Proposal: no undefined behavior

[densh]

Currently, a number of Scala language features that have strictly defined semantics on JVM but are undefined in Scala Native. While this provides greater flexibility in terms of implementation freedom, it burdens the users with harder to debug crashes.

To make the situation more manageable we are going to:

• Make null dereferencing deterministically crash with a the stack trace.
  • Field dereferencing on null throws NPE. (PR: Make field access on null throw NullPointerException #1391)
  • Method call on null throws NPE. (PR: Make calls on null throw NullPointerException #1394)
  • Array ops throw NPE on null array. (PR: Make calls on null throw NullPointerException #1394)
  • Throw on null throws NPE instead of original exception. (PR: Make casts crash with ClassCastException #1390)
• Make incorrect casts deterministically crash with a stack trace. (PR: Make casts crash with ClassCastException #1390)
• Make arithmetics error conditions throw arithmetic exceptions, the same as on JVM. (PRs: Make division by zero defined behaviour #1384 Fix undefined behaviour in integer shifts #1385 Bug compatibility with scala/bug/issues/11253  #1386 Fix undefined behaviour in float to integer conversions #1387)
• NIR's unreachable should crash with a stack trace. (PR: Make NIR's unreachable defined behavior #1392)
• Remove NIR's undefined value (PR: Remove undef value from NIR #1395)
• Make stack overflow deterministically crash with a stack trace. (See Proposal: automatically growable stack #1396)

[densh]

As of latest master (after #1397 merge) we don't have any expected undefined behavior in the NIR operations except for stack overflows and out-of-memory errors being reported differently than on JVM. I also opened #1396 to track progress on making stack overflows nicer and taking advantage of the fact that we might get rid off stack overflow as a concept altogether.