Saxon

What's happening here is a call on map:keys(parse-json('....')) where the call on parse-json() is preevaluated to a Literal map before the call on map:keys is type checked. The type checking code tries to determine the static type of the expression being passed as the first argument, and in this case the expression is a Literal, so it ends up trying to determine the item type of the map.

Perhaps rather than calling the method Expression.getItemType() there should be a method Expression.isItemTypeSubtypeOf(X) .That's a rather substantial change but the new method could be given a default implementation that calls getItemType(), and could have a custom implementation for literals.

Alternatively, and perhaps more simply, Literal could return true for implementsStaticTypeCheck(), in which case it would call Literal.staticTypeCheck(), which achieves much the same thing. Current expressions that implement this method are things like Block and Choose, which go on to check not just that the expression as a whole delivers the right type, but that every operand does so.

Added a fast path to TypeChecker.staticTypeCheck:

        if (supplied instanceof Literal && ((Literal)supplied).isInstance(req, th)) {
            return supplied;
        }

This is going to catch all cases where a function is called with a literal argument that matches the required type, so the benefit could be substantial.