Is it OK for a GAIT's nonisolated member to access its mutable isolated member?

Using Swift
3

FWIW, the behavior was defined in SE-0434:

First, let's consider the stored properties of struct s isolated to global actors. let properties of such types are implicitly treated as nonisolated within the current module if they have Sendable type, but var properties are not.
...
However, there is nothing unsafe about treating x as nonisolated . ... So, first off, there's no reason for Swift to require (unsafe) when marking x nonisolated .
...
We can do better than that, though. It should be possible to treat a var stored property of a global-actor-isolated value type as implicitly nonisolated under the same conditions that a let property can be.

A simpler example:

@MainActor struct S {
    var value: Int = 1

    nonisolated mutating func increase() {
        value += 1
    }
}