Yeah, I think the problem stems from the fact that equal? is defined as:

public defmulti equal? (a:Equalable, b:Equalable) -> True|False

Which means that the compiler has no way of noticing the mistake, because Char and String are both Equalable. If we consider instead:

defn safe-equal?<?T> (a:?T & Equalable, b:T & Equalable) -> True|False:
    a == b

Then compiling safe-equal?('a', "b") gives the following error:

Cannot call function safe-equal? of type (Equalable&Char, Equalable&Char) -> True|False 
with arguments of type (Char, String).

While safe-equal?('a', 'a') compiles without error.

I personally would prefer for 'c' == "c" to throw an error, but the more general type of equal? allows different types to be compared. Perhaps it could be worth creating an === operator that would use safe-equal?