New exception classes

Prefer the use of exceptions from the standard library over introducing new exception classes.

I feel this is not quite accurate.

Indeed, a gem should use standard library exceptions in case the gem is called with invalid inputs (e.g. TypeError if called with an Integer instead of a String, ArgumentError, IndexError, etc.). Typically these are never rescued.

But when something proper to the gem happens, instead of raising a RuntimeError it should have its own subclasses of StandardError for that gem (with potentially a base class for the gem). E.g. ::Parser::SyntaxError, ::Parser::ClobberingError. Typically these may be rescued by the dependents of the gem.

May I amend the guide?

Search the style guide for the quoted recommendation to prefer standard-library exceptions. Read the surrounding guidance and the issue’s distinction between invalid inputs and gem-specific failures, then determine whether the wording should be amended. Done means the guide clearly reflects that distinction and the rendered documentation remains consistent.