The error is about return type declarations resulting in a mismatch for the function signature.

The return type declarations are added with 2.0 (https://github.com/php-fig/http-message/commit/54df5d4d336354de566dc6bb5d99cc0954b117f8) and not part of mediawiki/core

RSS extension does not installs any further composer packages, so it not clear why this only happen when install that on your machine.

The error can happen when running "composer update" in the extension folder and not using the composer merge plugin from core, see more about it in T389005: Declaration of MWCallbackStream::write($string) must be compatible with Psr\Http\Message\StreamInterface::write(string $string)

Indeed, this is almost certainly not the fault of the linked commit.

CI would have been upset a long tine ago if so

Return types were added to the psr-message library in version 1.1, per
https://phabricator.wikimedia.org/T335073

That is the version that is installed on my installation as well. The error
in T335073 is the exact one I was getting.

Here is a partial list of composer libraries in my installation (I'm not home at the moment so I can only provide screenshots from the server control panel via my phone)

In T393824#10809509, @Oldiesmann wrote:

Return types were added to the psr-message library in version 1.1, per
https://phabricator.wikimedia.org/T335073

No they weren't. You've presumably got something (probably in an extension) installing version 2.0.

Just because you've got 1.1 in mediawiki core, doesn't mean somewhere else (likely a skin, or an extension) doesn't also have version 2.0 in its own vendor repository, which is also being included later.

https://github.com/php-fig/http-message/blob/1.1/src/StreamInterface.php#L55-L61

/**
 * Returns the current position of the file read/write pointer
 *
 * @return int Position of the file pointer
 * @throws \RuntimeException on error.
 */
public function tell();

https://github.com/php-fig/http-message/blob/1.1/src/StreamInterface.php#L110-L117

/**
 * Write data to the stream.
 *
 * @param string $string The string that is to be written.
 * @return int Returns the number of bytes written to the stream.
 * @throws \RuntimeException on failure.
 */
public function write(string $string);

Further update on this... I talked to a friend who knows a whole lot more about composer and PHP than I do and we tracked down the problem to the GoogleLogin extension, which apparently includes version 2.0 of psr/http-message. Disabling that extension fixes the error, so I will either have to modify my copy of the MWCallbackStream.php file or just not use that extension.

Or use https://github.com/wikimedia/composer-merge-plugin

Does the extension explicitly want 2.0, or is it just a loose dependancy that results in it picking the highest versions or can?

It isn't listed in composer.json for that extension at all - it's just included as one of the libraries shipped with that extension. I didn't install it via Composer either. Do I need to rerun composer update after each extension I install?

In T393824#10809794, @Reedy wrote:

Does the extension explicitly want 2.0, or is it just a loose dependancy that results in it picking the highest versions or can?

GoogleLogin has a dependency on google/apiclient, that has a dependency on guzzlehttp/psr7 in version 2.6. That dependency allows psr/http-message: ^1.1 || ^2.0 since 2.5.

See 427b6e95853b78db67d782143b3f84b0750165be how another extension has bypass this issue, when composer is running from within the extension folder and not using the merge plugin

I was just helping someone who had this randomly happen to them after doing a developer setup.

Since this seems to happen sometimes, would it make sense for us to just add the type-hints to MWCallbackStream?

public function write( string $string ): int {

Is all that's needed, it doesn't seem to cause any problems, and it's somewhat futureproofing us against Guzzle really bumping its required version.

Shortly after writing that, I found: T389272