Compiler confused about variable type inside while and block

[HertzDevil]

The following snippet produces a compilation error:

def scope(&)
  yield
end

def foo : Int32?
  if x = foo
    scope do
      while x
        x = foo
      end
    end
  end
  0
end

foo # BUG: trying to downcast (Int32 | Nil) (Crystal::MixedUnionType) <- Int32 (Crystal::IntegerType) (Exception)

The above pattern can be seen in IO#gets_peek, where foo is IO::Buffered#peek and scope is String.build. This is the root cause behind #8163, #13720, and #13792.

Changing #foo's return type to Int32 or removing it altogether makes the internal error go away. In the case of IO::Buffered#peek this is indeed possible: #13863