library(tidymodels)

set.seed(6735)
folds <- vfold_cv(mtcars, v = 5)

spline_rec <-
  recipe(mpg ~ ., data = mtcars) %>%
  step_ns(disp, deg_free = tune("disp")) %>%
  step_ns(wt, deg_free = tune("wt"))

lin_mod <-
  linear_reg() %>%
  set_engine("lm")

# only whole numbers
spline_grid <- expand.grid(disp = 2:5, wt = 2:5)

spline_res <- tune_grid(lin_mod, spline_rec, resamples = folds, 
                        grid = spline_grid, control = control_grid(save_pred = TRUE))
# this is fine
pred <- collect_predictions(spline_res)

# now with degrees of freedom not part of the tuning
# this should warn or error informatively
collect_predictions(spline_res, parameters = tibble(disp = 1.1, wt = 2.2))
#> # A tibble: 0 × 7
#> # ℹ 7 variables: id <chr>, .pred <dbl>, .row <int>, disp <dbl>, wt <dbl>,
#> #   mpg <dbl>, .config <chr>