glob / wildcard matching issue

[muesli]

  $ touch a                                                                                                                                                                           
  $ touch b                                                                                                                                                                           
  $ touch c                                                                                                                                                                           

  $ ls -l                                                                                                                                                                             
  total 0
  -rw-r--r-- 1 muesli muesli 0 Feb  2 11:40 a
  -rw-r--r-- 1 muesli muesli 0 Feb  2 11:40 b
  -rw-r--r-- 1 muesli muesli 0 Feb  2 11:40 c

  $ ls bar*                                                                                                                                                                           
  a  b  c

As you can see the non-matching 'bar*' glob essentially translates to an empty string and "ls" is executed. This is potentially dangerous.

[xiaq]

We can have wildcards that expand to nothing throw an error by default, and add a nothing-ok modifier (cf #298) to suppress this failing behavior.

PS. @muesli to be precise, your bar* did not evaluate to an empty string (that would be ls "") but rather nothing (giving ls no argument at all).

[xiaq]

Spec for new (just implemented) behavior: https://github.com/elves/elvish/wiki/Language-Specification#wildcards