Feature: Stricter addition

[TimTaylor]

The binary addition operator currently has the following implementation

`+.glue` <- function(e1, e2) {
  glue(e1, e2, .envir = parent.frame())
}

Would it be desirable to be a little stricter here and maybe error if one of the inputs is not a character / glue object. It feels like the following would be better to error:

1L + glue::glue("y")
#> 1y

^{Created on 2023-01-11 with reprex v2.0.2}

[hadley]

Reprex:

1L + glue::glue("y")
#> 1y

^{Created on 2023-01-25 with reprex v2.0.2}

[hadley]

And shouldn't it be vectorised?

"(" + glue::glue(c("y", "z")) + ")"
#> Error: All unnamed arguments must be length 1

^{Created on 2023-01-26 with reprex v2.0.2}

[hadley]

And this seems wrong:

glue::glue("{x}", x = "{x}") + "y"
#> Error in eval(parse(text = text, keep.source = FALSE), envir): object 'x' not found

^{Created on 2023-01-26 with reprex v2.0.2}

We're interpolating the string twice

[jennybc]

Note to self, making the last example even more obvious

glue::glue("{x}", x = "{wut}")
#> {wut}
glue::glue("{x}", x = "{wut}") + "y"
#> Error in eval(parse(text = text, keep.source = FALSE), envir): object 'wut' not found

^{Created on 2023-03-17 with reprex v2.0.2.9000}

[jennybc]

I'm about to merge #297 because it's definitely progress.

But just want to note this is also connected to #246. When that gets addressed, make sure to test +.

With #297:

library(glue)

str(as_glue("foo") + character())
#>  'glue' chr "foo"

str(glue("foo{x}", x = character()))
#>  'glue' chr(0)

With glue v1.6.2:

library(glue)

str(as_glue("foo") + character())
#>  'glue' chr(0)

str(glue("foo{x}", x = character()))
#>  'glue' chr(0)