Reloading `stanreg` objects cannot be used with `bridgesampling::bridge_sampler`

Summary:

Reloading stanreg objects cannot be used with bridgesampling::bridge_sampler.

Description:

When saving a stanreg model, with a defined diagnostic_file, and reloading it into a new R session, bridgesampling::bridge_sampler does not run even though the diagnostic file is available.

Reproducible Steps:

library(rstanarm)

count_data <- data.frame(
  counts = c(18,17,15,20,10,20,25,13,12),
  outcome = gl(3,1,9),
  treatment = gl(3,3)
)

fit <- stan_glm(
  counts ~ treatment + outcome, 
  data = count_data, 
  family = poisson(link="log"),
  diagnostic_file = "df.csv"
)

bridgesampling::bridge_sampler(fit)
#> Iteration: 1
#> Iteration: 2
#> Iteration: 3
#> Iteration: 4
#> Iteration: 5
#> Bridge sampling estimate of the log marginal likelihood: -38.62193
#> Estimate obtained in 5 iteration(s) via method "normal".


saveRDS(fit,"fit.rds")

Close R and restart.

library(rstanarm)
fit <- readRDS("fit.rds")
bridgesampling::bridge_sampler(fit)
#> Error in .local(object, ...) : 
#>  the model object is not created or not valid

RStanARM Version:

packageVersion("rstanarm")
#> [1] '2.18.2'

R Version:

getRversion()
#> [1] '3.5.3'

Operating System:

Windows 10

Start by reproducing the failure with the supplied rstanarm example: save and reload the stanreg object, then call bridgesampling::bridge_sampler with the diagnostic file present. The payload names no source files or tests, so trace the reload path and bridge_sampler integration before deciding where to work. Done means the reloaded object runs bridge_sampler successfully.