Poor type inference for `reduce`

TypeScript Version: 3.0.0-dev.20180705

Code

function toStrings(arr: ReadonlyArray<object>): string[] {
	return arr.reduce((acc, obj) => {
		acc.push(obj.toString());
		return acc;
	}, [] as string[]);
}

Expected behavior:

No error.

Actual behavior:

src/a.ts:2:2 - error TS2322: Type 'object' is not assignable to type 'string[]'.
  Property 'length' is missing in type '{}'.

2  return arr.reduce((acc, obj) => {
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3  	acc.push(obj.toString());
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
4  	return acc;
  ~~~~~~~~~~~~~
5  }, [] as string[]);
  ~~~~~~~~~~~~~~~~~~~~

src/a.ts:3:7 - error TS2339: Property 'push' does not exist on type 'object'.

3  	acc.push(obj.toString());

No error if arr is ReadonlyArray<number> or some other non-object type.
No error if I explicitly specify arr.reduce<string[]>.
No error if I remove the first two overloads to reduce, which are non-generic.

Start by reproducing the reported TypeScript 3.0 behavior with the provided toStrings example, then inspect how the non-generic reduce overloads are selected for ReadonlyArray. Compare the result with ReadonlyArray, an explicit string[] type argument, and the overloads removed. Done means the original example reports no error without regressing those comparisons.