ast: improve error message when inheritance is used without ref

[simonvonhackewitz]

fix #3563

How it looks now

> cat /tmp/impr_err_msg.fz
parent is
  func String => abstract

child : parent is
  redef func => "not working because ref keyword missing"


main(x parent) =>
  say (x.func)

main child
> FUZION_DISABLE_ANSI_ESCAPES=true fz /tmp/impr_err_msg.fz

/tmp/impr_err_msg.fz:11:6: error 1: Incompatible types when passing argument in a call
main child
-----^^^^^
Actual type for argument #1 'x' does not match expected type.
In call to          : 'main'
expected formal type: 'parent'
actual type found   : 'child'
assignable to       : 'child'
for value assigned  : 'child'
To solve this you could:
  • make  'parent' a reference by adding the 'ref' keyword, so all its heirs can be used in place of it
  • change the type of the target 'x' to 'child'
  • convert the type of the assigned value to 'parent'.

one error.