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

• OS version and name: Windows
• Poetry version: 0.12.16
• Link of a Gist with the contents of your pyproject.toml file: N/A, requires access to a proprietary pypi instance

Issue

My constraints aren't resolving.

A rough look at my dependency tree

• A: `B = "^0.7.5" via poetry
• B: "C~=0.2.0dev16" via setup.py

Poetry complains that it cannot resolve B's dependency on C

[SolverProblemError]
Because B (0.7.5) depends on C(>=0.2.0,<0.3.0) which doesn't match any versions, B is forbidden.
So, because no versions of B match >0.7.5,<0.8.0
 and A depends on B (^0.7.5), version solving failed.

I traced the problem down into semver/__init__.py:parse_single_constraint where the constraint

• ~=0.2.0dev16
  gets compiled into
• >=0.2.0,<0.3.0

In contrast, the constraint

• ~=2.0.dev0
  correctly gets compiled into
• >=2.0.dev0,<3.0.0

The problem seems to be

        if precision == 2:
            low = version
            high = version.stable.next_major
        else:
            low = Version(version.major, version.minor, 0)
            high = version.stable.next_minor

where if the precision is 1 or 3, then the pre-release is dropped from low, disqualifying them from resolving the constraint.