Problem

Provider values are unescaped shell-style before reaching the test, which is
defensible on its own — the parser has deliberate handling for \t, \n and
$'…' from printf '%q'. What is not defensible is the last row:

function provider_shapes() {
  echo 'plain'
  echo 'mid\dle'
  echo 'C:\'
  echo 'two\\back'
}

A value ending in a lone backslash does not arrive at all: ${1-<unset>} shows
the argument is unset, so the test runs against nothing and passes or fails for
a reason unrelated to the data. Silent, like the assert_equals backslash bug
reported today in #1108.

Windows paths (C:\), a trailing path separator, or a regex fragment ending in
an escape all hit it.

Where it is

bashunit::runner::parse_data_provider_args alone returns C: for that input —
the trailing backslash is dropped but the argument survives:

input=[C:\]                  -> arg=[C:]
input=[ends-with-backslash\] -> arg=[ends-with-backslash]

So the parser loses the backslash and something downstream loses the whole
argument. Both halves want fixing; the second is the user-visible one.

Nothing in docs/data-providers.md describes escape handling, so there is also
a question of what the intended contract is: literal values, or shell-unescaped
ones. Whichever it is, an argument silently vanishing is not it.