Missing use-before-def error for redeclared property present in base class

[ajafff]

Bug Report

🔎 Search Terms

• used before extends
• property initialization base
• useDefineForClassFields initialization

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about useDefineForClassFields

⏯ Playground Link

Playground link with relevant code

💻 Code

// @useDeclareForClassFields: true
class Base {
    a = 1;
    b = 1;
    c = 1;
    d = 1;
    e = 1;
}

class Derived extends Base {
    a = 2;
    b;
    declare c: number;
    d: number = this.a /*2*/ + this.b /*undefined*/ + this.c /*1*/ + this.d /*1*/ + this.e /*1*/; // expected error on 'b', all other are valid
    e;

    constructor() {
        super();
        this.b = 2;
        this.e = 2;
    }
}

🙁 Actual behavior

No error in the code above. Use-before-def allows access to properties of the base class, even if the current class redeclares that property.
This is correct until you enable useDefineForClassFields. Then the redeclaration of b overrides the base class property before the access. I added comments containing the runtime values.

🙂 Expected behavior

• a is allowed because it's initialized
• b is a use-before-def error
• c is allowed because of the declare modifier
• d is allowed to access the base class property in the initializer becaues at that point its value is not overridden yet
• e is allowed because its value is overridden after accessing it.

/cc @sandersn

[ajafff]

There's a very similar issue when a base class property is redeclared as parameter property. Note that this one requires target: ESNext AND useDefineForClassFields:

class Base {
    a = 1;
}

class Derived extends Base {
    b = this.a /*undefined*/;

    constructor(public a: number) {
        super();
    }
}

For this case the error logic needs to be chaned to: if emitting native class fields, it's not allowed to use parameter properties in other property's initializers, regardless of the presence of a base class property with the same name.