On recent master:

 (cl-substitute ?\_ ?\/ "foo/bar")

=> [102 111 111 95 98 97 114]

But this should return: "foo_bar"

This seems to be a bug in commit 8c8ff13e "Clean up 'cl-' prefixes for
local variables", which replaced variable names without considering
the semantics. In this case the cl-seq was being used to name a new
variable to distinguish it from `seq' -- see attached patch.

[The commit that introduced the semantically significant variable is
de5a89254 "Don't mutate strings in cl-substitute"

The point of the `n' in nsubstitute is to mark the function as one
that possibly mutates its argument, (when the implementation judges
that is more efficient and the user prefers to reuse string storage
rather than copy it)

https://www.lispworks.com/documentation/HyperSpec/Body/f_sbs_s.htm

I haven't understand this commit message yet but I hope it does not
preculde a comprehension of the semantics of the "n!"-variant.] -- Madhu