Give a more helpful error message for certain decorators with too many arguments

src/compiler/checker.ts

@@ -16381,6 +16381,12 @@ namespace ts {
                 return resolveUntypedCall(node);
             }
 
+            if (isPotentiallyUncalledDecorator(node, callSignatures)) {
+                const nodeStr = getTextOfNode(node.expression, /*includeTrivia*/ false);
+                error(node, Diagnostics._0_accepts_too_few_arguments_to_be_used_as_a_decorator_here_Did_you_mean_to_call_it_first_and_write_0, nodeStr);
+                return resolveErrorCall(node);
+            }
+
             const headMessage = getDiagnosticHeadMessageForDecoratorResolution(node);
             if (!callSignatures.length) {
                 let errorInfo: DiagnosticMessageChain;
@@ -16393,6 +16399,18 @@ namespace ts {
             return resolveCall(node, callSignatures, candidatesOutArray, headMessage);
         }
 
+        /**
+         * Sometimes, we have a decorator that could accept zero arguments,
+         * but is receiving too many arguments as part of the decorator invocation.
+         * In those cases, a user may have meant to *call* the expression before using it as a decorator.
+         */
+        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));
+        }
+
         /**
          * This function is similar to getResolvedSignature but is exclusively for trying to resolve JSX stateless-function component.
          * The main reason we have to use this function instead of getResolvedSignature because, the caller of this function will already check the type of openingLikeElement's tagName

src/compiler/diagnosticMessages.json

@@ -907,6 +907,10 @@
         "category": "Error",
         "code": 1328
     },
+    "'{0}' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@{0}()'?": {
+        "category": "Error",
+        "code": 1329
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",

tests/baselines/reference/decoratorOnClassMethod6.errors.txt

@@ -1,4 +1,4 @@
-tests/cases/conformance/decorators/class/method/decoratorOnClassMethod6.ts(4,5): error TS1241: Unable to resolve signature of method decorator when called as an expression.
+tests/cases/conformance/decorators/class/method/decoratorOnClassMethod6.ts(4,5): error TS1329: 'dec' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@dec()'?
 
 
 ==== tests/cases/conformance/decorators/class/method/decoratorOnClassMethod6.ts (1 errors) ====
@@ -7,5 +7,5 @@ tests/cases/conformance/decorators/class/method/decoratorOnClassMethod6.ts(4,5):
     class C {
         @dec ["method"]() {}
         ~~~~
-!!! error TS1241: Unable to resolve signature of method decorator when called as an expression.
+!!! error TS1329: 'dec' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@dec()'?
     }

tests/baselines/reference/decoratorOnClassProperty11.errors.txt

@@ -1,4 +1,4 @@
-tests/cases/conformance/decorators/class/property/decoratorOnClassProperty11.ts(4,5): error TS1240: Unable to resolve signature of property decorator when called as an expression.
+tests/cases/conformance/decorators/class/property/decoratorOnClassProperty11.ts(4,5): error TS1329: 'dec' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@dec()'?
 
 
 ==== tests/cases/conformance/decorators/class/property/decoratorOnClassProperty11.ts (1 errors) ====
@@ -7,5 +7,5 @@ tests/cases/conformance/decorators/class/property/decoratorOnClassProperty11.ts(
     class C {
         @dec prop;
         ~~~~
-!!! error TS1240: Unable to resolve signature of property decorator when called as an expression.
+!!! error TS1329: 'dec' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@dec()'?
     }

