[varLib] Fill in the forward-mapped location

Lib/fontTools/varLib/__init__.py

@@ -743,7 +743,8 @@ def load_designspace(designspace):
 			assert axis_name in axes, "Location axis '%s' unknown for '%s'." % (axis_name, obj_name)
 		for axis_name,axis in axes.items():
 			if axis_name not in loc:
-				loc[axis_name] = axis.default
+				# NOTE: `axis.default` is always user-space, but `obj.location` always design-space.
+				loc[axis_name] = axis.map_forward(axis.default)
 			else:
 				v = axis.map_backward(loc[axis_name])
 				assert axis.minimum <= v <= axis.maximum, "Location for axis '%s' (mapped to %s) out of range for '%s' [%s..%s]" % (axis_name, v, obj_name, axis.minimum, axis.maximum)

Tests/varLib/data/VarLibLocationTest.designspace

@@ -0,0 +1,26 @@
+<?xml version='1.0' encoding='utf-8'?>
+<designspace format="4.0">
+    <axes>
+        <axis default="100" maximum="900" minimum="100" name="weight" tag="wght">
+            <map input="500" output="105"/>
+            <map input="300" output="57"/>
+            <map input="900" output="158"/>
+            <map input="100" output="0"/>
+        </axis>
+        <axis default="50" maximum="100" minimum="0" name="width" tag="wdth" />
+    </axes>
+    <sources>
+        <source filename="A.ufo">
+            <location>
+                <dimension name="weight" xvalue="0" />
+                <dimension name="width" xvalue="50" />
+            </location>
+        </source>
+    </sources>
+    <instances>
+        <instance filename="C.ufo" familyname="C" stylename="CCC">
+            <location>
+            </location>
+        </instance>
+    </instances>
+</designspace>

Tests/varLib/varLib_test.py

@@ -1,6 +1,6 @@
 from fontTools.misc.py23 import *
 from fontTools.ttLib import TTFont, newTable
-from fontTools.varLib import build
+from fontTools.varLib import build, load_designspace
 from fontTools.varLib.mutator import instantiateVariableFont
 from fontTools.varLib import main as varLib_main, load_masters
 from fontTools.varLib import set_default_weight_width_slant
@@ -728,6 +728,13 @@ class BuildTest(unittest.TestCase):
             ("B", "D"): 40,
         }
 
+    def test_designspace_fill_in_location(self):
+        ds_path = self.get_test_input("VarLibLocationTest.designspace")
+        ds = DesignSpaceDocument.fromfile(ds_path)
+        ds_loaded = load_designspace(ds)
+
+        assert ds_loaded.instances[0].location == {"weight": 0, "width": 50}
+
 
 def test_load_masters_layerName_without_required_font():
     ds = DesignSpaceDocument()