:: is sometimes a variable name

[nbaum]

:: behaves like an identifier except it can't be assigned to.

f(::) = :: works and defines the identity function.

f(::Any) = :: works, but raises UndefVarError when called.

:: = 42 is invalid syntax. (Unexpected "=".)

I think that :: should be semantically equivalent to ::Any although possibly not parsed the same: macros might find the difference useful.

[simonster]

(::)(::) = (::)(::)
(::)((::)::typeof(::)) = ::
((::)(::)(::)::typeof(::))(Type){((::)(::)(::)::typeof(::))(typeof(typeof(::)))(typeof((::)(::)))}

[nbaum]

Your scientists were so preoccupied with whether or not they could, they didn’t stop to think if they should.

[fredrikekre]

same as #18912 and fixed by #22947