immediately following the declaration.
-tests/cases/compiler/callOverloads5.ts(3,7): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/callOverloads5.ts(2,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/callOverloads5.ts(3,7): error TS2774: Class declaration cannot implement overload list for 'Foo'.
 
 
 ==== tests/cases/compiler/callOverloads5.ts (4 errors) ====
     function Foo():Foo; // error
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads5.ts:2:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads5.ts:3:7: and here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/callOverloads5.ts:3:7: Consider adding a 'declare' modifier to this class.
     function Foo(s:string):Foo; // error
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads5.ts:1:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads5.ts:3:7: and here.
-             ~~~
 !!! error TS2391: Function implementation is missing or not immediately following the declaration.
+             ~~~
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/callOverloads5.ts:3:7: Consider adding a 'declare' modifier to this class.
     class Foo { // error
           ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/callOverloads5.ts:1:10: 'Foo' was also declared here.
-!!! related TS6204 tests/cases/compiler/callOverloads5.ts:2:10: and here.
+!!! error TS2774: Class declaration cannot implement overload list for 'Foo'.
+!!! related TS6503 tests/cases/compiler/callOverloads5.ts:3:7: Consider adding a 'declare' modifier to this class.
     	bar1(s:string);
     	bar1(n:number);
         bar1(a:any) { /*WScript.Echo(a);*/ }
diff --git a/tests/baselines/reference/classOverloadForFunction.errors.txt b/tests/baselines/reference/classOverloadForFunction.errors.txt
index 70e4d5d7b4f9d..60ac12f96fe8e 100644
--- a/tests/baselines/reference/classOverloadForFunction.errors.txt
+++ b/tests/baselines/reference/classOverloadForFunction.errors.txt
@@ -1,14 +1,14 @@
-tests/cases/compiler/classOverloadForFunction.ts(1,7): error TS2300: Duplicate identifier 'foo'.
-tests/cases/compiler/classOverloadForFunction.ts(2,10): error TS2300: Duplicate identifier 'foo'.
+tests/cases/compiler/classOverloadForFunction.ts(1,7): error TS2774: Class declaration cannot implement overload list for 'foo'.
+tests/cases/compiler/classOverloadForFunction.ts(2,10): error TS2775: Function with bodies can only merge with classes that are ambient.
 
 
 ==== tests/cases/compiler/classOverloadForFunction.ts (2 errors) ====
     class foo { };
           ~~~
-!!! error TS2300: Duplicate identifier 'foo'.
-!!! related TS6203 tests/cases/compiler/classOverloadForFunction.ts:2:10: 'foo' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'foo'.
+!!! related TS6503 tests/cases/compiler/classOverloadForFunction.ts:1:7: Consider adding a 'declare' modifier to this class.
     function foo() {}
              ~~~
-!!! error TS2300: Duplicate identifier 'foo'.
-!!! related TS6203 tests/cases/compiler/classOverloadForFunction.ts:1:7: 'foo' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/classOverloadForFunction.ts:1:7: Consider adding a 'declare' modifier to this class.
     
\ No newline at end of file
diff --git a/tests/baselines/reference/classOverloadForFunction2.errors.txt b/tests/baselines/reference/classOverloadForFunction2.errors.txt
index 447be14c4f97d..54904e7307eeb 100644
--- a/tests/baselines/reference/classOverloadForFunction2.errors.txt
+++ b/tests/baselines/reference/classOverloadForFunction2.errors.txt
@@ -1,16 +1,16 @@
-tests/cases/compiler/classOverloadForFunction2.ts(1,10): error TS2300: Duplicate identifier 'bar'.
 tests/cases/compiler/classOverloadForFunction2.ts(1,10): error TS2391: Function implementation is missing or not immediately following the declaration.
-tests/cases/compiler/classOverloadForFunction2.ts(2,7): error TS2300: Duplicate identifier 'bar'.
+tests/cases/compiler/classOverloadForFunction2.ts(1,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/classOverloadForFunction2.ts(2,7): error TS2774: Class declaration cannot implement overload list for 'bar'.
 
