cryptic error when `expect_equal(x, y, tolerance = -Inf)`

Start by reproducing the provided MRE through testthat::expect_equal(), then trace the tolerance handling through testthat:::waldo_compare() and waldo::compare(), as shown in the backtrace. Identify where tolerance = -Inf becomes NaN and add regression coverage that verifies the resulting diagnostic is informative rather than cryptic.

Hi,

This is not really a critical bug...

I was writing tests and hoped expect_equal(actual, expected, tolerance = -Inf) would use the "lowest possible tolerance". I did not even expect it to work. I am reporting because of the cryptic error message.

MRE:

# Success
test_that("1 = 1 with minimum tolerance", {
  expect_equal(1,1,tolerance = -Inf)
})

# Cryptic Error message
test_that("1 = 1.1 with minimum tolerance", {
  expect_equal(1,1.1,tolerance = -Inf) # does not work
})

# Success
test_that("1 = 1.1 with maximum tolerance", {
  expect_equal(1,1.1,tolerance = Inf)
})

error message

── Warning: 1 = 1.1 with minimum tolerance ───────────────────────────────────
NaNs produced
Backtrace:
    ▆
 1. └─testthat::expect_equal(1, 1.1, tolerance = -Inf)
 2.   └─testthat:::expect_waldo_equal_(msg, act, exp, info, ..., tolerance = tolerance)
 3.     └─testthat:::waldo_compare(...)
 4.       └─waldo::compare(x, y, ..., x_arg = x_arg, y_arg = y_arg)
 5.         └─waldo:::compare_structure(x, y, paths = c(x_arg, y_arg), opts = opts)
 6.           └─waldo:::compare_vector(x, y, paths = paths, opts = opts)
 7.             └─waldo:::compare_numeric(...)
 8.               └─waldo:::min_digits(x, y, tolerance)
 9.                 └─waldo:::digits(tolerance)

> packageVersion("testthat")
[1] ‘3.3.2’

> packageVersion("waldo")
[1] ‘0.6.2’

> version
               _                           
platform       x86_64-pc-linux-gnu         
arch           x86_64                      
os             linux-gnu                   
system         x86_64, linux-gnu           
status                                     
major          4                           
minor          6.1                         
year           2026                        
month          06                          
day            24                          
svn rev        90187                       
language       R                           
version.string R version 4.6.1 (2026-06-24)
nickname       Happy Hop