re-seq is relying on undefined behavior of subs

The behavior of subs is undefined in the following case: (subs "" 1).

Yet, re-seq calls subs like this in the following expression (re-seq #"a" "a").

In the expression (re-seq #"a" "") a call is made like (subs "" -1) which is also undefined.

re-seq doesn't actually use the result of these undefined calls, but we are relying on the undefined behavior that this doesn't throw.

Solution in patch: only make call to subs when needed.

The patch also optimizes the case where match-data is nil.

CLJS-2979-2.patch contains various optimizations which lead to the following speedups:

tested with

Credits to Mike Fikes and Jesse Wertheim for providing many optimization suggestions.