 
 ==== tests/cases/compiler/classOverloadForFunction2.ts (3 errors) ====
     function bar(): string;
              ~~~
-!!! error TS2300: Duplicate identifier 'bar'.
-!!! related TS6203 tests/cases/compiler/classOverloadForFunction2.ts:2:7: 'bar' was also declared here.
-             ~~~
 !!! error TS2391: Function implementation is missing or not immediately following the declaration.
+             ~~~
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/classOverloadForFunction2.ts:2:7: Consider adding a 'declare' modifier to this class.
     class bar {}
           ~~~
-!!! error TS2300: Duplicate identifier 'bar'.
-!!! related TS6203 tests/cases/compiler/classOverloadForFunction2.ts:1:10: 'bar' was also declared here.
\ No newline at end of file
+!!! error TS2774: Class declaration cannot implement overload list for 'bar'.
+!!! related TS6503 tests/cases/compiler/classOverloadForFunction2.ts:2:7: Consider adding a 'declare' modifier to this class.
\ No newline at end of file
diff --git a/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt b/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
index d4792e6b2d0e4..2ed1e339c19b1 100644
--- a/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
+++ b/tests/baselines/reference/duplicateIdentifiersAcrossContainerBoundaries.errors.txt
@@ -1,5 +1,5 @@
-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(9,21): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(12,18): error TS2774: Class declaration cannot implement overload list for 'f'.
 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(37,12): error TS2300: Duplicate identifier 'x'.
 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(41,16): error TS2300: Duplicate identifier 'x'.
 
@@ -15,14 +15,14 @@ tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts(41,16): er
     module M {
         export function f() { }
                         ~
-!!! error TS2300: Duplicate identifier 'f'.
-!!! related TS6203 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts:12:18: 'f' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts:12:18: Consider adding a 'declare' modifier to this class.
     }
     module M {
         export class f { } // error
                      ~
-!!! error TS2300: Duplicate identifier 'f'.
-!!! related TS6203 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts:9:21: 'f' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'f'.
+!!! related TS6503 tests/cases/compiler/duplicateIdentifiersAcrossContainerBoundaries.ts:12:18: Consider adding a 'declare' modifier to this class.
     }
     
     module M {
diff --git a/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt
index 8dca0b9ffcc3c..e784c0a48032c 100644
--- a/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt
+++ b/tests/baselines/reference/duplicateIdentifiersAcrossFileBoundaries.errors.txt
@@ -1,8 +1,8 @@
-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(3,7): error TS2774: Class declaration cannot implement overload list for 'C2'.
+tests/cases/compiler/file1.ts(4,10): error TS2775: Function with bodies can only merge with classes that are ambient.
 tests/cases/compiler/file1.ts(8,12): error TS2300: Duplicate identifier 'x'.
-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(3,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/file2.ts(4,7): error TS2774: Class declaration cannot implement overload list for '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'.
 
@@ -12,12 +12,12 @@ tests/cases/compiler/file2.ts(8,16): error TS2300: Duplicate identifier 'x'.
     class C1 { }
     class C2 { }
           ~~
-!!! error TS2300: Duplicate identifier 'C2'.
-!!! related TS6203 tests/cases/compiler/file2.ts:3:10: 'C2' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'C2'.
+!!! related TS6503 tests/cases/compiler/file1.ts:3:7: Consider adding a 'declare' modifier to this class.
     function f() { }
              ~
-!!! error TS2300: Duplicate identifier 'f'.
-!!! related TS6203 tests/cases/compiler/file2.ts:4:7: 'f' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/file2.ts:4:7: Consider adding a 'declare' modifier to this class.
     var v = 3;
     
     class Foo {
@@ -38,12 +38,12 @@ tests/cases/compiler/file2.ts(8,16): error TS2300: Duplicate identifier 'x'.
     interface C1 { } // error -- cannot merge interface with non-ambient class
     function C2() { } // error -- cannot merge function with non-ambient class
              ~~
-!!! error TS2300: Duplicate identifier 'C2'.
-!!! related TS6203 tests/cases/compiler/file1.ts:3:7: 'C2' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/file1.ts:3:7: Consider adding a 'declare' modifier to this class.
     class f { } // error -- cannot merge function with non-ambient class
           ~
-!!! error TS2300: Duplicate identifier 'f'.
-!!! related TS6203 tests/cases/compiler/file1.ts:4:10: 'f' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'f'.
+!!! related TS6503 tests/cases/compiler/file2.ts:4:7: Consider adding a 'declare' modifier to this class.
     var v = 3;
     
