🐞 bug report

Affected Package

@angular/core, ES2015 application; ReflectionCapabilities

Is this a regression?

Nope, I think it's an issue that has been there for a long time.

Description

Consider you have the following TypeScript file. Component A extends another component B, but
does have a public property defined in A.

Problem is that TypeScript in ES2015 mode creates a pass-through constructor just in order to initialize the public property that causes the compiler to not inherit metadata from B.

Here is the code (below some more details about the issue)

@Component({
   ...
})
export class CdkBaseComponent {
   constructor (myService: SomeService) { ... }
}

@Component({
   ...
})
export class EnhancedComponent extends CdkBaseComponent {
   somePublicProperty = "my_default_value"
}

This TypeScript code turns into:

// ...

export class EnhancedComponent extends CdkBaseComponent {
    constructor() {
        super(...arguments);
        this.somePublicProperty = "my_default_value";
    }
}
EnhancedComponent.decorators = [
    { type: Component, args: [{
                template: 'Enhanced Component Template',
            },] },
];

As you can see, TypeScript generated a pass-through constructor that just passes ...arguments to the superclass. This means that @angular/core detects a constructor for the class and doesn't bother checking the superclass metadata.

Maybe there should be a ES2015 variant for delegate constructors?

🔬 Minimal Reproduction

Code above; or some easy to check-out TypeScript file with tsconfig.

https://gist.github.com/DevVersion/c84f196d68a549e3cbbaacdfccd3aab5

Run ngc -p tsconfig-test.json and inspect output.

🌍 Your Environment

Angular Version:

7.1.0

Anything else relevant?

This is an issue we run into with Angular Material and the CDK.

Potential abandoned fix or related issues:

• Refactor ReflectionCapabilities to rely on metadata for identifying 'pass-through' constructors #22642
• Inheriting constructor with Jit mode fails when using closure compiler or property initializers #13211

A workaround would be to explicitly create a constructor with parameters, but in the long-term, this makes the source-code unnecessarily large.

cc. @jelbourn