Equality test fails if a nil value is used

[nickbattle]

The eq clause of an invariant type produces a function of type T * T +> bool. Therefore when two values of the type are compared, the arguments to eq are converted to type T before the equality is evaluated (like any function call).

But if one of the argument values is nil, this cannot be converted to T unless it is also an optional type. That currently causes a runtime exception; it should simply return false. For example:

types
  T = nat
  eq a = b == a <> b;

functions
  f: [T] -> bool
  f(a) == a = nil;

> p f(1)
org.overture.interpreter.runtime.ValueException: Cannot convert nil (nil) to nat

The solution is to check for a nil value before calling the eq function and return false if the underlying type is not optional.

[nickbattle]

Fix now available in ncb/development.