Unable to export requirements.txt from unpinned dependencies (PySide6)

[omars44]

Possibly related to: #1729

When trying to export requirements.txt with this dependency and no lock file, an exceptions is thrown

resolvelib.resolvers.RequirementsConflicted: Requirements conflict: NamedRequirement(name='PySide6', marker=None, extras=set(), specifier=<SpecifierSet('')>, editable=False, prerelease=False)

This however works normally when pinned:

dependencies = [
    "PySide6>=6.4.2",
]

pyproject.toml

[project]
name = 'example'
dynamic = ['version']
description = ''
authors = [
    {name = 'User', email = 'user@example.com'},
]
dependencies = [
    'PySide6'
]
requires-python = '>=3.9,<3.10'
license = {text = 'MIT'}

[project.optional-dependencies]
dev = [
    'mkdocs-material',
    'mkdocstrings',
    'mkdocstrings-python',
    'mkdocs-git-revision-date-plugin',
    'bump2version',
    'build'
]

[build-system]
requires = ['pdm-pep517']
build-backend = 'pdm.pep517.api'

[project.scripts]
pdm = 'pdm.core:main'

[tool.coverage.run]
branch = true
parallel = true
omit = [
  'example/__about__.py',
]

[tool.coverage.report]
exclude_lines = [
  'no cov',
  'if __name__ == .__main__.:',
  'if TYPE_CHECKING:',
]

[tool.mypy]
disallow_untyped_calls = true
disallow_untyped_defs = true
disallow_incomplete_defs = true
check_untyped_defs = true
disallow_subclassing_any = true
warn_no_return = true
strict_optional = true
no_implicit_optional = true
disallow_any_generics = true
disallow_any_unimported = true
warn_redundant_casts = true
warn_unused_configs = true
show_traceback = true
always_true = 'MYPYC'


[tool.pytest]
testpaths='tests/'

• I have searched the issue tracker and believe that this is not a duplicate.

Steps to reproduce

1. create a an empty directory
2. create the above pyproject.toml
3. try to export to a requirements file

pdm export -v -o requirements.txt

Actual behavior

Requirements Conflict exception is thrown

pdm export -v -o requirements.txt
The exported requirements file is no longer cross-platform. Using it on other platforms may cause unexpected result.
STATUS: Resolving packages from lockfile...
Traceback (most recent call last):
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/resolvelib/resolvers.py", line 372, in resolve
    self._add_to_criteria(self.state.criteria, r, parent=None)
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/resolvelib/resolvers.py", line 173, in _add_to_criteria
    raise RequirementsConflicted(criterion)
resolvelib.resolvers.RequirementsConflicted: Requirements conflict: NamedRequirement(name='PySide6', marker=None, extras=set(), specifier=<SpecifierSet('')>, editable=False, prerelease=False)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/user/.local/bin/pdm", line 8, in <module>
    sys.exit(main())
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/pdm/core.py", line 259, in main
    return Core().main(args)
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/pdm/core.py", line 191, in main
    raise cast(Exception, err).with_traceback(traceback)
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/pdm/core.py", line 186, in main
    f(project, options)
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/pdm/cli/commands/export.py", line 71, in handle
    candidates = resolve_candidates_from_lockfile(
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/pdm/cli/actions.py", line 142, in resolve_candidates_from_lockfile
    mapping, *_ = resolve(
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/pdm/resolver/core.py", line 35, in resolve
    result = resolver.resolve(requirements, max_rounds)
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/resolvelib/resolvers.py", line 521, in resolve
    state = resolution.resolve(requirements, max_rounds=max_rounds)
  File "/home/user/.local/share/pdm/venv/lib/python3.9/site-packages/resolvelib/resolvers.py", line 374, in resolve
    raise ResolutionImpossible(e.criterion.information)
resolvelib.resolvers.ResolutionImpossible: [RequirementInformation(requirement=NamedRequirement(name='PySide6', marker=None, extras=set(), specifier=<SpecifierSet('')>, editable=False, prerelease=False), parent=None)]

Expected behavior

Either a complain about a missing lock file
or lock and then successfully produce the requirements.txt

Environment Information

pdm info && pdm info --env
PDM version:
  2.4.5
Python Interpreter:
  /usr/bin/python3 (3.9)
Project Root:
  /home/user/dev/python/example-cookiecutter/example
Project Packages:
  /home/user/dev/python/example-cookiecutter/example/__pypackages__/3.9
{
  "implementation_name": "cpython",
  "implementation_version": "3.9.2",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "5.10.0-21-rt-amd64",
  "platform_system": "Linux",
  "platform_version": "#1 SMP PREEMPT_RT Debian 5.10.162-1 (2023-01-21)",
  "python_full_version": "3.9.2",
  "platform_python_implementation": "CPython",
  "python_version": "3.9",
  "sys_platform": "linux"
}