Added test and adjusted reporting logic.

src/compiler/checker.ts

@@ -16382,7 +16382,7 @@ namespace ts {
             }
 
             if (isPotentiallyUncalledDecorator(node, callSignatures)) {
-                error(node, Diagnostics.This_function_cannot_be_used_as_a_decorator_Did_you_mean_to_call_it_first);
+                error(node, Diagnostics.This_value_has_type_0_which_accepts_too_few_arguments_to_be_used_as_a_decorator_here_Did_you_mean_to_call_it_first, typeToString(funcType));
                 return resolveErrorCall(node);
             }
 
@@ -16406,6 +16406,7 @@ namespace ts {
         function isPotentiallyUncalledDecorator(decorator: Decorator, signatures: Signature[]) {
             return signatures.length && every(signatures, signature =>
                 signature.minArgumentCount === 0 &&
+                !signature.hasRestParameter &&
                 signature.parameters.length < getEffectiveArgumentCount(decorator, /*args*/ undefined, signature));
         }
 

src/compiler/diagnosticMessages.json

@@ -907,7 +907,7 @@
         "category": "Error",
         "code": 1328
     },
-    "This function cannot be used as a decorator. Did you mean to call it first?": {
+    "This value has type '{0}' which accepts too few arguments to be used as a decorator here. Did you mean to call it first?": {
         "category": "Error",
         "code": 1329
     },

tests/cases/compiler/potentiallyUncalledDecorators.ts

@@ -0,0 +1,74 @@
+// @target: esnext
+// @module: esnext
+// @experimentalDecorators: true
+
+// Angular-style Input/Output API:
+declare function Input(bindingPropertyName?: string): any;
+class FooComponent {
+    @Input foo: string;
+}
+
+// Glimmer-style tracked API:
+declare const tracked: PropertyDecorator & { (...watchedProperties: string[]): any; }
+
+class Person {
+    @tracked person; any;
+}
+
+class MultiplyByTwo {
+    args: any;
+    @tracked('args')
+    get multiplied() {
+        return this.args.number * 2;
+    }
+}
+
+// Other fun stuff.
+
+interface OmniDecorator extends MethodDecorator, ClassDecorator, PropertyDecorator {
+}
+
+declare function noArgs(): OmniDecorator;
+declare function allRest(...args: any[]): OmniDecorator;
+declare function oneOptional(x?: any): OmniDecorator;
+declare function twoOptional(x?: any, y?: any): OmniDecorator;
+declare function threeOptional(x?: any, y?: any, z?: any): OmniDecorator;
+declare function oneOptionalWithRest(x?: any, ...args: any[]): OmniDecorator;
+
+@noArgs
+class A {
+    @noArgs foo: any;
+    @noArgs bar() { }
+}
+
+@allRest
+class B {
+    @allRest foo: any;
+    @allRest bar() { }
+}
+
+@oneOptional
+class C {
+    @oneOptional foo: any;
+    @oneOptional bar() { }
+}
+
+@twoOptional
+class D {
+    @twoOptional foo: any;
+    @twoOptional bar() { }
+}
+
+@threeOptional
+class E {
+    @threeOptional foo: any;
+    @threeOptional bar() { }
+}
+
+@oneOptionalWithRest
+class F {
+    @oneOptionalWithRest foo: any;
+    @oneOptionalWithRest bar() { }
+}
+
+export { };