Specificity alteration under precompilation

[timholy]

Noticed in JuliaGraphics/Gtk.jl#311. Here's a reasonably short repro:

__precompile__(false)

module Convert

import Base: convert

export GBoxed

abstract type GBoxed  end

convert(::Type{GBoxed}, unbox::Ptr{GBoxed}) = GBoxedUnkown(unbox)
convert{T <: GBoxed}(::Type{GBoxed}, unbox::Ptr{T}) = GBoxedUnkown(unbox)
convert{T <: GBoxed}(::Type{T}, unbox::Ptr{GBoxed}) = convert(T, convert(Ptr{T}, unbox))
convert{T <: GBoxed}(::Type{T}, unbox::Ptr{T}) = T(unbox)

abstract type GdkEvent <: GBoxed end

convert(::Type{GdkEvent}, evt::Ptr{GdkEvent}) = -1

end

Because the method tables for convert are very long, this filter_gboxed.jl script is handy:

using Convert

isgboxed_method(m::Method) = _isgboxed(Base.unwrap_unionall(m.sig).parameters[2])
_isgboxed(::Type{Type{T}}) where T<:GBoxed = true
function _isgboxed(t::Type)
    p = Base.unwrap_unionall(t).parameters
    length(p) != 1 && return false
    _isgboxed(p[1])
end
_isgboxed(TV::TypeVar) = TV.ub <: GBoxed
_isgboxed(arg) = false

mt = methods(convert)
collect(Iterators.filter(isgboxed_method, mt))

As written, the output is

5-element Array{Any,1}:
 convert(::Type{Convert.GdkEvent}, evt::Ptr{Convert.GdkEvent}) in Convert at /tmp/Convert.jl:18         
 convert(::Type{T}, unbox::Ptr{T}) where T<:Convert.GBoxed in Convert at /tmp/Convert.jl:14             
 convert(::Type{Convert.GBoxed}, unbox::Ptr{Convert.GBoxed}) in Convert at /tmp/Convert.jl:11           
 convert(::Type{Convert.GBoxed}, unbox::Ptr{T}) where T<:Convert.GBoxed in Convert at /tmp/Convert.jl:12
 convert(::Type{T}, unbox::Ptr{Convert.GBoxed}) where T<:Convert.GBoxed in Convert at /tmp/Convert.jl:13

However, if you change it to __precompile__(true) then I get

 convert(::Type{Convert.GBoxed}, unbox::Ptr{Convert.GBoxed}) in Convert at /tmp/Convert.jl:11           
 convert(::Type{T}, unbox::Ptr{T}) where T<:Convert.GBoxed in Convert at /tmp/Convert.jl:14             
 convert(::Type{Convert.GdkEvent}, evt::Ptr{Convert.GdkEvent}) in Convert at /tmp/Convert.jl:18         
 convert(::Type{Convert.GBoxed}, unbox::Ptr{T}) where T<:Convert.GBoxed in Convert at /tmp/Convert.jl:12
 convert(::Type{T}, unbox::Ptr{Convert.GBoxed}) where T<:Convert.GBoxed in Convert at /tmp/Convert.jl:13

The second method should come after the third.

[timholy]

Seems similar to #21382.