$score() has surprising behaviour when passing the argument predict_sets

[sebffischer]

library(mlr3verse)
#> Loading required package: mlr3

resample(
  tsk("mtcars"), lrn("regr.rpart", predict_sets = "train"), rsmp("holdout")
)$score(msr("regr.mse"), predict_sets = "train")
#> INFO  [17:51:35.145] [mlr3] Applying learner 'regr.rpart' on task 'mtcars' (iter 1/1)
#> Warning: Measure 'regr.mse' needs predict sets 'test', but learner 'regr.rpart'
#> only predicted on sets 'train'
#>    task_id learner_id resampling_id iteration regr.mse
#>     <char>     <char>        <char>     <int>    <num>
#> 1:  mtcars regr.rpart       holdout         1      NaN
#> Hidden columns: task, learner, resampling, prediction

^{Created on 2024-03-19 with reprex v2.0.2}