fix: Poetry tilde on a short version converts to the wrong upper bound

[deepspace28]

What's broken

pdm import -f poetry converts every tilde constraint to PEP 440's ~=. That only means the same thing when all three components are present. With a short version it either widens the constraint or produces a specifier that isn't valid at all:

Poetry	means	~= equivalent	pdm imported
~1.2.3	>=1.2.3 <1.3.0	>=1.2.3, ==1.2.*	correct
~1.2	>=1.2.0 <1.3.0	>=1.2, ==1.*	<2.0, too wide
~1	>=1.0.0 <2.0.0	— not a valid specifier	import aborts

~= requires at least two release segments, so the one-component case doesn't just import wrongly, it raises.

Repro

foo = "~1.2" and bar = "~1" in a Poetry project:

before:
dependencies = ["foo~=1.2"]                 # accepts 1.9.9, which Poetry refuses
bar          -> MetaConvertError: dependencies: Invalid specifier for bar: ~=1

after:
dependencies = ["foo<1.3,>=1.2", "bar<2,>=1"]

The same path handles requires-python, so python = "~3" aborts the whole import today:

before: InvalidPyVersion: Invalid specifier: ~=3
after:  requires-python = "<4,>=3"

The fix

A tilde allows patch-level changes when a minor version is given and minor-level changes otherwise, so the bumped position is the minor when there is one and the major when there isn't. _tilde_upper_bound computes it and the tilde gets an explicit upper bound, the way _caret_upper_bound already does for ^ (#3848).

Verification

test_convert_poetry_tilde_constraint is parametrised over Poetry's own tilde table, ~1.2.3 through ~0, checked against the table in their dependency-specification docs, plus test_convert_poetry_tilde_python_constraint for the requires-python path. Reverting only src/pdm/formats/poetry.py turns 8 of the 9 new cases red; the one that stays green is ~1.2.3, which ~= already handled correctly.

One assertion in test_convert_poetry had the old bound baked in. The fixture is Poetry's own pyproject.toml with python = "~2.7 || ^3.4", so requires-python was asserting the bug — ~2.7 stops below 2.8, and the expectation now carries the resulting !=2.8.*.

tests/test_formats.py passes, 68 tests. Full suite is 1378 passed / 7 failed on this box, and those 7 are identical on a clean checkout — they need symlink privileges, POSIX echo/cat on PATH, or a working python3.14. ruff check and ruff format --check clean. Windows 11, CPython 3.11.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.52%. Comparing base (a7e9d00) to head (6a468d3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3858   +/-   ##
=======================================
  Coverage   88.51%   88.52%           
=======================================
  Files         121      121           
  Lines       13281    13288    +7     
  Branches     2257     2257           
=======================================
+ Hits        11756    11763    +7     
  Misses        960      960           
  Partials      565      565           

Flag	Coverage Δ
unittests	88.41% <100.00%> (+<0.01%)	⬆️

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