Don't generate a PaintColrLayers for a list of 1

Lib/fontTools/colorLib/builder.py

@@ -524,7 +524,12 @@ class LayerCollector:
             layerGlyph, paint = paint
             return buildPaintGlyph(self, layerGlyph, paint)
         elif isinstance(paint, list):
-            # implicit PaintColrLayers
+            # implicit PaintColrLayers for a list of > 1
+            if len(paint) == 0:
+                raise ValueError("An empty list is hard to paint")
+            elif len(paint) == 1:
+                return self.build(paint[0])
+            else:
                 return self.buildColrLayers(paint)
         elif isinstance(paint, collections.abc.Mapping):
             kwargs = dict(paint)

Tests/colorLib/builder_test.py

@@ -643,7 +643,7 @@ def assertNoV0Content(colr):
 
 
 def test_build_layerv1list_empty():
-    # Nobody uses PaintColorLayers (format 8), no layerlist
+    # Nobody uses PaintColrLayers (format 8), no layerlist
     colr = builder.buildCOLR(
         {
             "a": {
@@ -651,7 +651,8 @@ def test_build_layerv1list_empty():
                 "paint": {"format": 1, "paletteIndex": 2, "alpha": 0.8},
                 "glyph": "b",
             },
-            "b": {
+            # A list of 1 shouldn't become a PaintColrLayers
+            "b": [{
                 "format": 4, # PaintGlyph
                 "paint":  {
                     "format": 2,
@@ -664,7 +665,7 @@ def test_build_layerv1list_empty():
                     "p2": (2, 2),
                 },
                 "glyph": "bb",
-            },
+            }],
         }
     )
 
@@ -836,7 +837,7 @@ class BuildCOLRTest(object):
                             "p1": (3, 4),
                             "p2": (2, 2),
                         },
-                    )
+                    ),
                 ],
             }
         )
@@ -860,7 +861,8 @@ class BuildCOLRTest(object):
                             "p1": (3, 4),
                             "p2": (2, 2),
                         },
-                    )
+                    ),
+                    ("f", {"format": 1, "paletteIndex": 2, "alpha": 0.8}),
                 ],
             }
         )