`stats::update()` weirdness

update.R

# Example 1: running at top level. This works.
model_str1 <- "speed~dist"
model1 <- lm(formula=model_str1, data=cars)
stats::update(model1, y = TRUE)
#> 
#> Call:
#> lm(formula = model_str1, data = cars, y = TRUE)
#> 
#> Coefficients:
#> (Intercept)         dist  
#>      8.2839       0.1656


# Example 2: model is created in a function, then update() is called at the top
# level. This throws an error.
mymodel <- function() {
  model_str <- "speed~dist"
  model <- lm(formula=model_str, data=cars)
}
update(mymodel(), y = TRUE)
#> Error in stats::model.frame(formula = model_str, data = cars, drop.unused.levels = TRUE): object 'model_str' not found


# Example 3: Same as example 2, but there is a variable with the same name as
# one inside of mymodel(), and it causes incorrect result.
model_str <- "dist~speed"
update(mymodel(), y = TRUE)  # <- This generates the wrong result!!
#> 
#> Call:
#> lm(formula = model_str, data = cars, y = TRUE)
#> 
#> Coefficients:
#> (Intercept)        speed  
#>     -17.579        3.932