lintr::lint(text = "function() function(x) {\n  x\n}")
#> <text>:1:1: style: [brace_linter] Any function spanning multiple lines should use curly braces.
#> function() function(x) {
#> ^~~~~~~~~~~~~~~~~~~~~~~~

# always allowed
function() {
  function(x) {
    x
  }
}

# could optionally be allowed
function() function(x) {
  x
}