abs(Union[int, Decimal]) is inferred as object

I'm afraid I'm not sure whether I'm reporting a bug or requesting a feature. I'm happy to leave that up for you all to triage. I'll start with what I'm seeing and what I'd like to see, and then get a little bit into the why.

Here's a minimal reproduction:

from decimal import Decimal
from typing import Union

def check(x: Decimal, y: Union[Decimal, int]) -> bool:
    return x < -abs(y)

With Python 3.7.3 (from Debian buster) and mypy 0.770 (from PyPI), checking this code returns an error:

deccomp.py:6: error: Unsupported operand type for unary - ("object")  [operator]
Found 1 error in 1 file (checked 1 source file)

The return type of abs is inferred as object, I think because that's the common supertype of int and Decimal. It would be nice if, one way or another, the return type of abs could be inferred as some higher numeric type.

The context here is I'm working on accounting software where I want to be careful to do decimal math throughout. In other words, I never want to deal with the decimal.FloatOperation signal. For functions that do basic arithmetic or comparisons across numbers, it's fine to accept arguments that are either int or Decimal, and it's convenient for callers if I can annotate that argument type rather than requiring them to convert their integer arguments to Decimal all the time.

Inizia con la riproduzione minima in deccomp.py ed eseguila con mypy per confermare che abs(y) venga inferito come object e causi l'errore di unary-minus. Esamina come viene tipizzato abs per Union[Decimal, int]; il lavoro è completato quando l'esempio supera il type-check senza l'errore dell'operando object, preservando al contempo un'aritmetica sicura per Decimal.