[ruby-core:113550] [Ruby master Bug#19681] The final classpath of partially named modules is sometimes inconsistent once permanently named

Issue #19681 has been updated by Eregon (Benoit Daloze).


fxn (Xavier Noria) wrote in #note-10:
> @Eregon, as I said, there is no requirement that names are reachable constant paths.

For `Module#name` in general, no, but once there is a permanent/non-anonymous (= starts with `#<`) name, then that is a requirement and a guarantee, until possibly broken by remove_const/const_set (very rare and would be a bug of whoever does that).

Everyone when they see `undefined method 'bar' for #<M::C:0x00007fe82b0d1450> (NoMethodError)` expect they can refer to that class via `M::C` (e.g., in a debugger/in irb/in code).
I would go as far as to say every non-trivial Ruby code out there relies on permanent module names to be a way to refer to that module.
Otherwise you couldn't even rely on `Hash` to refer to the Hash class.

----------------------------------------
Bug #19681: The final classpath of partially named modules is sometimes inconsistent once permanently named
https://bugs.ruby-lang.org/issues/19681#change-103179

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Backport: 3.0: WONTFIX, 3.1: REQUIRED, 3.2: REQUIRED
----------------------------------------
Reported to me by @fxn 

```ruby
m = Module.new

class m::C; end
p m::C.name # => "#<Module:0x000000010789fbe0>::C"

m::D = m::C
p m::D.name # => "#<Module:0x000000010789fbe0>::C"

M = m
p M::C.name # => "M::D"
```

Expected behavior:

```ruby
p M::C.name # => "M::C"
```

### Reason

When the parent is assigned its permanent classpath, we iterate over its `const_table` to recursively give a permanent name to all the constant it owns.

However, `const_table` is an `id_table` so it doesn't retain the insertion order, which means that if the constant was aliased,
we can no longer distinguish between the original name and its aliases, and whichever comes first in the `const_table` will be used as the permanent name.

### Potential solution

I have a tentative fix for it in https://github.com/ruby/ruby/pull/7829. Instead of relying on the `const_table` key, it extract the original name from the temporary classpath.
It does feel a bit wrong to do a string search in such a place, but it does work.

 



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