[erlang-bugs] Cover transforms some binary comprehensions incorrectly

[Alexander Burkov]

In some cases cover transforms binary comprehensions into non-working ones.
Please consider example below:
---cut---
-module(wrong_transform).
-compile(export_all).

good() ->
    << <<$,, X/binary>> || X <- [ <<"a">>, <<"b">> ]>>.

bad() ->
    << <<",", X/binary>> || X <- [ <<"a">>, <<"b">> ]>>.
---cut---

With disabled cover, outputs of this functions are equal, `bad() =:=
good()`. But if you do `cover:compile_beam/1` (see decompile.erl) it blows
with `badarg` in `bad/0` function. Meanwhile `good/0` works well.

I've patched a bit `cover.erl` to dump AST forms before and after
`cover:compile_beam/1` and found that for some reason it surrounds terms
inside of comprehension with `begin ... end` pattern. It is wrong in case
of list (I can't explain why, but it throws badarg, so I guess it is
definitely an issue :)

Also I will appreciate if someone explain me why we do need this cover's
trick with `begin .. end`.

Thank you!
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