     module Foo {
diff --git a/tests/baselines/reference/funClodule.errors.txt b/tests/baselines/reference/funClodule.errors.txt
index a86c3ed89bf14..b009797c27bde 100644
--- a/tests/baselines/reference/funClodule.errors.txt
+++ b/tests/baselines/reference/funClodule.errors.txt
@@ -1,9 +1,8 @@
-tests/cases/compiler/funClodule.ts(15,10): error TS2300: Duplicate identifier 'foo3'.
-tests/cases/compiler/funClodule.ts(16,8): error TS2300: Duplicate identifier 'foo3'.
-tests/cases/compiler/funClodule.ts(19,7): error TS2300: Duplicate identifier 'foo3'.
+tests/cases/compiler/funClodule.ts(15,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/funClodule.ts(19,7): error TS2774: Class declaration cannot implement overload list for 'foo3'.
 
 
-==== tests/cases/compiler/funClodule.ts (3 errors) ====
+==== tests/cases/compiler/funClodule.ts (2 errors) ====
     declare function foo();
     declare module foo {
         export function x(): any;
@@ -20,18 +19,12 @@ tests/cases/compiler/funClodule.ts(19,7): error TS2300: Duplicate identifier 'fo
     
     function foo3() { }
              ~~~~
-!!! error TS2300: Duplicate identifier 'foo3'.
-!!! related TS6203 tests/cases/compiler/funClodule.ts:16:8: 'foo3' was also declared here.
-!!! related TS6204 tests/cases/compiler/funClodule.ts:19:7: and here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/funClodule.ts:19:7: Consider adding a 'declare' modifier to this class.
     module foo3 {
-           ~~~~
-!!! error TS2300: Duplicate identifier 'foo3'.
-!!! related TS6203 tests/cases/compiler/funClodule.ts:15:10: 'foo3' was also declared here.
-!!! related TS6204 tests/cases/compiler/funClodule.ts:19:7: and here.
          export function x(): any { }
     }
     class foo3 { } // Should error
           ~~~~
-!!! error TS2300: Duplicate identifier 'foo3'.
-!!! related TS6203 tests/cases/compiler/funClodule.ts:15:10: 'foo3' was also declared here.
-!!! related TS6204 tests/cases/compiler/funClodule.ts:16:8: and here.
\ No newline at end of file
+!!! error TS2774: Class declaration cannot implement overload list for 'foo3'.
+!!! related TS6503 tests/cases/compiler/funClodule.ts:19:7: Consider adding a 'declare' modifier to this class.
\ No newline at end of file
diff --git a/tests/baselines/reference/multipleDefaultExports01.errors.txt b/tests/baselines/reference/multipleDefaultExports01.errors.txt
index 37865bc30e0af..a60cf2a97774d 100644
--- a/tests/baselines/reference/multipleDefaultExports01.errors.txt
+++ b/tests/baselines/reference/multipleDefaultExports01.errors.txt
@@ -1,34 +1,34 @@
-tests/cases/conformance/es6/modules/m1.ts(1,22): error TS2300: Duplicate identifier 'default'.
 tests/cases/conformance/es6/modules/m1.ts(1,22): error TS2323: Cannot redeclare exported variable 'default'.
 tests/cases/conformance/es6/modules/m1.ts(1,22): error TS2528: A module cannot have multiple default exports.
-tests/cases/conformance/es6/modules/m1.ts(5,25): error TS2300: Duplicate identifier 'default'.
+tests/cases/conformance/es6/modules/m1.ts(1,22): error TS2774: Class declaration cannot implement overload list for 'default'.
 tests/cases/conformance/es6/modules/m1.ts(5,25): error TS2323: Cannot redeclare exported variable 'default'.
 tests/cases/conformance/es6/modules/m1.ts(5,25): error TS2528: A module cannot have multiple default exports.
+tests/cases/conformance/es6/modules/m1.ts(5,25): error TS2775: Function with bodies can only merge with classes that are ambient.
 tests/cases/conformance/es6/modules/m1.ts(10,16): error TS2528: A module cannot have multiple default exports.
 
 
 ==== tests/cases/conformance/es6/modules/m1.ts (7 errors) ====
     export default class foo {
                          ~~~
-!!! error TS2300: Duplicate identifier 'default'.
-!!! related TS6203 tests/cases/conformance/es6/modules/m1.ts:5:25: 'default' was also declared here.
-                         ~~~
 !!! error TS2323: Cannot redeclare exported variable 'default'.
                          ~~~
 !!! error TS2528: A module cannot have multiple default exports.
 !!! related TS6204 tests/cases/conformance/es6/modules/m1.ts:10:16: and here.
+                         ~~~