tests/baselines/reference/potentiallyUncalledDecorators.errors.txt

@@ -0,0 +1,174 @@
+tests/cases/compiler/potentiallyUncalledDecorators.ts(4,5): error TS1329: 'Input' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@Input()'?
+tests/cases/compiler/potentiallyUncalledDecorators.ts(35,1): error TS1329: 'noArgs' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@noArgs()'?
+tests/cases/compiler/potentiallyUncalledDecorators.ts(37,5): error TS1329: 'noArgs' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@noArgs()'?
+tests/cases/compiler/potentiallyUncalledDecorators.ts(38,5): error TS1329: 'noArgs' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@noArgs()'?
+tests/cases/compiler/potentiallyUncalledDecorators.ts(41,1): error TS1238: Unable to resolve signature of class decorator when called as an expression.
+  Type 'OmniDecorator' is not assignable to type 'typeof B'.
+    Type 'OmniDecorator' provides no match for the signature 'new (): B'.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(43,5): error TS1236: The return type of a property decorator function must be either 'void' or 'any'.
+  Unable to resolve signature of property decorator when called as an expression.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(44,5): error TS1241: Unable to resolve signature of method decorator when called as an expression.
+  Type 'OmniDecorator' has no properties in common with type 'TypedPropertyDescriptor<() => void>'.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(47,1): error TS1238: Unable to resolve signature of class decorator when called as an expression.
+  Type 'OmniDecorator' is not assignable to type 'typeof C'.
+    Type 'OmniDecorator' provides no match for the signature 'new (): C'.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(49,5): error TS1329: 'oneOptional' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@oneOptional()'?
+tests/cases/compiler/potentiallyUncalledDecorators.ts(50,5): error TS1329: 'oneOptional' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@oneOptional()'?
+tests/cases/compiler/potentiallyUncalledDecorators.ts(53,1): error TS1238: Unable to resolve signature of class decorator when called as an expression.
+  Type 'OmniDecorator' is not assignable to type 'typeof D'.
+    Type 'OmniDecorator' provides no match for the signature 'new (): D'.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(55,5): error TS1236: The return type of a property decorator function must be either 'void' or 'any'.
+  Unable to resolve signature of property decorator when called as an expression.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(56,5): error TS1241: Unable to resolve signature of method decorator when called as an expression.
+  Type 'OmniDecorator' has no properties in common with type 'TypedPropertyDescriptor<() => void>'.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(59,1): error TS1238: Unable to resolve signature of class decorator when called as an expression.
+  Type 'OmniDecorator' is not assignable to type 'typeof E'.
+    Type 'OmniDecorator' provides no match for the signature 'new (): E'.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(61,5): error TS1236: The return type of a property decorator function must be either 'void' or 'any'.
+  Unable to resolve signature of property decorator when called as an expression.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(62,5): error TS1241: Unable to resolve signature of method decorator when called as an expression.
+  Type 'OmniDecorator' has no properties in common with type 'TypedPropertyDescriptor<() => void>'.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(65,1): error TS1238: Unable to resolve signature of class decorator when called as an expression.
+  Type 'OmniDecorator' is not assignable to type 'typeof F'.
+    Type 'OmniDecorator' provides no match for the signature 'new (): F'.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(67,5): error TS1236: The return type of a property decorator function must be either 'void' or 'any'.
+  Unable to resolve signature of property decorator when called as an expression.
+tests/cases/compiler/potentiallyUncalledDecorators.ts(68,5): error TS1241: Unable to resolve signature of method decorator when called as an expression.
+  Type 'OmniDecorator' has no properties in common with type 'TypedPropertyDescriptor<() => void>'.
+
+
+==== tests/cases/compiler/potentiallyUncalledDecorators.ts (19 errors) ====
+    // Angular-style Input/Output API:
+    declare function Input(bindingPropertyName?: string): any;
+    class FooComponent {
+        @Input foo: string;
+        ~~~~~~
+!!! error TS1329: 'Input' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@Input()'?
+    }
+
