Options: treat array option -Dopt= and -Dopt=[] as equivalent

Currently the former will be parsed as [''], while the latter is parsed
as [] in python. This makes for some obnoxious special handling
depending on what the user passes. This is even more obnoxious since for
string type arguments this doesn't require special handling.

docs/markdown/Build-options.md

@@ -61,6 +61,9 @@ empty. The `value` parameter specifies the default value of the option
 and if it is unset then the values of `choices` will be used as the
 default.
 
+As of 0.47.0 -Dopt= and -Dopt=[] both pass an empty list, before this -Dopt=
+would pass a list with an empty string.
+
 This type is available since version 0.44.0
 
 ### Features

docs/markdown/snippets/empty-array-opts.md

@@ -0,0 +1,5 @@
+## Array options treat -Dopt= and -Dopt=[] as equivalent
+
+Prior to this change passing -Dopt= to an array opt would be interpreted as
+[''] (an array with an empty string), now -Dopt= is the same as -Dopt=[], an
+empty list.

mesonbuild/coredata.py

@@ -154,6 +154,8 @@ class UserArrayOption(UserOption):
         if isinstance(value, str):
             if value.startswith('['):
                 newvalue = ast.literal_eval(value)
+            elif value == '':
+                newvalue = []
             else:
                 if self.shlex_split:
                     newvalue = shlex.split(value)

run_unittests.py

@@ -1786,6 +1786,26 @@ int main(int argc, char **argv) {
         changed = get_opt()
         self.assertDictEqual(changed, expected)
 
+    def test_array_option_empty_equivalents(self):
+        """Array options treat -Dopt=[] and -Dopt= as equivalent."""
+        def get_opt():
+            opts = self.introspect('--buildoptions')
+            for x in opts:
+                if x.get('name') == 'list':
+                    return x
+            raise Exception(opts)
+
+        expected = {
+            'name': 'list',
+            'description': 'list',
+            'type': 'array',
+            'value': [],
+        }
+        tdir = os.path.join(self.unit_test_dir, '18 array option')
+        self.init(tdir, extra_args='-Dlist=')
+        original = get_opt()
+        self.assertDictEqual(original, expected)
+
     def opt_has(self, name, value):
         res = self.introspect('--buildoptions')
         found = False