• [x ] I am on the latest Poetry version.
• [x ] I have searched the issues of this repo and believe that this is not a duplicate.
• [x ] If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: OSX 10.14.5
• Poetry version: 1.0.0b4
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/tonysyu/82b3fc28b01e0fec6e53cece29adc7db

Issue

While testing out the extras export fix for #1313, I tried setting up a new project (with the pyproject.toml linked above) and ran into the following issue:

$ poetry --version
Poetry version 1.0.0b4

$ poetry add -v3 requests[security]
The currently activated Python version 2.7.13 is not supported by the project (^3.6).
Trying to find and use a compatible version.
Using python3 (3.6.5)
Using version ^2.22.0 for requests

Updating dependencies
Resolving dependencies... (13.7s)
Resolving dependencies... (13.8s)
[RuntimeError]
maximum recursion depth exceeded while calling a Python object

• Running the same command on 1.0.0b3 works as expected
• Running poetry add requests (without the extras) works as expected