`assert_unreachable` function

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KotlinIsland/basedtyping#28
would need to invert the unreachable error code when it sees an assert_unreachable call, such that it's raised if the code is reachable. perhaps a seperate error code?

when warn-unreachable is enabled

if False:
  assert_unreachable() # no error
if False:
  print(1) #error: unreachable
  assert_unreachable() # no error
else:
  assert_unreachable() # error

When warn-unreachable is disabled

if False:
  print(1) # no error
  assert_unreachable() # no error
else:
  assert_unreachable() # error

[KotlinIsland]

Would lead up be allowed? eg:

a: float = 1

if not isinstance(a, float):
    message = "LOOL!"
    logger.warning(DuckTypingError(message))
    assert_unreachable(message)

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i don't think so

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when warn-unreachable is enabled

if False:
  print(1) #error: unreachable
  assert_unreachable() # no error
else:
  assert_unreachable() # error

when warn-unreachable is disabled

if False:
  print(1) # no error
  assert_unreachable() # no error
else:
  assert_unreachable() # error

[KotlinIsland]

python/typing#735

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assert_never exists now