Strange type validation on when_then expressions

[coastalwhite]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

This succeeds:

df = pl.DataFrame({
    'a': [False, True],
    'b': [{ 'c': 1 }, None]
}).select(pl.when(pl.col.a).then(pl.struct({ 'b': 1 })).otherwise(pl.col.b))

This fails:

df = pl.DataFrame({
    'a': [False, True],
    'b': [{ 'c': 1 }, None]
}).select(pl.when(pl.col.a).then(pl.struct({ 'x': 1 })).otherwise(pl.col.b))

with a polars.exceptions.ColumnNotFoundError: x.

Log output

No response

Issue description

Strange error and inconsistent behavior.

Expected behavior

Consistent behavior

Installed versions

Replace this line with the output of pl.show_versions(). Leave the backticks in place.