Improve decorator on overload error message (fixes #6064)

src/compiler/checker.ts

@@ -15544,7 +15544,12 @@ namespace ts {
                 return false;
             }
             if (!nodeCanBeDecorated(node)) {
-                return grammarErrorOnFirstToken(node, Diagnostics.Decorators_are_not_valid_here);
+                if (node.kind === SyntaxKind.MethodDeclaration && !ts.nodeIsPresent((<MethodDeclaration>node).body)) {
+                    return grammarErrorOnFirstToken(node, Diagnostics.A_decorator_can_only_decorate_a_method_implementation_not_an_overload);
+                }
+                else {
+                    return grammarErrorOnFirstToken(node, Diagnostics.Decorators_are_not_valid_here);
+                }
             }
             else if (node.kind === SyntaxKind.GetAccessor || node.kind === SyntaxKind.SetAccessor) {
                 const accessors = getAllAccessorDeclarations((<ClassDeclaration>node.parent).members, <AccessorDeclaration>node);

src/compiler/diagnosticMessages.json

@@ -795,6 +795,10 @@
         "category": "Error",
         "code": 1248
     },
+    "A decorator can only decorate a method implementation, not an overload.": {
+        "category": "Error",
+        "code": 1249
+    }, 
     "'with' statements are not allowed in an async function block.": {
         "category": "Error",
         "code": 1300

tests/baselines/reference/decoratorOnClassMethodOverload1.errors.txt

@@ -0,0 +1,13 @@
+tests/cases/conformance/decorators/class/method/decoratorOnClassMethodOverload1.ts(4,5): error TS1249: A decorator can only decorate a method implementation, not an overload.
+
+
+==== tests/cases/conformance/decorators/class/method/decoratorOnClassMethodOverload1.ts (1 errors) ====
+    declare function dec<T>(target: any, propertyKey: string, descriptor: TypedPropertyDescriptor<T>): TypedPropertyDescriptor<T>;
+    
+    class C {
+        @dec
+        ~
+!!! error TS1249: A decorator can only decorate a method implementation, not an overload.
+        method()
+        method() { }
+    }

tests/baselines/reference/decoratorOnClassMethodOverload1.js

@@ -0,0 +1,16 @@
+//// [decoratorOnClassMethodOverload1.ts]
+declare function dec<T>(target: any, propertyKey: string, descriptor: TypedPropertyDescriptor<T>): TypedPropertyDescriptor<T>;
+
+class C {
+    @dec
+    method()
+    method() { }
+}
+
+//// [decoratorOnClassMethodOverload1.js]
+var C = (function () {
+    function C() {
+    }
+    C.prototype.method = function () { };
+    return C;
+}());

tests/cases/conformance/decorators/class/method/decoratorOnClassMethodOverload1.ts

@@ -0,0 +1,9 @@
+// @target: ES5
+// @experimentaldecorators: true
+declare function dec<T>(target: any, propertyKey: string, descriptor: TypedPropertyDescriptor<T>): TypedPropertyDescriptor<T>;
+
+class C {
+    @dec
+    method()
+    method() { }
+}