1555
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Ran 03 Jun 2026 03:12PM UTC

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Committed 03 Jun 2026 03:54AM UTC coverage: 76.495% (+0.01%) from 76.483%

Job # 1555

Build Type

push

buildkite

Committed by

web-flow

Commit Message

Don't rewrap other unioned types in flatten_type_union (#61980)

Fixes #61914, using the suggestion by Opus 4.6. (Thanks to @JeffBezanson
for confirmation that the original issue was real)

Here's a simpler example I used to understand the problem:
```
T = TypeVar(:T)
a = UnionAll(T, Union{Vector{T}, Int64})
```

Now, when we do `Union{a, T}`, we get `Union{Int64, Vector, T}` like
expected, since the `T` on the RHS is not bound by the UnionAll, and the
UnionAll of Union was rewritten to Union of UnionAll, leaving us with
`UnionAll(T, Vector{T}) == Vector`.

`Union{T, a}` gave us the wrong answer because `flatten_type_union`,
when doing the UnionAll of Union transform, would re-wrap every Union
component from the beginning of the `out` with the UnionAll's variable,
even if those components were not originally under the UnionAll. The
`out` array would go from `T, Vector{T}, Int64` to `UnionAll(T, T) =>
Any, UnionAll(T, Vector{T}) => Vector, UnionAll(T, Int64) => Int64`,
leaving us with `Any`.

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