strictFunctionTypes restricts generic type

Question

The problem seems to be specific to how strictFunctionTypes affects generic class type.

Here is a class that closely reproduces what happens and cannot be simplified

Answer 1

This is at it's core an issue of variance. So first a variance primer:

About variance

Given a generic type Foo<T>, and two related types Animal and Dog extends Animal. There are four possible relationships between Foo<Animal> and Foo<Dog>:

1. Covariance - The arrow of inheritance points in the same direction for Foo<Animal> and Foo<Dog> as it does for Animal and Dog, so Foo<Dog> is a sub type of Foo<Animal>, which also means Foo<Dog> is assignable to Foo<Animal>

type CoVariant<T> = () => T
declare var coAnimal: CoVariant<Animal>
declare var coDog: CoVariant<Dog>
coDog = coAnimal; //