[ruby-core:99210] [Ruby master Bug#17034] Unexpected behavior in #max for beginless range

Issue #17034 has been updated by citizen428 (Michael Kohl).


jeremyevans0 (Jeremy Evans) wrote in #note-2:
> I think a `RangeError` is more correct.  Without having a beginning, you cannot know the increment value, and therefore cannot know the maximum value.  People that want the end of the range should use `Range#end`, not `Range#max`.  However, I can see where `Range#max` returning the end for an inclusive beginless range could potentially be more useful.

The current PR only returns the last value if it's an inclusive beginless range of integers:

``` ruby
(..2).max
#=> 2
```

Exclusive range (`(...2).max`): TypeError (cannot exclude end value with non Integer begin value)
Non-integer range (`(...2.0).max`): TypeError (cannot exclude non Integer end value)
Non-numeric range (`(...'c').max`): TypeError (cannot get the maximum of beginless range with custom comparison method)

That said I'm happy to change it to a `RangeError` if that's the overall consensus. 

----------------------------------------
Bug #17034: Unexpected behavior in #max for beginless range
https://bugs.ruby-lang.org/issues/17034#change-86590

* Author: citizen428 (Michael Kohl)
* Status: Open
* Priority: Normal
* ruby -v: 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
When calling `max` on a beginless range, a non-intuitive error gets raised:

``` ruby
r = ..9
r.max
# ArgumentError: comparison of NilClass with 9 failed
```

There's a check for `NIL_P(RANGE_BEG(range))` but it's inside another check which is false for the example case above:

``` ruby
if (rb_block_given_p() || (EXCL(range) && !nm) || argc) {
        if (NIL_P(RANGE_BEG(range))) {
            rb_raise(rb_eRangeError, "cannot get the maximum of beginless range with custom comparison method");
        }
        return rb_call_super(argc, argv);
    }
```

The first part of the condition is false since there is no block, and even though I'm not sure what `EXCL` does the second part of the condition will be false due to `!nm` (`nm` will be true because of `FIXNUM_P(e)`). So I think the error gets raised here:

``` ruby
int c = OPTIMIZED_CMP(b, e, cmp_opt);
```


I think this is not ideal. Possible solutions:

1. Return `e` (`RANGE_END(range)` for beginless ranges or
2. return a `RangeError` with a message like "cannot get the maximum of beginless range" (similar to `.min`).


Happy to provide a patch if people want this changed and can agree on what the new behavior should be.

---Files--------------------------------
range-max-beginless.patch (1.26 KB)


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