Merge pull request #18811 from Microsoft/uncalledDecorator

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

src/compiler/checker.ts

@@ -16548,6 +16548,12 @@ 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;
@@ -16560,6 +16566,18 @@ 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 { };
+    

tests/baselines/reference/potentiallyUncalledDecorators.js

@@ -0,0 +1,189 @@
+//// [potentiallyUncalledDecorators.ts]
+// 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;
+declare const anyDec: any;
+
+@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() { }
+}
+
+@anyDec
+class G {
+    @anyDec foo: any;
+    @anyDec bar() { }
+}
+
+export { };
+
+
+//// [potentiallyUncalledDecorators.js]
+var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
+    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
+    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
+    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
+    return c > 3 && r && Object.defineProperty(target, key, r), r;
+};
+class FooComponent {
+}
+__decorate([
+    Input
+], FooComponent.prototype, "foo", void 0);
+class Person {
+}
+__decorate([
+    tracked
+], Person.prototype, "person", void 0);
+class MultiplyByTwo {
+    get multiplied() {
+        return this.args.number * 2;
+    }
+}
+__decorate([
+    tracked('args')
+], MultiplyByTwo.prototype, "multiplied", null);
+let A = class A {
+    bar() { }
+};
+__decorate([
+    noArgs
+], A.prototype, "foo", void 0);
+__decorate([
+    noArgs
+], A.prototype, "bar", null);
+A = __decorate([
+    noArgs
+], A);
+let B = class B {
+    bar() { }
+};
+__decorate([
+    allRest
+], B.prototype, "foo", void 0);
+__decorate([
+    allRest
+], B.prototype, "bar", null);
+B = __decorate([
+    allRest
+], B);
+let C = class C {
+    bar() { }
+};
+__decorate([
+    oneOptional
+], C.prototype, "foo", void 0);
+__decorate([
+    oneOptional
+], C.prototype, "bar", null);
+C = __decorate([
+    oneOptional
+], C);
+let D = class D {
+    bar() { }
+};
+__decorate([
+    twoOptional
+], D.prototype, "foo", void 0);
+__decorate([
+    twoOptional
+], D.prototype, "bar", null);
+D = __decorate([
+    twoOptional
+], D);
+let E = class E {
+    bar() { }
+};
+__decorate([
+    threeOptional
+], E.prototype, "foo", void 0);
+__decorate([
+    threeOptional
+], E.prototype, "bar", null);
+E = __decorate([
+    threeOptional
+], E);
+let F = class F {
+    bar() { }
+};
+__decorate([
+    oneOptionalWithRest
+], F.prototype, "foo", void 0);
+__decorate([
+    oneOptionalWithRest
+], F.prototype, "bar", null);
+F = __decorate([
+    oneOptionalWithRest
+], F);
+let G = class G {
+    bar() { }
+};
+__decorate([
+    anyDec
+], G.prototype, "foo", void 0);
+__decorate([
+    anyDec
+], G.prototype, "bar", null);
+G = __decorate([
+    anyDec
+], G);

