[Bug] Updating config group from a config group of another config group only works with full path

[Yann-CV]

🐛 Bug

Description

When trying to modify a config group which is child of another config groups as well a child of another config group, the update is only possible by overriding the existing value (add a +).

It ends up with a mix of the existing and new values.

(I guess the example will make more sense than my explanation sorry)

Checklist

• I checked on the latest version of Hydra
• I created a minimal repro (See this for tips).

To reproduce

test_hydra2/config.yaml

defaults:
  - level1: level11/level11_ini
  - _self_

test_hydra2/config/level1/level11/level11_ini.yaml

defaults:
  - /level1/level11/level111@subparams: level111_ini
  - _self_

p1: 1
p2: 2

test_hydra2/config/level1/level11/level111/level111_ini.yaml

defaults:
  - ../../level12@out: level12_ini
  - _self_

test_hydra2/config/level1/level12/level12_ini.yaml

o1: 1
o2: 2

test_hydra2/config/level1/level12/level12_end.yaml

o1: 10
o2: 3

test_hydra2/main.py

import hydra
from omegaconf import DictConfig

@hydra.main(version_base=None, config_path="config/", config_name="config")
def run_test(cfg: DictConfig) -> None:
    print(cfg)

if __name__ == "__main__":
    run_test()

uv run python -m test_hydra2.main

{'level1': {'subparams': {'out': {'o1': 1, 'o2': 2}}, 'p1': 1, 'p2': 2}}

uv run python -m test_hydra2.main level1/level12@level1.subparams.out=level12_end

Could not override 'level1/level12@level1.subparams.out'. No match in the defaults list.
To append to your default list use +level1/level12@level1.subparams.out=level12_end

uv run python -m test_hydra2.main +level1/level12@level1.subparams.out=level12_end

{'level1': {'subparams': {'out': {'o1': 10, 'o2': 2, 'o3': 3}}, 'p1': 1, 'p2': 2}}

** Stack trace/error message **

Could not override 'level1/level12@level1.subparams.out'. No match in the defaults list.
To append to your default list use +level1/level12@level1.subparams.out=level12_end

Expected Behavior

{'level1': {'subparams': {'out': {'o1': 10, 'o3': 3}}, 'p1': 1, 'p2': 2}}

System information

• Hydra Version : 1.3.2
• Python version : 3.13.3
• Virtual environment type and version : uv 0.9.18
• Operating system : Ubuntu 24.04

Additional context

Add any other context about the problem here.

The way to fix is to use the complete path for the out param

test_hydra2/config/level1/level11/level111/level111_ini.yaml

defaults:
  - /level1/level12@out: level12_ini
  - _self_