Initialization problems with `preferunits` for physical constants on Julia 1.8

Hi, I struggle with a subtle issue concerning the definition of physical constants in unitful.jl after preferunits has been set.

I set up a small package on https://github.com/j-fu/UTest.jl which can be dev'd for testing.

In the root directory it contains utest.jl which can be called either with

julia --project=. --compile=min utest.jl`

or with

julia --project=. utest.jl`

The correct output is

Unitful.upreferred(1 * u"m") = 100 cm
ustrip(upreferred(u"ε0")) = 8.85418781762039e-18
ustrip(upreferred(Unitful.ε0)) = 8.85418781762039e-18
ustrip(upreferred(PhysicalConstants.CODATA2018.ε_0)) = 8.8541878128e-18

This is obtained on all older Julia versions, and on 1.8 (beta, rc3) with --compile=min

On 1.8 (beta, rc3) and 1.9 (f1991edd30) without --compile=min one gets

Unitful.upreferred(1 * u"m") = 100 cm
ustrip(upreferred(u"ε0")) = 8.85418781762039e-12
ustrip(upreferred(Unitful.ε0)) = 8.85418781762039e-12
ustrip(upreferred(PhysicalConstants.CODATA2018.ε_0)) = 8.8541878128e-18

One gets the correct result with optimized code if after modification, the --compile=min has been called first. The problem occurs only for physical constants, not for units. PhysicalConstants.jl behaves in the right way, though.

I see three possibilities here:

• Problem with the architecture of Unitful
• Problem with code loading in Julia 1.8 when optimizing
• Evaluating upreferred into the global context in a package using Unitful is a bad idea, even if preferunits is called before loading the package. If this is the case, it needs to be documented. (in fact I struggled with this situation in more obvious situations, so I have an Idea what to write into a PR...)

Start with the reproducible utest.jl script in the linked UTest.jl package and compare Julia 1.8/1.9 runs with and without --compile=min. Trace preferunits, upreferred(u"ε0"), Unitful.ε0, and PhysicalConstants.CODATA2018.ε_0; done means physical constants produce the same correct result under optimized and minimal compilation.