#39258's stricter assignability breaks vue-next

[sandersn]

This may be intended, but I want a bug to track this in case others run into it.

// @strict: true
export interface Ref<T = any> { value: T }
type UnwrapRef<T> = T extends Ref<infer V> ? V : never
declare function readonly<T>(target: T): UnwrapRef<T>

const toReadonly = <T extends unknown>(value: T): T =>
    // readonly(value as T & Record<any,any>)
  isObject(value) ? readonly(value) : value

declare function isObject(val: unknown): val is Record<any, any>

This is an error without --strict on 4.0, but not with --strict. In 4.1, it's an error for both.

Expected behavior:

No error and value: Record<any, any> in readonly(value).

Actual behavior:

Error and value: T & Record<any, any> in readonly(value):

Type 'T | UnwrapRef<T & Record<any, any>>' is not assignable to type 'T'.
  'T | UnwrapRef<T & Record<any, any>>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'.
    Type 'UnwrapRef<T & Record<any, any>>' is not assignable to type 'T'.
      'UnwrapRef<T & Record<any, any>>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'.
        Type 'unknown' is not assignable to type 'T'.
          'unknown' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'.

Also I'm not sure where the last two lines of the error come from. They don't seem to follow from the previous lines.

Workaround

readonly(value as Record<any, any>) to restore the old type of value: casting to subtype is allowed.

[typescript-bot]

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in this issue running against the nightly TypeScript. If something changes, I will post a new comment.

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Issue body code block by @sandersn

❌ Failed: -

• Type 'T | UnwrapRef<T & Record<any, any>>' is not assignable to type 'T'. 'T | UnwrapRef<T & Record<any, any>>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'. Type 'UnwrapRef<T & Record<any, any>>' is not assignable to type 'T'. 'UnwrapRef<T & Record<any, any>>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'. Type 'unknown' is not assignable to type 'T'. 'unknown' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'.

Historical Information

Issue body code block by @sandersn

Version	Reproduction Outputs
Nightly	❌ Failed: - Type 'T | UnwrapRef<T & Record<any, any>>' is not assignable to type 'T'. 'T | UnwrapRef<T & Record<any, any>>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'. Type 'UnwrapRef<T & Record<any, any>>' is not assignable to type 'T'. 'UnwrapRef<T & Record<any, any>>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'. Type 'unknown' is not assignable to type 'T'. 'unknown' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'unknown'.
3.5.3, 3.6.4, 3.7.5, 3.8.3, 3.9.5	👍 Compiled

[orta]

So, we think this is a real compiler error that used to be masked - 4.0 and and below would effectively give an any for the T side of the compiler error previously

const toReadonly = <T extends unknown>(value: T): T =>
    // readonly(value as T & Record<any,any>)
  isObject(value) ? readonly(value) : value

Effectively gives either { value: 123} or 123 which aren't assignable - thus the error is legit 👍🏻

[sandersn]

My workaround also made it into vue-next yesterday: vuejs/core#2219