Need robust unreachable expression detection.

[otac0n]

Here are just a few I can come up with:

a = #ERROR{ "any" } 'unreachable'
b = [a-z]<0, 1, 'unreachable'>
c = '' / 'unreachable'
d = &!!&!!&&&!&!&'' 'unreachable'
e = ""<100> #ERROR{ "any" } / 'unreachable'

Note that it is impossible to solve this problem completely:

a = !{ true } 'unreachable'

So, we are only trying to detect provably unreachable expressions.

Detecting something like these would eventually be nice, but will not be included in the first version of the feature:

a = !'a' 'a' 'unreachable'
b = ![a-z] 'a' 'unreachable'