+!!! error TS2774: Class declaration cannot implement overload list for 'default'.
+!!! related TS6503 tests/cases/conformance/es6/modules/m1.ts:1:22: Consider adding a 'declare' modifier to this class.
     
     }
     
     export default function bar() {
                             ~~~
-!!! error TS2300: Duplicate identifier 'default'.
-!!! related TS6203 tests/cases/conformance/es6/modules/m1.ts:1:22: 'default' was also declared here.
-                            ~~~
 !!! error TS2323: Cannot redeclare exported variable 'default'.
                             ~~~
 !!! error TS2528: A module cannot have multiple default exports.
 !!! related TS2753 tests/cases/conformance/es6/modules/m1.ts:10:16: Another export default is here.
+                            ~~~
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/conformance/es6/modules/m1.ts:1:22: Consider adding a 'declare' modifier to this class.
     
     }
     
diff --git a/tests/baselines/reference/multipleExportDefault5.errors.txt b/tests/baselines/reference/multipleExportDefault5.errors.txt
index b03ba7f7ac612..e857d084bb9bf 100644
--- a/tests/baselines/reference/multipleExportDefault5.errors.txt
+++ b/tests/baselines/reference/multipleExportDefault5.errors.txt
@@ -1,19 +1,19 @@
-tests/cases/conformance/externalModules/multipleExportDefault5.ts(1,25): error TS2300: Duplicate identifier 'default'.
 tests/cases/conformance/externalModules/multipleExportDefault5.ts(1,25): error TS2323: Cannot redeclare exported variable 'default'.
-tests/cases/conformance/externalModules/multipleExportDefault5.ts(2,22): error TS2300: Duplicate identifier 'default'.
+tests/cases/conformance/externalModules/multipleExportDefault5.ts(1,25): error TS2775: Function with bodies can only merge with classes that are ambient.
 tests/cases/conformance/externalModules/multipleExportDefault5.ts(2,22): error TS2323: Cannot redeclare exported variable 'default'.
+tests/cases/conformance/externalModules/multipleExportDefault5.ts(2,22): error TS2774: Class declaration cannot implement overload list for 'default'.
 
 
 ==== tests/cases/conformance/externalModules/multipleExportDefault5.ts (4 errors) ====
     export default function bar() { }
                             ~~~
-!!! error TS2300: Duplicate identifier 'default'.
-!!! related TS6203 tests/cases/conformance/externalModules/multipleExportDefault5.ts:2:22: 'default' was also declared here.
-                            ~~~
 !!! error TS2323: Cannot redeclare exported variable 'default'.
+                            ~~~
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/conformance/externalModules/multipleExportDefault5.ts:2:22: Consider adding a 'declare' modifier to this class.
     export default class C {}
                          ~
-!!! error TS2300: Duplicate identifier 'default'.
-!!! related TS6203 tests/cases/conformance/externalModules/multipleExportDefault5.ts:1:25: 'default' was also declared here.
+!!! error TS2323: Cannot redeclare exported variable 'default'.
                          ~
-!!! error TS2323: Cannot redeclare exported variable 'default'.
\ No newline at end of file
+!!! error TS2774: Class declaration cannot implement overload list for 'default'.
+!!! related TS6503 tests/cases/conformance/externalModules/multipleExportDefault5.ts:2:22: Consider adding a 'declare' modifier to this class.
\ No newline at end of file
diff --git a/tests/baselines/reference/nameCollisions.errors.txt b/tests/baselines/reference/nameCollisions.errors.txt
index 484444226b42e..0b2b5054018fb 100644
--- a/tests/baselines/reference/nameCollisions.errors.txt
+++ b/tests/baselines/reference/nameCollisions.errors.txt
@@ -7,10 +7,10 @@ tests/cases/compiler/nameCollisions.ts(24,9): error TS2300: Duplicate identifier
 tests/cases/compiler/nameCollisions.ts(25,14): error TS2300: Duplicate identifier 'f'.
 tests/cases/compiler/nameCollisions.ts(27,14): error TS2300: Duplicate identifier 'f2'.
 tests/cases/compiler/nameCollisions.ts(28,9): error TS2300: Duplicate identifier 'f2'.
-tests/cases/compiler/nameCollisions.ts(33,11): error TS2300: Duplicate identifier 'C'.