tests/baselines/reference/potentiallyUncalledDecorators.symbols

@@ -0,0 +1,200 @@
+=== tests/cases/compiler/potentiallyUncalledDecorators.ts ===
+// Angular-style Input/Output API:
+declare function Input(bindingPropertyName?: string): any;
+>Input : Symbol(Input, Decl(potentiallyUncalledDecorators.ts, 0, 0))
+>bindingPropertyName : Symbol(bindingPropertyName, Decl(potentiallyUncalledDecorators.ts, 1, 23))
+
+class FooComponent {
+>FooComponent : Symbol(FooComponent, Decl(potentiallyUncalledDecorators.ts, 1, 58))
+
+    @Input foo: string;
+>Input : Symbol(Input, Decl(potentiallyUncalledDecorators.ts, 0, 0))
+>foo : Symbol(FooComponent.foo, Decl(potentiallyUncalledDecorators.ts, 2, 20))
+}
+
+// Glimmer-style tracked API:
+declare const tracked: PropertyDecorator & { (...watchedProperties: string[]): any; }
+>tracked : Symbol(tracked, Decl(potentiallyUncalledDecorators.ts, 7, 13))
+>PropertyDecorator : Symbol(PropertyDecorator, Decl(lib.es5.d.ts, --, --))
+>watchedProperties : Symbol(watchedProperties, Decl(potentiallyUncalledDecorators.ts, 7, 46))
+
+class Person {
+>Person : Symbol(Person, Decl(potentiallyUncalledDecorators.ts, 7, 85))
+
+    @tracked person; any;
+>tracked : Symbol(tracked, Decl(potentiallyUncalledDecorators.ts, 7, 13))
+>person : Symbol(Person.person, Decl(potentiallyUncalledDecorators.ts, 9, 14))
+>any : Symbol(Person.any, Decl(potentiallyUncalledDecorators.ts, 10, 20))
+}
+
+class MultiplyByTwo {
+>MultiplyByTwo : Symbol(MultiplyByTwo, Decl(potentiallyUncalledDecorators.ts, 11, 1))
+
+    args: any;
+>args : Symbol(MultiplyByTwo.args, Decl(potentiallyUncalledDecorators.ts, 13, 21))
+
+    @tracked('args')
+>tracked : Symbol(tracked, Decl(potentiallyUncalledDecorators.ts, 7, 13))
+
+    get multiplied() {
+>multiplied : Symbol(MultiplyByTwo.multiplied, Decl(potentiallyUncalledDecorators.ts, 14, 14))
+
+        return this.args.number * 2;
+>this.args : Symbol(MultiplyByTwo.args, Decl(potentiallyUncalledDecorators.ts, 13, 21))
+>this : Symbol(MultiplyByTwo, Decl(potentiallyUncalledDecorators.ts, 11, 1))
+>args : Symbol(MultiplyByTwo.args, Decl(potentiallyUncalledDecorators.ts, 13, 21))
+    }
+}
+
+// Other fun stuff.
+
+interface OmniDecorator extends MethodDecorator, ClassDecorator, PropertyDecorator {
+>OmniDecorator : Symbol(OmniDecorator, Decl(potentiallyUncalledDecorators.ts, 19, 1))
+>MethodDecorator : Symbol(MethodDecorator, Decl(lib.es5.d.ts, --, --))
+>ClassDecorator : Symbol(ClassDecorator, Decl(lib.es5.d.ts, --, --))
+>PropertyDecorator : Symbol(PropertyDecorator, Decl(lib.es5.d.ts, --, --))
+}
+
+declare function noArgs(): OmniDecorator;
+>noArgs : Symbol(noArgs, Decl(potentiallyUncalledDecorators.ts, 24, 1))
+>OmniDecorator : Symbol(OmniDecorator, Decl(potentiallyUncalledDecorators.ts, 19, 1))
+
+declare function allRest(...args: any[]): OmniDecorator;
