Also return True if the default source is the only one containing fea…

…tures.

This allows us to use variable features more easily by putting them in the
default UFO only.

Lib/ufo2ft/featureCompiler.py

@@ -446,7 +446,8 @@ def setupFeatures(self):
 
 
 def _featuresCompatible(designSpaceDoc: DesignSpaceDocument) -> bool:
-    """Returns whether the features of the individual source UFOs are the same.
+    """Returns True when the features of the individual source UFOs are the same,
+    or when only the default source has features.
 
     NOTE: Only compares the feature file text inside the source UFO and does not
     follow imports. This will suffice as long as no external feature file is
@@ -463,5 +464,13 @@ def transform(f: SourceDescriptor) -> str:
         text = re.sub(r"\s+", " ", text)
         return text
 
-    first = transform(designSpaceDoc.sources[0])
-    return all(transform(s) == first for s in designSpaceDoc.sources[1:])
+    sources = sorted(
+        designSpaceDoc.sources, key=lambda source: source != designSpaceDoc.default
+    )
+    assert sources[0] == designSpaceDoc.default
+
+    transformed = [transform(s) for s in designSpaceDoc.sources]
+    first = transformed[0]
+    return all(s == first for s in transformed[1:]) or all(
+        not s for s in transformed[1:]
+    )