Key Path Expressions as Functions and Identity Key Path

The SE-0249 Proposal gives the following code as an example, but it doesn't compile.

[1, nil, 3, nil, 5].compactMap(\.self)
// error: cannot convert value of type 'WritableKeyPath<Int, Int>' to expected argument type '(Int?) throws -> ElementOfResult?'

Is this a known bug?

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Yes, it's a known limitation, but I don't have a bug number offhand. I'll let others fill in the "why".

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SR-12897 is the bug you are looking for.

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Duplicate:

https://bugs.swift.org/browse/SR-12387

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