Regression: ERROR TS2564: Property 'module/A#c' has no initializer and is not assigned in the constructor before 'this' is used or returned.

[jtenner]

Link: here

Minimal reproduction: (as far as I can reduce it)

class B {}
class A {
  b: B;
  c: B;
  constructor() {
    // this should be safe?
    this.b = new B();
    this.c = this.b;
  }

}

export function fib(n: i32): i32 {
  let a = new A();
  return 0;
}

Error:
;; ERROR TS2564: Property 'module/A#c' has no initializer and is not assigned in the constructor before 'this' is used or returned.
;;    :
;;  4 │ c: B;
;;    │ ~
;;    └─ in module.ts(4,3)
;;    :
;;  7 │ this.c = this.b;
;;    │          ~~~~~~
;;    └─ in module.ts(7,14)

[dcodeIO]

This is in fact a relatively strict check atm, but it is intended. Imagine that this.b is a property that executes arbitrary code (accessing this) when accessed, then in that code this.c has not yet been assigned.

Edit: Makes me realize that this.b = ... before is similar, but doesn't diagnose an error. I guess what we need there is to detect if something is an actual getter or setter, and only then diagnose 🤔

[ppppqp]

I was tracking this bug for some time and realized it is actually a feature #1349. In brief, the compiler checks if the class is fully initialized before handling a call on this in the constructor. It is indeed a different behavior than Typescript (so a bit confusing), but as mentioned in the above PR, it can be overcome with an assertion.

class B {}
class A {
  b: B;
  c!: B; // add assertion
  constructor() {
    this.b = new B();
    this.c = this.b;
  }

}

/** Calculates the n-th Fibonacci number. */
export function fib(n: i32): i32 {
  let a = new A();
  return 0;
}

This compiles just fine. A smarter way to handle this is to only check the accessed properties rather than all properties on this. What do you think?