define method `Base.convert(::Type{Nothing}, ::MyType)` causes latency/recompilation when `using` in second time

[songjhaha]

Here is a small example to reproudce this problem:

1. create a package and define a new Type;
2. define convert method for convert this new Type to Type Nothing;
3. estimate loading time with using MyPkg twice, the second time using is very very slow and looks like doing recompilation;

Julia Version: 1.9.3
OS: I reproduce this problem in both Win10 && Unbuntu

module MyPkg

struct MyCls end

Base.convert(::Type{Nothing}, x::MyCls) = 1

end # module MyPkg

julia> @time using MyPkg
  0.015626 seconds (8.70 k allocations: 582.849 KiB)

julia> @time using MyPkg
  2.311770 seconds (1.30 M allocations: 91.081 MiB, 3.68% gc time, 99.92% compilation time: 100% of which was recompilation)

[johnnychen94]

This doesn't occur at Julia 1.10.0-beta2. Possibly already fixed by #49349

using SnoopCompileCore
invalidations = @snoor using myy

using SnoopCompile
length(uinvalidated(invalidations)) 

There were 2659 invalidations in Julia 1.9.3, and only 3 invalidations in Julia 1.10.0-beta2.
I believe the latency for the second loading is due to these invalidations.

See also some pre-1.9 PRs:

• fix invalidations due to convert(Nothing, ::Any) #39434
• make convert(Union{},x) directly ambiguous #46000

[KristofferC]

You also wouldn't notice this in 1.10 due to #49525