types/struct: fix context when calculating default value

sx/types/struct.py

@@ -140,7 +140,7 @@ class StructType(G[T], Type[T]):
         stream.seek(pos + n, os.SEEK_SET)
 
     def default(self, context: Context) -> T:
-        return self.cls()
+        return self.cls(_sx_context_=context)
 
     def get_sizes(self, context: Context, value: O[Any], n: str) -> List[Mapping[str, int]]:
         sizes = []
@@ -178,7 +178,7 @@ class StructType(G[T], Type[T]):
             raise ValueError('path element for struct must be string')
         if field not in self.fields:
             raise ValueError(f'field {field!r} invalid for {self.cls.__name__}')
-    
+
         child = self.fields[field]
         with self.enter():
             if self.union:
@@ -220,11 +220,17 @@ class Struct:
     def __init__(self, **kwargs) -> None:
         super().__init__()
 
+        ctx = kwargs.pop('_sx_context_', None)
+
         st = self._sx_type_
         with st.enter():
             for k, t in st.fields.items():
                 if k not in kwargs:
-                    v = sx.default(t)
+                    if ctx:
+                        with ctx.enter(k, t):
+                            v = ctx.default(to_type(t))
+                    else:
+                        v = sx.default(t)
                 else:
                     v = kwargs.pop(k)
                 setattr(self, k, v)
@@ -273,7 +279,7 @@ class Struct:
                 val = next(v for v in val.__metadata__ if isinstance(v, Type))
             fields[name] = val
             proxy._sx_fields_[name] = []
-        
+
         del localns
         for name, exprs in proxy._sx_fields_.items():
             count = len(exprs)