Generic object can be widened unsafely

[awerlogus]

Bug Report

🔎 Search Terms generic type object wide

🕗 Version & Regression Information 4.4.0-dev.20210805

💻 Code

const obj = { prop: 5, field: '' }


// Expected error: other properties of 'data' can have types of not a number
// Currently: no error
const func1 = <T extends { prop: number }>(data: T): Record<string, number> => data

// No error
const result1 = func1(obj)

// Got: number | undefined
// Real value: string
const field  = result1.field


// Type checker must make us rewrite func1 into
const func2 = <T extends { prop: number } & Record<string, number>>(data: T): Record<string, number> => data

// Error as expected: Type '{ prop: number; field: string; }' is not assignable to type 'Record<string, number>'.
const result2 = func2(obj)

🙁 Actual behavior

T extends { prop: number } is assignable to Record<string, number>

🙂 Expected behavior

T extends { prop: number } is not assignable to Record<string, number>

[RyanCavanaugh]

Record<string, number> has the same soundness hole as optional properties. I don't see how we could change this without breaking a huge amount of working code.

[awerlogus]

How about adding some new tsconfig option?

[fatcerberus]

I don’t think such an option would be feasible. T extends X means precisely “T is assignable to X”. If { prop: number } were not assignable to Record<string, number> then you also couldn’t do this:

const rec: Record<string, number> = {
    prop: 42
};

Or for that matter, even initializing it with {} would have to be an error (because every object is assignable to {}).

[awerlogus]

@fatcerberus it's incorrect to say that if A is assignable to B and B is assignable to C, then A is always assignable to C.

Counterexample:
A: { a: number, b: string }
B: { a: number }
C: Record<string, number>

[fatcerberus]

As Ryan pointed out, the same kind of unsoundness exists for all types with optional properties, so this isn’t unique to Record.

[awerlogus]

Yes, and this doesn't mean that we can leave everything as it is because of it. It just means that the problem, needs to be solved, simply takes on a larger outline.

[awerlogus]

Duplicate of #39915