Require explicit import or module qualification when extending constructors

[nickrobinson251]

The behaviour for extending functions is:

julia> using Base: run

julia> run(x) = 1
ERROR: error in method definition: function Base.run must be explicitly imported to be extended

i.e. one must instead write Base.run(x) = 1 (or have used import Base: run)

However, the behaviour is different for constructors:

julia> using Base: Int

julia> Int(::String) = 99
Int64

julia> Int("1")
99

I'd have expected this to error, just like the function extension case above, and for Base.Int(::String) = 99 to be required (or import).

I found this behaviour very surprising (given the behaviour for functions).
And ran into it in practice, where some private code was engaging in piracy that i'd expected to not be possible (thinking that importing names with using had protected against this, and not know the constructors had different behaviour).

Is this behaviour intended?
If not, is this something that would be practicable to change in v1.x? (I supposed we'd have to check with PkgEval to what extent this behaviour is relied upon in practice?)

This seems to have been the behaviour in all 1.x releases.

I think it would be preferable for the behaviour of constructors to be consistent with functions.

On Julia Slack, @MasonProtter points out you can also make types callable without qualification, e.g.

julia> (::String)(x) = println("boo!")

julia> "hello"(1)
boo!

[vtjnash]

We may remove the former error, in v2 if not sooner. See #39235

[KristofferC]

Dup of #25744

[nalimilan]

Should we tag #25744 as 2.0 material not to forget it?

[KristofferC]

Sure, why not.