bug#38587: base64-decode-region breaks encoding

[Juri Linkov <juri@linkov.net>]

>> Maybe an additional CODING arg for base64-decode-region?
>
> BASE64 is defined on a sequence of bytes.  It doesn't make sense to
> apply it to characters.

But isn't UTF-8 a multibyte encoding represented by a sequence of bytes
(e.g. when saved to a file)?

Then why base64-encode-region couldn't use the buffer's coding
to convert the region to a sequence of bytes?

Also why base64-encode-region accepts region's characters
only from the charsets ‘eight-bit-control’ and ‘eight-bit-graphic’,
but not other UTF-8 characters?

> The input of base64-encode-region needs to be encoded into bytes and the
> output of base64-decode-region needs to be decoded into characters.  If
> you do that, you get a full reversible operation.

I guess base64-encode-region already encodes the region into bytes,
but only partially - it signals an error on some characters,
I don't understand why it can't encode all of them.

>> Or it would be enough to use the coding system of the
>> output buffer?
>
> The coding system of the output buffer has nothing to do with the coding
> of the data produced by base64-decode-region, just like
> process-coding-system is independent from the coding system of the
> process buffer.

It's understandable that the coding system of the output buffer
is not necessarily the same as expected from the output of
base64-decode-region.

But is it still possible to tell base64-decode-region
about the expected output coding system?  Maybe using
a prefix arg: C-u M-x base64-decode-region could ask
for a coding, defaulting to the buffer's coding.

For example, in Ruby

  require 'base64'
  Base64.decode64(Base64.encode64("☃"))
  => "\xE2\x98\x83"

indeed outputs ASCII not encoded to UTF-8.
But it's possible to force encoding with:

  Base64.decode64(Base64.encode64("☃")).force_encoding('UTF-8')
  => "☃"

Is there an equivalent of force_encoding('UTF-8') in Emacs?
I tried to call after base64-decode-region on its output:

  (decode-coding-region (point-min) (point-max) 'binary)

but it doesn't work, neither this:

  (encode-coding-region (point-min) (point-max) 'utf-8)

Also this doesn't work on the string output:

  (decode-coding-string (base64-decode-string (base64-encode-string "ä"))
                        'utf-8)

Maybe I'm doing something wrong?