API/REGR: Convert to float for index union

[TomAugspurger]

This restores the 0.24.x behavior of Index.union(other) between
Float and (U)Int indexes. These are again floating dtype.

left	right	output of left.union(right)
int	float	float64
int	uint	object
float	uint	float64

#26778 (comment)

Closes #26778

[jreback]

looks good. can you add a sub-section in whatsnew to explain the change.

+1 on the actual change semantics.

[codecov]

Codecov Report

Merging #27034 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #27034 +/- ## ========================================== - Coverage 91.99% 91.99% -0.01%  ========================================== Files 180 180 Lines 50774 50799 +25 ========================================== + Hits 46711 46733 +22  - Misses 4063 4066 +3

Flag	Coverage Δ
#multiple	90.63% <100%> (ø)	⬆️
#single	41.85% <46.15%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/numeric.py	97.6% <100%> (+0.26%)	⬆️
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️
pandas/core/frame.py	96.89% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.94% <0%> (+0.1%)	⬆️

---

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[codecov]

Codecov Report

Merging #27034 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #27034 +/- ## ========================================== - Coverage 92.04% 92% -0.04%  ========================================== Files 180 180 Lines 50714 50765 +51 ========================================== + Hits 46679 46706 +27  - Misses 4035 4059 +24

Flag	Coverage Δ
#multiple	90.64% <100%> (-0.04%)	⬇️
#single	41.83% <58.33%> (-0.13%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/numeric.py	97.46% <100%> (+0.12%)	⬆️
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️
pandas/core/internals/managers.py	96.09% <0%> (-0.77%)	⬇️
pandas/core/internals/blocks.py	94.61% <0%> (-0.53%)	⬇️
pandas/core/panel.py	17.61% <0%> (-0.2%)	⬇️
pandas/core/frame.py	96.89% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.94% <0%> (+0.1%)	⬆️
pandas/core/indexing.py	93.48% <0%> (+0.18%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[TomAugspurger]

looks good. can you add a sub-section in whatsnew to explain the change.

This is restoring the 0.24.2 behavior. I added a note in the user guide docs.

[jreback]

looks good. can you add a sub-section in whatsnew to explain the change.

This is restoring the 0.24.2 behavior. I added a note in the user guide docs.

That may be, but this should be up front about that in the whatsnew. IOW should be highly visible.

[TomAugspurger]

The CI failure from codecov is unrelated I think (cc @mroeschke. May just be a hiccup)

[jreback]

thanks @TomAugspurger