Crystal 1.15 seems to have symbol Index reserved

[wolfgang371]

With Crystal 1.15.0 [7b9e2ef80] (2025-01-09), LLVM: 18.1.6, Default target: x86_64-unknown-linux-gnu running the following ...

alias Index = Int32 # `Index` seems to be reserved now...?

... I get...

In /snap/crystal/2355/share/crystal/src/crystal/event_loop/epoll.cr:6:1
 6 | class Crystal::EventLoop::Epoll < Crystal::EventLoop::Polling
     ^
Error: abstract `def Crystal::EventLoop::Polling#system_add(fd : Int32, index : Index)` must be implemented by Crystal::EventLoop::Epoll

However, there was no issue on prior releases, e.g. 1.14: https://play.crystal-lang.org/#/r/hm4t

[straight-shoota]

This is a regression from refactoring the internal EventLoop API in #14996.
The new API introduces the mentioned abstract method which has a parameter with type restriction Index. But this type doesn't resolve to anything in stdlib, so the compiler doesn't check for implementations. By introducing a Index type in the top level this path now resolves and forces an implementation of the abstract def which obviously cannot exist.

The fix is simple, adjust the path to point to Arena::Index, the actual target of this restriction.

But I'm wondering if we shouldn't have caught this issue with a compiler error (or warning). (see #15359)

[Blacksmoke16]

But I'm wondering if we shouldn't have caught this issue with a compiler error (or warning).

Yea I'm surprised this didn't originally error due to an "Undefined constant" or something.

[wolfgang371]

Index binds with higher prio to the higher level symbol:

alias Index = Int32 # (un-)comment to trigger error

# src/crystal/event_loop/polling/arena.cr
class Crystal::EventLoop::Polling::Arena(T, BLOCK_BYTESIZE)
    # ...
    struct Index
        # ...
    end
end

# src/crystal/event_loop.cr
abstract class Crystal::EventLoop
# ...
end

# src/crystal/event_loop/polling.cr
abstract class Crystal::EventLoop::Polling < Crystal::EventLoop
    # ...
    protected abstract def system_add(fd : Int32, index : Index) : Nil
    # ...
end