Sorting an array removes type inference

TypeScript Version: 3.7.0-dev.20191011

Search Terms: type inference

Code

function getAllValuesUnsorted(m: ReadonlyArray<ReadonlyArray<string>>): ReadonlyArray<string> {
	// OK
	return Array.from(flatten(m.values()))
}

function getAllValuesSorted(m: ReadonlyArray<ReadonlyArray<string>>): ReadonlyArray<string> {
	// Type 'unknown[]' is not assignable to type 'readonly string[]'.
	return Array.from(flatten(m.values())).sort()
}

function* flatten<T>(a: Iterable<Iterable<T>>): Iterable<T> {
	for (const xs of a)
		for (const x of xs)
			yield x
}

Expected behavior:

No error.

Actual behavior:

a.ts:6:2 - error TS2322: Type 'unknown[]' is not assignable to type 'readonly string[]'.
  Type 'unknown' is not assignable to type 'string'.

Start with the TypeScript compiler's type inference and generic array method handling, using the reproducer in the issue as the first test case. Investigate why adding sort() changes the inferred result from string[] to unknown[], then add a regression test that accepts the sorted expression without an error.