Reference extras in tool.poetry.extras

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• I have searched the documentation and believe that my question is not covered.

Feature Request

Currently we can define extras like this

[tool.poetry]
name = "awesome"

[tool.poetry.dependencies]
# These packages are mandatory and form the core of this package’s distribution.
mandatory = "^1.0"

# A list of all of the optional dependencies, some of which are included in the
# below `extras`. They can be opted into by apps.
psycopg2 = { version = "^2.7", optional = true }
mysqlclient = { version = "^1.3", optional = true }

[tool.poetry.extras]
mysql = ["mysqlclient"]
pgsql = ["psycopg2"]
databases = ["mysqlclient", "psycopg2"]

I have a monorepo where I use a single pyproject.toml file to describe different runtime options (by using extras). It would be cool if the last line in the above could be changed to the following

databases = ["mysql", "pgsql"]

I.e. being able to extend other extras. The same goes for the upcoming tool.poetry.group feature.

Start by reviewing the pyproject.toml examples for tool.poetry.extras and the referenced upcoming tool.poetry.group feature. Determine how nested extras should be represented and resolved, then define completion criteria for accepting names of other extras while preserving existing dependency behavior.