Add a --no-skip-missing-interpreters option

doc/config.rst

@@ -45,6 +45,12 @@ and will first lookup global tox settings in this section::
     always overrides this setting if passed on the invokation.
     **Default:** ``False``
 
+    .. versionadded:: (unreleased)
+
+    The ``--no-skip-missing-interpreters`` command line option can also be used
+    to override the configuration value to *ensure* failure on missing
+    interpreters.
+
 .. confval:: envlist=CSV
 
     Determining the environment list that ``tox`` is to operate on

tests/test_z_cmdline.py

@@ -330,6 +330,26 @@ def test_skip_unknown_interpreter(cmd, initproj):
     ])
 
 
+def test_no_skip_missing_interpreters(cmd, initproj):
+    initproj("interp123-0.5", filedefs={
+        'tests': {'test_hello.py': "def test_hello(): pass"},
+        'tox.ini': '''
+            [tox]
+            skip_missing_interpreters = True
+            [testenv:python]
+            basepython=xyz_unknown_interpreter
+            [testenv]
+            changedir=tests
+        '''
+    })
+    result = cmd.run("tox", "--no-skip-missing-interpreters")
+    # Unlike `test_skip_unknown_intepreter`, this should fail
+    assert result.ret
+    result.stdout.fnmatch_lines([
+        "*ERROR*InterpreterNotFound*xyz_unknown_interpreter*",
+    ])
+
+
 def test_unknown_dep(cmd, initproj):
     initproj("dep123-0.7", filedefs={
         'tests': {'test_hello.py': "def test_hello(): pass"},

tox/config.py

@@ -387,7 +387,13 @@ def tox_addoption(parser):
     parser.add_argument("--alwayscopy", action="store_true",
                         help="override alwayscopy setting to True in all envs")
     parser.add_argument("--skip-missing-interpreters", action="store_true",
+                        default=None,
                         help="don't fail tests for missing interpreters")
+    parser.add_argument(
+        '--no-skip-missing-interpreters', action='store_false',
+        dest='skip_missing_interpreters',
+        help='Ensure failure when interpreters are missing.',
+    )
     parser.add_argument("--workdir", action="store",
                         dest="workdir", metavar="PATH", default=None,
                         help="tox working directory")
@@ -749,9 +755,10 @@ def __init__(self, config, inipath):
         else:
             config.toxworkdir = config.toxinidir.join(config.option.workdir, abs=True)
 
-        if not config.option.skip_missing_interpreters:
-            config.option.skip_missing_interpreters = \
-                reader.getbool("skip_missing_interpreters", False)
+        if config.option.skip_missing_interpreters is None:
+            config.option.skip_missing_interpreters = reader.getbool(
+                'skip_missing_interpreters', False,
+            )
 
         # determine indexserver dictionary
         config.indexserver = {'default': IndexServerConfig('default')}