Range#size raises compile time error

[straight-shoota]

Range#size raises a compile time error when either the begin or end type of the range are Nil. This method overrides Enumerable#size.

Raising at compile time effectively makes this method undefined for Range(T, Nil) and Range(Nil, T) and subsequently for Enumerable(T) and Enumerable(Nil), respectively. This breaks the expectation that a subtype inherits all methods of a parent type. When a range type with Nil parameter is instantiated anywhere in a program, it makes it impossible to call Enumerable#size because then not all inheriting types implement that method.

0.. 
[1, 2, 3].as(Enumerable(Int32)).size # Error: Can't calculate size of an open range

There is no way to work around that, except changing the argument type of Range from Nil (it can be nilable, which raises at runtime when the value is nil).

To fix this, Range#size must not raise at compile time.
The existing runtime error for nil values should be sufficient.

This compile time error was introduced in #8829.
The issue was first reported in Sija/debug.cr#7

[Sija]

Btw, several other Range methods are doing the same thing, see:

• Range#each
• Range#reverse_each
• Range#sample

If we're going to proceed with removing the compile checks from #size, it makes sense to remove them from the other methods as well.

[devnote-dev]

Isn't there a way to have this raise exclusively for Range? Having this checked at compile time makes it easier to catch accidental footguns and you are guaranteed to not run into this further down development, whereas raising at runtime does the opposite.

[straight-shoota]

No that's not possible. Range#size is an implementation of Enumerable#size and will always be considered when a call to that method is made.

Considering that this issue hasn't been noticed in 3 years, I figure the practical relevance of footgun prevention is negligible.