coredata: Passing an option is supposed to set it, not flip the default

With the current logic passing `--debug` will actually be parsed as
`-Ddebug=false`, which is absolutely not what is expected.

There is no case in which the presence of a boolean option in `--foo`
form will mean 'I want feature foo disabled', regardless of the
*default* value of that option.

Also includes a test.

Closes https://github.com/mesonbuild/meson/issues/4686

mesonbuild/coredata.py

@@ -1001,9 +1001,10 @@ class BuiltinOption(T.Generic[_T, _U]):
         return self.opt_type(self.description, **keywords)
 
     def _argparse_action(self) -> T.Optional[str]:
-        if self.default is True:
-            return 'store_false'
-        elif self.default is False:
+        # If the type is a boolean, the presence of the argument in --foo form
+        # is to enable it. Disabling happens by using -Dfoo=false, which is
+        # parsed under `args.projectoptions` and does not hit this codepath.
+        if isinstance(self.default, bool):
             return 'store_true'
         return None
 

run_unittests.py

@@ -3842,6 +3842,14 @@ recommended as it is not supported on some platforms''')
         self.assertEqual(opts['debug'], True)
         self.assertEqual(opts['optimization'], '2')
         self.assertEqual(opts['buildtype'], 'debugoptimized')
+        # Setting both buildtype and debug on the command-line should work
+        # Also test that --debug is parsed as -Ddebug=true
+        self.new_builddir()
+        self.init(testdir, extra_args=['-Dbuildtype=debugoptimized', '--debug'])
+        opts = self.get_opts_as_dict()
+        self.assertEqual(opts['debug'], True)
+        self.assertEqual(opts['optimization'], '2')
+        self.assertEqual(opts['buildtype'], 'debugoptimized')
 
     @skipIfNoPkgconfig
     @unittest.skipIf(is_windows(), 'Help needed with fixing this test on windows')