when nimvm silently gives wrong results (implicit discard causing bugs); nimvm gives redefinition error in valid code

[timotheecour]

when nimvm accepts invalid code (implicit discard causing bugs) or silently gives wrong results (example 2) and other errors.

Example

proc main(): int =
  when nimvm: doAssert false
  else: 123 # returns 0, implicitly discarded
  # else: result = 123 # returns 123
echo main()

Current Output

0

Expected Output

returning 123 is probably not what we want here because of the way nimvm works. However it should give error:
Error: expression '123' is of type 'int literal(123)' and has to be discarded
just like it would with if false instead of when nimvm

Example 2

when true: # D20200420T210320
  proc bar(): int =
    when nimvm: result = 2
    else: 1
  doAssert bar() == 1 # fails

Example 3

this example is a different bug, it gives: Error: redefinition of 'y'

when true: # D20200420T210337:here
  proc bar() =
    when nimvm:
      let y = 1
    else:
      let y = 2
  bar()

Additional Information

• devel bb777fe 1.1.1