Pedantic: deny mutable type generalization

Search Terms

type generalization
type expanding

Suggestion

Currently we have a vulnerability in type safety of mutable variables. You can generalize the type of some variable and change its value by passing there something not compatible with the old type. Examples:

const a: Array<number> = [1, 2]

const b: Array<number | string> = a

b.push('a is not array of numbers anymore')

const a = { data: 0 }

function b (param: { data: string | number }) {
  param.data = 'a is broken now'
}

b(a)

So I suggest adding a new compiler option under 'pedantic' label which will deny such type conversions:

const a: Array<number> = [1, 2]

// Error: type 'Array<number>' is not assignable to 'Array<number | string>'
const b: Array<number | string> = a

const a: Array<number> = [1, 2]

// OK: 'ReadonlyArray' won't let us mutate the state of 'a'
const b: ReadonlyArray<number | string> = a

const a = { foo: 0, bar: 'string' }

function b (param: { readonly foo: number | string, bar: string }) {
  param.bar = String(param.foo)
}

// OK
// property 'foo: number' is assignable to 'readonly foo: number | string'
// property 'bar: string' is assignable to 'bar: string'
b(a) 

function c (param: { readonly foo: number | string, bar: number | string }) {
  param.bar = String(param.foo)
}

// Error: property 'bar: string' is not assignable to 'bar: number | string'
c(a)

La issue non indica alcun file né alcun punto di ingresso dei test. Inizia esaminando come vengono gestite le opzioni del compilatore e l’assegnabilità, quindi confronta gli esempi di array e oggetti mutabili con i casi readonly. Il lavoro è completato quando un’opzione pedantic rifiuta le generalizzazioni mutabili non sicure consentendo al contempo le conversioni readonly descritte.