config: Prevent .tox in envlist

Fixes #1684

docs/changelog/1684.feature.rst

@@ -0,0 +1 @@
+Prevent .tox in envlist - by :user:`jayvdb`

src/tox/config/__init__.py

@@ -1475,13 +1475,19 @@ def _getenvdata(self, reader, config):
         if not env_list:
             env_list = all_envs
 
+        provision_tox_env = config.provision_tox_env
+        if config.provision_tox_env in env_list:
+            msg = "provision_tox_env {} cannot be part of envlist".format(provision_tox_env)
+            raise tox.exception.ConfigError(msg)
+
         package_env = config.isolated_build_env
         if config.isolated_build is True and package_env in all_envs:
             all_envs.remove(package_env)
 
         if config.isolated_build is True and package_env in env_list:
             msg = "isolated_build_env {} cannot be part of envlist".format(package_env)
             raise tox.exception.ConfigError(msg)
+
         return env_list, all_envs, _split_env(from_config), envlist_explicit
 
     @staticmethod

tests/unit/config/test_config.py

@@ -3479,6 +3479,22 @@ def test_commands_with_backslash(self, newconfig):
         assert envconfig.commands[0] == ["some", r"hello\world"]
 
 
+def test_provision_tox_env_cannot_be_in_envlist(newconfig, capsys):
+    inisource = """
+            [tox]
+            envlist = py36,.tox
+        """
+    with pytest.raises(
+        tox.exception.ConfigError,
+        match="provision_tox_env .tox cannot be part of envlist",
+    ):
+        newconfig([], inisource)
+
+    out, err = capsys.readouterr()
+    assert not err
+    assert not out
+
+
 def test_isolated_build_env_cannot_be_in_envlist(newconfig, capsys):
     inisource = """
             [tox]