fix union{} ambiguity

[Roger-luo]

See JuliaLang/julia#47840

[codecov]

Codecov Report

Base: 94.33% // Head: 94.09% // Decreases project coverage by -0.23% ⚠️

Coverage data is based on head (26b93f7) compared to base (b550b7b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #527      +/-   ##
==========================================
- Coverage   94.33%   94.09%   -0.24%     
==========================================
  Files           9        9              
  Lines        1270     1270              
==========================================
- Hits         1198     1195       -3     
- Misses         72       75       +3     

Impacted Files	Coverage Δ
src/display.jl	100.00% <ø> (ø)
src/utilities.jl	95.60% <0.00%> (-0.74%)	⬇️
src/conversions.jl	99.13% <0.00%> (-0.29%)	⬇️

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[Roger-luo]

I'm wondering if someone could review this? The test coverage drop does not seem related to this PR.

[johnnychen94]

will make a release later

[aplavin]

@Roger-luo @johnnychen94 this PR doesn't have any tests to actually confirm the fix, and AFAIK the fix doesn't work.
Union{} <: T <: Color is exactly the same as T <: Color.

[Roger-luo]

@aplavin yeah I think you are right, I just realize Union{} <: T is always true, thus this won't work unless we type pirate Union{} this MIME type.

But this does not work for show since lots of backends will call the corresponding show by checking if there is a valid show overloaded for the richest MIME type, e.g https://github.com/fonsp/Pluto.jl/blob/0f0b6f596fd4cf27b2665e1d93ddd44d96fe2a99/src/runner/PlutoRunner.jl#L1148 as a result we will see MIME"text/plain" displayed as picture if we overload

MIME"image/svg+xml" => MIME"text/plain"

I don't have a good way to fix this now.

[aplavin]

I agree, and think this PR should be reverted so that not to confuse future readers.

[johnnychen94]

@aplavin Thanks for identifying this. I didn't think carefully when merging it -- was checking it on the phone and did a quick merge. Reverted in 9518578

This Union{} would be a nightmare to JuliaImages. Almost every function is written in such a way.

[johnnychen94]

I realize that I shouldn't make the release on the master branch since there are plenty of breaking changes made by @kimikage since v0.12.8. Thus I've made a backport release branch https://github.com/JuliaGraphics/Colors.jl/tree/release-0.12 with a new patch release v0.12.10 to supersede v0.12.9.

v0.12.10 includes #521 (@brenhinkeller) and #525(@williamjsdavis) as bug fixes.

I've also submitted a yank patch for v0.12.9 to General JuliaRegistries/General#74192