Parse difference for KSH, no semicolon required after double brackets

[masalomon]

For bugs

• Rule Id SC1049, and possibly others.
• My shellcheck version (shellcheck --version or "online"): online
• I tried on shellcheck.net and verified that this is still a problem on the latest commit

Here's a snippet or screenshot that shows the problem:

#! /bin/ksh
if [[ -z "" ]] then echo yes ; fi

Here's what shellcheck currently says:

SC1049: Did you forget the 'then' for this 'if'?
SC1073: Couldn't parse this if expression. Fix to allow more checks.
SC1010: Use semicolon or linefeed before 'then' (or quote to make it literal).
SC1050: Expected 'then'.
SC1072: Unexpected keyword/token. Fix any mentioned problems and try again.

Here's what I wanted or expected to see:

Nothing. This is valid in Ksh, as far back as ksh88. The semicolon is not required before the then if the if condition uses double brackets.

See related StackOverflow question: https://stackoverflow.com/q/48107383/1313898