parser: figure out what to do about bash's extended globbing

[mvdan]

If bash's extended globbing is on, some programs are suddenly valid:

shopt -s extglob

echo foo@(bar)

Normally, this fails as ( is a token:

 $ echo 'echo @(foo)' | bash
bash: line 1: syntax error near unexpected token `('
bash: line 1: `echo @(foo)'
 $ echo 'echo @(foo)' | shfmt
1:7: a command can only contain words and redirects

I see two possible options:

• Change the behaviour of the parser whenever a shopt statement is parsed (ugh)
• Always parse extended globbing stuff in the bash mode

The first option would make the parser even more complex, as it would change behaviour not only depending on the context, but also depending on what statements it has parsed.

The second option is somewhat cleaner, but I see some minor disadvantages:

• Some invalid bash programs will be accepted (which is okay for shfmt)
• If this parser is ever used for anything serious, another package (like an interpreter/executor) would have to take care of erroring if it finds an extended globbing node when not in extglob mode

[mvdan]

I went for the latter, which is the only option that doesn't make us understand shopt and otherwise evaluate the program.

See 3917ecf.