[ruby-core:121087] [Ruby master Misc#21035] Clarify or redefine Module#autoload? and Module#const_defined?

Issue #21035 has been updated by Eregon (Benoit Daloze).


My POV is there is no autoloads being unregistered or ignored (the autoload exists until it's resolved, and importantly other threads must never see any difference between "before autoloading C" and "during autoloading C").
Rather it's just that `defined?` and `const_defined?` pretend the constant doesn't exist for the autoloading thread because lots of code can run in an autoload, and the code there should see there is no such constant yet since code during that autoload is supposed to define that constant.
If the code there would refer to the autoload constant it would be a sort of cycle and in fact IIRC it will throw a NameError in that case, which is consistent with "the constant looks not defined for the autoloading thread, until it defines that constant" whether asking though `defined?` or `const_defined?` or accessing the constant.

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Misc #21035: Clarify or redefine Module#autoload? and Module#const_defined?
https://bugs.ruby-lang.org/issues/21035#change-112007

* Author: fxn (Xavier Noria)
* Status: Closed
----------------------------------------
The documentation for `Module#autoload?` says:

> Returns filename to be loaded if name is registered as autoload in the namespace of mod or one of its ancestors.

As a user, if I declare an autoload, I expect this API:

```ruby
module M
  autoload :Foo, 'foo'

  constants            # => [:Foo]
  const_defined?(:Foo) # => true
  autoload?(:Foo)      # => 'foo'
end
```

That it is indeed how it generally works. Even if the autoload path does not exist. But there is an edge case.

While `constants` does include always `:Foo` as far as I can tell, the return value of `const_defined?` and `autoload?` depends on the stack of features being loaded: The autoload path is resolved and if seen to be in the stack of features being loaded, the predicates return `false` and `nil`, respectively. Do you think that is intuitive?

I find that logic totally unexpected. I just defined an autoload, therefore, I think it would be natural for `autoload?` to return what I just configured. Why should `const_defined?` return nothing but `true`? And why is it not consistent with `constants`?

To me, it would make more sense that in the previous example `const_defined?` returns `true`, and `autoload?` returns `foo` unconditionally (and instantly, nowadays it takes a relative long time due to the lookup).

Now, if the autoload is triggered in a lookup **then** I would expect `Kernel#require` logic to apply. But not when calling some simple predicates.

Please, note that the present behavior is not documented, so on paper the change would not be backwards incompatible.

If, on the other side, it is preferred to keep the behavior as it is, I guess it should be documented with precision (accounting for symlinks, relative paths in `$LOAD_PATH`, etc.)




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