+>allRest : Symbol(allRest, Decl(potentiallyUncalledDecorators.ts, 26, 41))
+>args : Symbol(args, Decl(potentiallyUncalledDecorators.ts, 27, 25))
+>OmniDecorator : Symbol(OmniDecorator, Decl(potentiallyUncalledDecorators.ts, 19, 1))
+
+declare function oneOptional(x?: any): OmniDecorator;
+>oneOptional : Symbol(oneOptional, Decl(potentiallyUncalledDecorators.ts, 27, 56))
+>x : Symbol(x, Decl(potentiallyUncalledDecorators.ts, 28, 29))
+>OmniDecorator : Symbol(OmniDecorator, Decl(potentiallyUncalledDecorators.ts, 19, 1))
+
+declare function twoOptional(x?: any, y?: any): OmniDecorator;
+>twoOptional : Symbol(twoOptional, Decl(potentiallyUncalledDecorators.ts, 28, 53))
+>x : Symbol(x, Decl(potentiallyUncalledDecorators.ts, 29, 29))
+>y : Symbol(y, Decl(potentiallyUncalledDecorators.ts, 29, 37))
+>OmniDecorator : Symbol(OmniDecorator, Decl(potentiallyUncalledDecorators.ts, 19, 1))
+
+declare function threeOptional(x?: any, y?: any, z?: any): OmniDecorator;
+>threeOptional : Symbol(threeOptional, Decl(potentiallyUncalledDecorators.ts, 29, 62))
+>x : Symbol(x, Decl(potentiallyUncalledDecorators.ts, 30, 31))
+>y : Symbol(y, Decl(potentiallyUncalledDecorators.ts, 30, 39))
+>z : Symbol(z, Decl(potentiallyUncalledDecorators.ts, 30, 48))
+>OmniDecorator : Symbol(OmniDecorator, Decl(potentiallyUncalledDecorators.ts, 19, 1))
+
+declare function oneOptionalWithRest(x?: any, ...args: any[]): OmniDecorator;
+>oneOptionalWithRest : Symbol(oneOptionalWithRest, Decl(potentiallyUncalledDecorators.ts, 30, 73))
+>x : Symbol(x, Decl(potentiallyUncalledDecorators.ts, 31, 37))
+>args : Symbol(args, Decl(potentiallyUncalledDecorators.ts, 31, 45))
+>OmniDecorator : Symbol(OmniDecorator, Decl(potentiallyUncalledDecorators.ts, 19, 1))
+
+declare const anyDec: any;
+>anyDec : Symbol(anyDec, Decl(potentiallyUncalledDecorators.ts, 32, 13))
+
+@noArgs
+>noArgs : Symbol(noArgs, Decl(potentiallyUncalledDecorators.ts, 24, 1))
+
+class A {
+>A : Symbol(A, Decl(potentiallyUncalledDecorators.ts, 32, 26))
+
+    @noArgs foo: any;
+>noArgs : Symbol(noArgs, Decl(potentiallyUncalledDecorators.ts, 24, 1))
+>foo : Symbol(A.foo, Decl(potentiallyUncalledDecorators.ts, 35, 9))
+
+    @noArgs bar() { }
+>noArgs : Symbol(noArgs, Decl(potentiallyUncalledDecorators.ts, 24, 1))
+>bar : Symbol(A.bar, Decl(potentiallyUncalledDecorators.ts, 36, 21))
+}
+
+@allRest
+>allRest : Symbol(allRest, Decl(potentiallyUncalledDecorators.ts, 26, 41))
+
+class B {
+>B : Symbol(B, Decl(potentiallyUncalledDecorators.ts, 38, 1))
+
+    @allRest foo: any;
+>allRest : Symbol(allRest, Decl(potentiallyUncalledDecorators.ts, 26, 41))
+>foo : Symbol(B.foo, Decl(potentiallyUncalledDecorators.ts, 41, 9))
+
+    @allRest bar() { }
+>allRest : Symbol(allRest, Decl(potentiallyUncalledDecorators.ts, 26, 41))
+>bar : Symbol(B.bar, Decl(potentiallyUncalledDecorators.ts, 42, 22))
