why covariant array?

[Guest0x0]

https://github.com/WebAssembly/gc/blob/main/proposals/gc/MVP.md#composite-types says:

Array types support depth subtyping

array <fieldtype1> <: array <fieldtype2> iff <fieldtype1> <: <fieldtype2>

But arrays are mutable in the proposal and should be invariant.

[vouillon]

See the next line:

• Field types are covariant if they are immutable, invariant otherwise