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Issue Description:

Quick Help in Xcode does not show type information for closure params, but does for usages of the param within the closure body.

Affected Xcode versions: This has been the case since Swift was released AFAIK, I never remember it working, but definitely Xcode 10, 11 & 12.

Example:

let _: (Int) -> Void = { num in // <- option-clicking 'num' here does not show type info
    print(num) // <- option-clicking here does show type info
}

Option-clicking closure params is something I want to do very often, as a means of exploring types of unfamiliar APIs, or to know which order the params come in, e.g. "does [1, 2, 3].enumerated() return (index, num) or (num, index)?"

Workarounds include: using the parameter in the function body and option-clicking, or manually looking up the documentation for the function.

Screenshots:

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