No pyproject.toml, nothing installed:
  freeze_output: |
    # empty
  expected: |
    python3dist(setuptools) >= 40.8
    python3dist(wheel)
  result: 0

Nothing installed yet:
  freeze_output: |
    # empty
  pyproject.toml: |
    # empty
  expected: |
    python3dist(setuptools) >= 40.8
    python3dist(wheel)
  result: 0

Insufficient version of setuptools:
  freeze_output: |
    setuptools==5
    wheel==1
  pyproject.toml: |
    # empty
  expected: |
    python3dist(setuptools) >= 40.8
    python3dist(wheel)
  result: 0

Empty pyproject.toml, empty setup.py:
  freeze_output: |
    setuptools==50
    wheel==1
  setup.py: |
  expected: |
    python3dist(setuptools) >= 40.8
    python3dist(wheel)
    python3dist(wheel)
  result: 0

Default build system, empty setup.py:
  freeze_output: |
    setuptools==50
    wheel==1
  pyproject.toml: |
    # empty
  setup.py: |
  expected: |
    python3dist(setuptools) >= 40.8
    python3dist(wheel)
    python3dist(wheel)
  result: 0

Erroring setup.py:
  freeze_output: |
    setuptools==50
    wheel==1
  setup.py: |
    exit(77)
  result: 77

Bad character in version:
  freeze_output: |
  pyproject.toml: |
    [build-system]
    requires = ["pkg == 0.$.^.*"]
  except: ValueError

Build system dependencies in pyproject.toml:
  freeze_output: |
    setuptools==50
    wheel==1
  pyproject.toml: |
    [build-system]
    requires = [
        "foo",
        "ne!=1",
        "ge>=1.2",
        "le <= 1.2.3",
        "lt < 1.2.3.4      ",
        "    gt > 1.2.3.4.5",
        "combo >2, <5, != 3.0.0",
        "invalid!!ignored",
        "py2 ; python_version < '2.7'",
        "py3 ; python_version > '3.0'",
        "pkg [extra-currently-ignored]",
    ]
  expected: |
    python3dist(foo)
    (python3dist(ne) < 1 or python3dist(ne) > 1.0)
    python3dist(ge) >= 1.2
    python3dist(le) <= 1.2.3
    python3dist(lt) < 1.2.3.4
    python3dist(gt) > 1.2.3.4.5
    ((python3dist(combo) < 3 or python3dist(combo) > 3.0) and python3dist(combo) < 5 and python3dist(combo) > 2)
    python3dist(py3)
    python3dist(pkg)
    python3dist(setuptools) >= 40.8
    python3dist(wheel)
  result: 0

Default build system, dependencies in setup.py:
  freeze_output: |
    setuptools==50
    wheel==1
  setup.py: |
    from setuptools import setup
    setup(
        name='test',
        version='0.1',
        setup_requires=['foo', 'bar!=2'],
    )
  expected: |
    python3dist(setuptools) >= 40.8
    python3dist(wheel)
    python3dist(wheel)
    python3dist(foo)
    (python3dist(bar) < 2 or python3dist(bar) > 2.0)
  result: 0