PHP_CodeSniffer

For this Sniff, you could have a look at Squiz_Sniffs_Formatting_OperatorBracketSniff.

And is it correct to throw a warning for example 2?
This is the most used way to loop over a result set:

while(($row = $GLOBALS['TYPO3_DB']->sql_fetch_assoc($res))) {
    // doo something
}

This is used in the whole core.
What does the core team say? Or are this rules / code examples made by the core team?

The CGL says:

Assignment in conditions should be avoided. However if it makes sense to do assignment in condition, it should be surrounded by the extra pair of brackets. Example:

if (($fields = $GLOBALS['TYPO3_DB']->sql_fetch_assoc($res))) {

// Do something

}

The following is allowed but not recommended:

if (FALSE !== ($fields = $GLOBALS['TYPO3_DB']->sql_fetch_assoc($res))) {

// Do something

}

The following is not allowed (missing the extra pair of brackets):

while ($fields = $GLOBALS['TYPO3_DB']->sql_fetch_assoc($res)) {

// Do something

}

So, example two is not wrong, but a warning is a hint to say something like: Hey, this is not wrong, but we recommend this way.