[ruby-core:120501] [Ruby master Feature#21000] A way to avoid loading constant required by a type check

["Dan0042 (Daniel DeLorme) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #21000 has been updated by Dan0042 (Daniel DeLorme).


Eregon (Benoit Daloze) wrote in #note-2:
> So `defined?(Foo::Bar)` will actually try to load `Foo`.

It would be a good idea to fix that right?

----------------------------------------
Feature #21000: A way to avoid loading constant required by a type check
https://bugs.ruby-lang.org/issues/21000#change-111294

* Author: Dan0042 (Daniel DeLorme)
* Status: Open
----------------------------------------
There is this pattern I encounter sometimes:

```ruby
if defined?(NameSpace::ClassName) and obj.is_a?(NameSpace::ClassName)
```

Searching in gems, the pattern is fairly common: https://pastebin.com/VGfjRWNu

I would like a way to avoid the repetition of `NameSpace::ClassName` above.  I can think of a number of ways to approach the issue, each with different tradeoffs...


### Pattern match ignores uninitialized constant 

Pattern match like `obj in XYZ` could return false if XYZ is not defined. The danger here is that a typo could go undetected and just silently ignore the error even when the constants is expected to be defined.

### Pattern match has special syntax to ignore uninitialized constant 

Pattern match such as `obj in XYZ?` (or some other syntax) could return false if XYZ is not defined. The downside is that we're adding yet more new syntax. But it could be `obj in defined?(XYZ)` and then it doesn't really feel like new syntax.

### Do not autoload constants required by pattern match

If we have `autoload :XYZ, "xyz"` then `obj in XYZ` could skip the autoload and return false. There is a possibility that `XYZ` might be defined as a regexp or other matcher that return true, but in general autoload is only used for classes/modules. And if the class/module is not yet loaded, obviously an object of that type cannot exist so we can avoid loading it. But this would only work for autoloaded constants, so can't be used to check a library that might not be loaded, ex: `obj in ActiveRecord::Base`

### defined?(mod) returns mod if it's a class/module

If XYZ is a module, `defined?(XYZ)` could return XYZ instead of returning "constant". So it can be used in expressions like 

```ruby
case obj
when nil
when defined?(XYZ)

if obj and defined?(XYZ) === obj

if defined?(Gem::Specification)&.respond_to?(:each)
```
Very versatile, with the downside that it's a small backward incompatibiliy.




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