+}
+
+@oneOptional
+>oneOptional : Symbol(oneOptional, Decl(potentiallyUncalledDecorators.ts, 27, 56))
+
+class C {
+>C : Symbol(C, Decl(potentiallyUncalledDecorators.ts, 44, 1))
+
+    @oneOptional foo: any;
+>oneOptional : Symbol(oneOptional, Decl(potentiallyUncalledDecorators.ts, 27, 56))
+>foo : Symbol(C.foo, Decl(potentiallyUncalledDecorators.ts, 47, 9))
+
+    @oneOptional bar() { }
+>oneOptional : Symbol(oneOptional, Decl(potentiallyUncalledDecorators.ts, 27, 56))
+>bar : Symbol(C.bar, Decl(potentiallyUncalledDecorators.ts, 48, 26))
+}
+
+@twoOptional
+>twoOptional : Symbol(twoOptional, Decl(potentiallyUncalledDecorators.ts, 28, 53))
+
+class D {
+>D : Symbol(D, Decl(potentiallyUncalledDecorators.ts, 50, 1))
+
+    @twoOptional foo: any;
+>twoOptional : Symbol(twoOptional, Decl(potentiallyUncalledDecorators.ts, 28, 53))
+>foo : Symbol(D.foo, Decl(potentiallyUncalledDecorators.ts, 53, 9))
+
+    @twoOptional bar() { }
+>twoOptional : Symbol(twoOptional, Decl(potentiallyUncalledDecorators.ts, 28, 53))
+>bar : Symbol(D.bar, Decl(potentiallyUncalledDecorators.ts, 54, 26))
+}
+
+@threeOptional
+>threeOptional : Symbol(threeOptional, Decl(potentiallyUncalledDecorators.ts, 29, 62))
+
+class E {
+>E : Symbol(E, Decl(potentiallyUncalledDecorators.ts, 56, 1))
+
+    @threeOptional foo: any;
+>threeOptional : Symbol(threeOptional, Decl(potentiallyUncalledDecorators.ts, 29, 62))
+>foo : Symbol(E.foo, Decl(potentiallyUncalledDecorators.ts, 59, 9))
+
+    @threeOptional bar() { }
+>threeOptional : Symbol(threeOptional, Decl(potentiallyUncalledDecorators.ts, 29, 62))
+>bar : Symbol(E.bar, Decl(potentiallyUncalledDecorators.ts, 60, 28))
+}
+
+@oneOptionalWithRest
+>oneOptionalWithRest : Symbol(oneOptionalWithRest, Decl(potentiallyUncalledDecorators.ts, 30, 73))
+
+class F {
+>F : Symbol(F, Decl(potentiallyUncalledDecorators.ts, 62, 1))
+
+    @oneOptionalWithRest foo: any;
+>oneOptionalWithRest : Symbol(oneOptionalWithRest, Decl(potentiallyUncalledDecorators.ts, 30, 73))
+>foo : Symbol(F.foo, Decl(potentiallyUncalledDecorators.ts, 65, 9))
+
+    @oneOptionalWithRest bar() { }
+>oneOptionalWithRest : Symbol(oneOptionalWithRest, Decl(potentiallyUncalledDecorators.ts, 30, 73))
+>bar : Symbol(F.bar, Decl(potentiallyUncalledDecorators.ts, 66, 34))
+}
+
+@anyDec
+>anyDec : Symbol(anyDec, Decl(potentiallyUncalledDecorators.ts, 32, 13))
+
+class G {
+>G : Symbol(G, Decl(potentiallyUncalledDecorators.ts, 68, 1))
+
+    @anyDec foo: any;
+>anyDec : Symbol(anyDec, Decl(potentiallyUncalledDecorators.ts, 32, 13))
+>foo : Symbol(G.foo, Decl(potentiallyUncalledDecorators.ts, 71, 9))
+
+    @anyDec bar() { }
+>anyDec : Symbol(anyDec, Decl(potentiallyUncalledDecorators.ts, 32, 13))
+>bar : Symbol(G.bar, Decl(potentiallyUncalledDecorators.ts, 72, 21))
+}
+
+export { };
+

