@tem I find the answer. I didn't read through SE-0035 carefully. It says the following (highlighting is added by me):
I propose we make it so that implicitly capturing an
inout
parameter into an escapable closure is an error.
Capturing an
inout
parameter, includingself
in amutating
method, becomes an error in an escapable closure literal, unless the capture is made explicit (and thereby immutable)
That explains it!