Result of calling langlib methods on immutable values

[MaryamZi]

Description:
When langlib methods that create new values are called on immutable values, will the resultant value also be immutable?

For example, where arr is

int[] & readonly arr = [1, 2, 1];

should the static type and the inherent type of the value created by

arr.filter(val => val > 1)

be int[] or int[] & readonly?

Based on #551 (comment), do we only consider the element type and not the read-only bit of the array type itself?

Related Issues:
#391
#551

[jclark]

A langlib function like filter is parameterized the member type, so the static type of the collection wouldn't pick up the readonly (unless we do #521). But readonly does get pushed down to the member type. So, for example, if the static type T of the array is readonly & (json[]), then the most precise type M such that the T is a subtype of M[] is would make M be readonly & json (since readonly & (json[]) is subtype of (readonly & json)[]).

So in general if the argument type is T[] & readonly the static type and inherent type of the return value would be (readonly & T)[].

[jclark]

#187 should say something about this.

[MaryamZi]

Noted, thank you.