-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(33,11): error TS2774: Class declaration cannot implement overload list for 'C'.
+tests/cases/compiler/nameCollisions.ts(34,14): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/nameCollisions.ts(36,14): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/nameCollisions.ts(37,11): error TS2774: Class declaration cannot implement overload list for 'C2'.
 
 
 ==== tests/cases/compiler/nameCollisions.ts (13 errors) ====
@@ -66,21 +66,21 @@ tests/cases/compiler/nameCollisions.ts(37,11): error TS2300: Duplicate identifie
     
         class C { }
               ~
-!!! error TS2300: Duplicate identifier 'C'.
-!!! related TS6203 tests/cases/compiler/nameCollisions.ts:34:14: 'C' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'C'.
+!!! related TS6503 tests/cases/compiler/nameCollisions.ts:33:11: Consider adding a 'declare' modifier to this class.
         function C() { } // error
                  ~
-!!! error TS2300: Duplicate identifier 'C'.
-!!! related TS6203 tests/cases/compiler/nameCollisions.ts:33:11: 'C' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/nameCollisions.ts:33:11: Consider adding a 'declare' modifier to this class.
     
         function C2() { }
                  ~~
-!!! error TS2300: Duplicate identifier 'C2'.
-!!! related TS6203 tests/cases/compiler/nameCollisions.ts:37:11: 'C2' was also declared here.
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/nameCollisions.ts:37:11: Consider adding a 'declare' modifier to this class.
         class C2 { } // error
               ~~
-!!! error TS2300: Duplicate identifier 'C2'.
-!!! related TS6203 tests/cases/compiler/nameCollisions.ts:36:14: 'C2' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'C2'.
+!!! related TS6503 tests/cases/compiler/nameCollisions.ts:37:11: Consider adding a 'declare' modifier to this class.
     
         function fi() { }
         interface fi { } // ok
diff --git a/tests/baselines/reference/staticClassMemberError.errors.txt b/tests/baselines/reference/staticClassMemberError.errors.txt
index b82eb78b1d645..d6139e543a025 100644
--- a/tests/baselines/reference/staticClassMemberError.errors.txt
+++ b/tests/baselines/reference/staticClassMemberError.errors.txt
@@ -1,7 +1,7 @@
 tests/cases/compiler/staticClassMemberError.ts(4,3): error TS2662: Cannot find name 's'. Did you mean the static member 'C.s'?
-tests/cases/compiler/staticClassMemberError.ts(9,10): error TS2300: Duplicate identifier 'Foo'.
 tests/cases/compiler/staticClassMemberError.ts(9,10): error TS2391: Function implementation is missing or not immediately following the declaration.
-tests/cases/compiler/staticClassMemberError.ts(10,7): error TS2300: Duplicate identifier 'Foo'.
+tests/cases/compiler/staticClassMemberError.ts(9,10): error TS2775: Function with bodies can only merge with classes that are ambient.
+tests/cases/compiler/staticClassMemberError.ts(10,7): error TS2774: Class declaration cannot implement overload list for 'Foo'.
 
 
 ==== tests/cases/compiler/staticClassMemberError.ts (4 errors) ====
@@ -17,13 +17,13 @@ tests/cases/compiler/staticClassMemberError.ts(10,7): error TS2300: Duplicate id
     // just want to make sure this one doesn't crash the compiler
     function Foo();
              ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/staticClassMemberError.ts:10:7: 'Foo' was also declared here.
-             ~~~
 !!! error TS2391: Function implementation is missing or not immediately following the declaration.
+             ~~~
+!!! error TS2775: Function with bodies can only merge with classes that are ambient.
+!!! related TS6503 tests/cases/compiler/staticClassMemberError.ts:10:7: Consider adding a 'declare' modifier to this class.
     class Foo {
           ~~~
-!!! error TS2300: Duplicate identifier 'Foo'.
-!!! related TS6203 tests/cases/compiler/staticClassMemberError.ts:9:10: 'Foo' was also declared here.
+!!! error TS2774: Class declaration cannot implement overload list for 'Foo'.
+!!! related TS6503 tests/cases/compiler/staticClassMemberError.ts:10:7: Consider adding a 'declare' modifier to this class.
      static bar;
     }
\ No newline at end of file