Failing test for unset conf (#17445)

* Initial sketch for better unset of confs

* Fail test for now

* Make choice'ed unset behave the same

---------

Co-authored-by: Carlos Zoido <mrgalleta@gmail.com>

conans/model/conf.py

@@ -322,7 +322,7 @@ def get(self, conf_name, default=None, check_type=None, choices=None):
         conf_value = self._values.get(conf_name)
         if conf_value:
             v = conf_value.value
-            if choices is not None and v not in choices:
+            if choices is not None and v not in choices and v is not None:
                 raise ConanException(f"Unknown value '{v}' for '{conf_name}'")
             # Some smart conversions
             if check_type is bool and not isinstance(v, bool):

test/unittests/model/test_conf.py

@@ -326,3 +326,22 @@ def test_conf_scope_patterns_bad(scope, conf, assert_message):
         c.loads(final_conf)
         c.validate()
     assert assert_message in str(exc_info.value)
+
+
+@pytest.mark.parametrize("choices", [None, ["Foo", "Bar"]])
+def test_unset_basic_same_behaviour(choices):
+    c = ConfDefinition()
+    assert c.get("user.company.cpu:jobs", choices=choices) is None
+
+    c2 = ConfDefinition()
+    c2.loads("user.company.cpu:jobs=!")
+    assert c2.get("user.company.cpu:jobs", choices=choices) is None
+
+    c3 = ConfDefinition()
+    c3.loads("user.company.cpu:jobs=4")
+
+    c4 = ConfDefinition()
+    c4.loads("user.company.cpu:jobs=!")
+    c3.update_conf_definition(c4)
+
+    assert c3.get("user.company.cpu:jobs", choices=choices) is None