tests/baselines/reference/potentiallyUncalledDecorators.types

@@ -0,0 +1,206 @@
+=== tests/cases/compiler/potentiallyUncalledDecorators.ts ===
+// Angular-style Input/Output API:
+declare function Input(bindingPropertyName?: string): any;
+>Input : (bindingPropertyName?: string) => any
+>bindingPropertyName : string
+
+class FooComponent {
+>FooComponent : FooComponent
+
+    @Input foo: string;
+>Input : (bindingPropertyName?: string) => any
+>foo : string
+}
+
+// Glimmer-style tracked API:
+declare const tracked: PropertyDecorator & { (...watchedProperties: string[]): any; }
+>tracked : PropertyDecorator & ((...watchedProperties: string[]) => any)
+>PropertyDecorator : PropertyDecorator
+>watchedProperties : string[]
+
+class Person {
+>Person : Person
+
+    @tracked person; any;
+>tracked : PropertyDecorator & ((...watchedProperties: string[]) => any)
+>person : any
+>any : any
+}
+
+class MultiplyByTwo {
+>MultiplyByTwo : MultiplyByTwo
+
+    args: any;
+>args : any
+
+    @tracked('args')
+>tracked('args') : any
+>tracked : PropertyDecorator & ((...watchedProperties: string[]) => any)
+>'args' : "args"
+
+    get multiplied() {
+>multiplied : number
+
+        return this.args.number * 2;
+>this.args.number * 2 : number
+>this.args.number : any
+>this.args : any
+>this : this
+>args : any
+>number : any
+>2 : 2
+    }
+}
+
+// Other fun stuff.
+
+interface OmniDecorator extends MethodDecorator, ClassDecorator, PropertyDecorator {
+>OmniDecorator : OmniDecorator
+>MethodDecorator : MethodDecorator
+>ClassDecorator : ClassDecorator
+>PropertyDecorator : PropertyDecorator
+}
+
+declare function noArgs(): OmniDecorator;
+>noArgs : () => OmniDecorator
+>OmniDecorator : OmniDecorator
+
+declare function allRest(...args: any[]): OmniDecorator;
+>allRest : (...args: any[]) => OmniDecorator
+>args : any[]
+>OmniDecorator : OmniDecorator
+
+declare function oneOptional(x?: any): OmniDecorator;
+>oneOptional : (x?: any) => OmniDecorator
+>x : any
+>OmniDecorator : OmniDecorator
+
+declare function twoOptional(x?: any, y?: any): OmniDecorator;
+>twoOptional : (x?: any, y?: any) => OmniDecorator
+>x : any
+>y : any
+>OmniDecorator : OmniDecorator
+
+declare function threeOptional(x?: any, y?: any, z?: any): OmniDecorator;
+>threeOptional : (x?: any, y?: any, z?: any) => OmniDecorator
+>x : any
+>y : any
+>z : any
+>OmniDecorator : OmniDecorator
+
+declare function oneOptionalWithRest(x?: any, ...args: any[]): OmniDecorator;
+>oneOptionalWithRest : (x?: any, ...args: any[]) => OmniDecorator
+>x : any
+>args : any[]
+>OmniDecorator : OmniDecorator
+
+declare const anyDec: any;
+>anyDec : any
+
+@noArgs
+>noArgs : () => OmniDecorator
+
+class A {
+>A : A
+
+    @noArgs foo: any;
+>noArgs : () => OmniDecorator
+>foo : any
+
+    @noArgs bar() { }
+>noArgs : () => OmniDecorator
+>bar : () => void
+}
+
+@allRest
+>allRest : (...args: any[]) => OmniDecorator
+
+class B {
+>B : B
+
+    @allRest foo: any;
+>allRest : (...args: any[]) => OmniDecorator
+>foo : any
+
+    @allRest bar() { }
+>allRest : (...args: any[]) => OmniDecorator
+>bar : () => void
+}
+
+@oneOptional
+>oneOptional : (x?: any) => OmniDecorator
+
+class C {
+>C : C
+
+    @oneOptional foo: any;
+>oneOptional : (x?: any) => OmniDecorator
+>foo : any
+
+    @oneOptional bar() { }
+>oneOptional : (x?: any) => OmniDecorator
+>bar : () => void
+}
+
+@twoOptional
+>twoOptional : (x?: any, y?: any) => OmniDecorator
+
+class D {
+>D : D
+
+    @twoOptional foo: any;
+>twoOptional : (x?: any, y?: any) => OmniDecorator
+>foo : any
+
+    @twoOptional bar() { }
+>twoOptional : (x?: any, y?: any) => OmniDecorator
+>bar : () => void
+}
+
+@threeOptional
+>threeOptional : (x?: any, y?: any, z?: any) => OmniDecorator
+
+class E {
+>E : E
+
+    @threeOptional foo: any;
+>threeOptional : (x?: any, y?: any, z?: any) => OmniDecorator
+>foo : any
+
+    @threeOptional bar() { }
+>threeOptional : (x?: any, y?: any, z?: any) => OmniDecorator
+>bar : () => void
+}
+
+@oneOptionalWithRest
+>oneOptionalWithRest : (x?: any, ...args: any[]) => OmniDecorator
+
+class F {
+>F : F
+
+    @oneOptionalWithRest foo: any;
+>oneOptionalWithRest : (x?: any, ...args: any[]) => OmniDecorator
+>foo : any
+
+    @oneOptionalWithRest bar() { }
+>oneOptionalWithRest : (x?: any, ...args: any[]) => OmniDecorator
+>bar : () => void
+}
+
+@anyDec
+>anyDec : any
+
+class G {
+>G : G
+
+    @anyDec foo: any;
+>anyDec : any
+>foo : any
+
+    @anyDec bar() { }
+>anyDec : any
+>bar : () => void
+}
+
+export { };
+

tests/cases/compiler/potentiallyUncalledDecorators.ts

@@ -0,0 +1,81 @@
+// @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;
+declare const anyDec: any;
+
+@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() { }
+}
+
+@anyDec
+class G {
+    @anyDec foo: any;
+    @anyDec bar() { }
+}
+
+export { };