bug(semantic): references on MemberExpression not recorded

[DonIsaac]

enum Foo { // <- foo creates a scope, just like a namespace
  A        // <- has a symbol (SymbolFlags::EnumMember)
}

// Foo has a reference, but A does not
console.log(Foo.A) 

(edit: it appears playground links are broken. When I go to copy a playground URL for this snippet, it takes me to the playground with the default react component snippet)

[armano2]

enums are special

const foo = 2;
const b = 2;
enum Foo {
  outer = foo, // foo == 2
  a = 1, // 1
  b = a, // b == Foo.a == 1  |  shadows const b
  c = b, // c == Foo.b == Foo.c == 1
}

you can read a little about some edge cases at typescript-eslint/typescript-eslint#325

---

typescript enum behaves like a object and block at the same time,

code above compiles to:

const foo = 2;
const b = 2;
var Foo;
(function (Foo) {
    Foo[Foo["outer"] = 2] = "outer";
    Foo[Foo["a"] = 1] = "a";
    Foo[Foo["b"] = 1] = "b";
    Foo[Foo["c"] = 1] = "c";
})(Foo || (Foo = {}));

---

from no unused vars persepctive we should not care if enum values are read or not.

---

MemberExpression can't be easily referenced, as you would need way more data about structures.

eg.

// file1.ts
export enum Foo {
  test = 1,
  unused = 2 // ??
}

// file2.ts
import { Foo } from 'file1.ts'

Foo.test

[overlookmotel]

I agree with @armano2 that the property of a MemberExpression should not be a reference (i.e. the A in Foo.A).

But the issue of what is a symbol is trickier.

In one sense A inside the enum is a symbol. But it doesn't quite map cleanly onto the usual scopes concept. e.g.:

const A = 123;
enum X {
    A = 1,
    B = A, // refers to `A = 1`
}
enum X {
    C = A, // refers to `A = 1`
}

TS playground

If we're treating A in enum X as a symbol, then that symbol should also be bound inside the body of the second enum X.

@armano2 How does typescript-eslint handle this case?

I think we probably do need it to be a symbol to make this kind of thing tractable:

const A = 123;
enum X {
   A = 1,
   B = A, // refers to `A = 1`
}
enum X {
   C = A, // refers to `A = 1`
   D = (() => {
     enum Y {
       Q = A, // refers to `A = 1`
     }
     return Y.Q;
   })(),
}

We don't seem to be getting this quite right at present: Oxc playground

TS playground

[armano2]

we are ignoring this case, and we are "marking" all identifiers in enum's as already in use

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/src/rules/no-unused-vars.ts#L516

[DonIsaac]

I agree that no-unused-vars should ignore this case. I'm hoping to add class/object/interface/etc members to the symbol table later, but that can be for a later time. I'll close this issue.