ruby-grape / grape / 30694876944

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Warn when a rescue_from handler can never run

Grape::Middleware::Error resolves a registered handler with #find, so within a
scope the first matching class wins. A handler registered for a class an
earlier one already covers is therefore dead code, and silently so:

    rescue_from StandardError do ... end   # wins
    rescue_from ArgumentError do ... end   # never runs

Swapping the two lines is all it takes, but nothing said so -- and the
neighbouring `rescue_from :all` behaves the other way round, since it is
consulted only after the registered handlers, so a specific class registered
after it still wins. Two documented ways of saying "catch everything",
ordering differently against a later specific handler.

Warn at definition time rather than reorder: which handler should win is the
author's call, and :all already covers "broad first, specific still wins".
The check lives in Grape::Util::ShadowedRescueHandlers, called from
#add_rescue_handlers where the scope's own registrations are known.

Compared within a scope only -- across scopes the nearest registration
deliberately wins, so an inner rescue_from StandardError shadowing an outer
rescue_from ArgumentError is the documented behaviour, not a mistake. Classes
sharing a handler object are skipped too, since `rescue_from A, B` registers
one handler for both and the entry that loses changes nothing. Exact-match
handlers (rescue_subclasses: false) are consulted before the subclass-matching
ones and never match a descendant, so they cannot shadow each other either.

README gains the ordering rule, which was only implied.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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