From version of 15 EAP there is a partial support for ligatures with editor.enable.optional.ligatures=true in bin/idea.properties file.

In OS X, the bin folder is located inside of the application itself, for example: /Applications/IntelliJ IDEA 16 EAP/ Context menu -> Show package content /Contents/bin/idea.properties.

Here is a proof:

As you can see, ligatures appears if there is a letter (not number) beside.

I created a test case for this:

 .=a .-a :=a =:=a ==a !=a ===a !==a =/=a <<-a <--a <-a b<->a b-> b--> b->> <=<a <<=a <==a b<=>a b=> b==> b=>> b>=> >>=a >>-a >-a b-< b-<< b=<< <~~a <~a b~~~a b~> b~~> <<<a <<a <=a b<>a b>= b>> b>>> <|a b<|>a b|> <$a b<$>a b$> <+a b<+>a b+> <*a b<*>a b*> \\a \\\a b{- -}a b// b/// /*a /**a b**/ b*/ </a <!--a www b--> b/> 0xF 9:45a m-x *ptr ;;a ;;;a ::a :::a !!a !!!a ??a ???a %%a %%%a &&a &&&a ||a |||a ..a ...a b..< a[]b --a ---a ++a +++a **a ***a ~=a ~-a -~a =~a ~@a ^=a ?=a /=a /==a |=a ||=a ##a ###a ####a a#{b a#[b a#(b a#?b a#_b a#_(b a*b a*A B*b A*B *a *A a* A* a-b a-A B-b A-B -a -A a- A- a+b a+A B+b A+B +a +A a+ A+ a:b a:A B:b A:B :a :A a: A: 

And this is how it looks in IntelliJ IDEA 16 EAP v144.2151.16:

As you can see, every ligature that based on 2 glyphs is working, except of any brackets <>[]{}()

But without letter beside there is nothing as I described above:

So, basically, we can add this info in repository readme.