We already have that situation with unary operators: as described in the thread Optional chaining on prefix operators, optional chaining works with postfix operators x?... but not prefix operators ...x?.
I don’t think it’s surprising to say, “optional chaining takes an optional on the left and a function on the right”.
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My actual use-case for this, besides a general sense that it ought to work for consistency, is dynamic casting:
(t as? T)?.foo() // valid
(t as? T)? == u // error
I’d like to be able to write the latter.