+    // 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;
+    declare const anyDec: any;
+
+    @noArgs
+    ~~~~~~~
+!!! error TS1329: 'noArgs' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@noArgs()'?
+    class A {
+        @noArgs foo: any;
+        ~~~~~~~
+!!! error TS1329: 'noArgs' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@noArgs()'?
+        @noArgs bar() { }
+        ~~~~~~~
+!!! error TS1329: 'noArgs' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@noArgs()'?
+    }
+
+    @allRest
+    ~~~~~~~~
+!!! error TS1238: Unable to resolve signature of class decorator when called as an expression.
+!!! error TS1238:   Type 'OmniDecorator' is not assignable to type 'typeof B'.
+!!! error TS1238:     Type 'OmniDecorator' provides no match for the signature 'new (): B'.
+    class B {
+        @allRest foo: any;
+        ~~~~~~~~
+!!! error TS1236: The return type of a property decorator function must be either 'void' or 'any'.
+!!! error TS1236:   Unable to resolve signature of property decorator when called as an expression.
+        @allRest bar() { }
+        ~~~~~~~~
+!!! error TS1241: Unable to resolve signature of method decorator when called as an expression.
+!!! error TS1241:   Type 'OmniDecorator' has no properties in common with type 'TypedPropertyDescriptor<() => void>'.
+    }
+
+    @oneOptional
+    ~~~~~~~~~~~~
+!!! error TS1238: Unable to resolve signature of class decorator when called as an expression.
+!!! error TS1238:   Type 'OmniDecorator' is not assignable to type 'typeof C'.
+!!! error TS1238:     Type 'OmniDecorator' provides no match for the signature 'new (): C'.
+    class C {
+        @oneOptional foo: any;
+        ~~~~~~~~~~~~
+!!! error TS1329: 'oneOptional' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@oneOptional()'?
+        @oneOptional bar() { }
+        ~~~~~~~~~~~~
+!!! error TS1329: 'oneOptional' accepts too few arguments to be used as a decorator here. Did you mean to call it first and write '@oneOptional()'?
+    }
+
+    @twoOptional
+    ~~~~~~~~~~~~
+!!! error TS1238: Unable to resolve signature of class decorator when called as an expression.
+!!! error TS1238:   Type 'OmniDecorator' is not assignable to type 'typeof D'.
+!!! error TS1238:     Type 'OmniDecorator' provides no match for the signature 'new (): D'.
+    class D {
+        @twoOptional foo: any;
+        ~~~~~~~~~~~~
+!!! error TS1236: The return type of a property decorator function must be either 'void' or 'any'.
+!!! error TS1236:   Unable to resolve signature of property decorator when called as an expression.
+        @twoOptional bar() { }
+        ~~~~~~~~~~~~
+!!! error TS1241: Unable to resolve signature of method decorator when called as an expression.
+!!! error TS1241:   Type 'OmniDecorator' has no properties in common with type 'TypedPropertyDescriptor<() => void>'.
+    }
+
+    @threeOptional
+    ~~~~~~~~~~~~~~
+!!! error TS1238: Unable to resolve signature of class decorator when called as an expression.
+!!! error TS1238:   Type 'OmniDecorator' is not assignable to type 'typeof E'.
+!!! error TS1238:     Type 'OmniDecorator' provides no match for the signature 'new (): E'.
+    class E {
+        @threeOptional foo: any;
+        ~~~~~~~~~~~~~~
+!!! error TS1236: The return type of a property decorator function must be either 'void' or 'any'.
+!!! error TS1236:   Unable to resolve signature of property decorator when called as an expression.
+        @threeOptional bar() { }
+        ~~~~~~~~~~~~~~
+!!! error TS1241: Unable to resolve signature of method decorator when called as an expression.
+!!! error TS1241:   Type 'OmniDecorator' has no properties in common with type 'TypedPropertyDescriptor<() => void>'.
+    }
+
+    @oneOptionalWithRest
+    ~~~~~~~~~~~~~~~~~~~~
+!!! error TS1238: Unable to resolve signature of class decorator when called as an expression.
+!!! error TS1238:   Type 'OmniDecorator' is not assignable to type 'typeof F'.
+!!! error TS1238:     Type 'OmniDecorator' provides no match for the signature 'new (): F'.
+    class F {
+        @oneOptionalWithRest foo: any;
+        ~~~~~~~~~~~~~~~~~~~~
+!!! error TS1236: The return type of a property decorator function must be either 'void' or 'any'.
+!!! error TS1236:   Unable to resolve signature of property decorator when called as an expression.
+        @oneOptionalWithRest bar() { }
+        ~~~~~~~~~~~~~~~~~~~~
+!!! error TS1241: Unable to resolve signature of method decorator when called as an expression.
+!!! error TS1241:   Type 'OmniDecorator' has no properties in common with type 'TypedPropertyDescriptor<() => void>'.
+    }
+
+    @anyDec
+    class G {
+        @anyDec foo: any;
+        @anyDec bar() { }
+    }
+
+    export { };
+