WIP: ´Fix attribute lookup for __set__ only descriptors

This makes the first half of the test in test_descr.set_and_no_get pass.
The second part using metaclassses still fail.

I am pretty sure this is not the proper approach, but I don't know what
that would be.

See IronLanguages#1722.

Src/IronPython/Runtime/Binding/MetaUserObject.Members.cs

@@ -104,7 +104,9 @@ public TResult Bind(CodeContext context, string name) {
  foundSlot = FindSlot(context, name, sdo, out systemTypeResolution, out extensionMethodResolution);
  _extensionMethodRestriction = extensionMethodResolution;
 
- if (sdo.PythonType.HasDictionary && (foundSlot == null || !foundSlot.IsSetDescriptor(context, sdo.PythonType))) {
+ if (sdo.PythonType.HasDictionary && (foundSlot == null ||
+ !foundSlot.IsSetDescriptor(context, sdo.PythonType) ||
+ !foundSlot.IsGetDescriptor(context, sdo.PythonType))) {
  MakeDictionaryAccess();
  }
 

Src/IronPython/Runtime/Types/PythonTypeSlot.cs

@@ -103,6 +103,10 @@ internal virtual bool GetAlwaysSucceeds {
  }
  }
 
+ internal virtual bool IsGetDescriptor(CodeContext context, PythonType owner) {
+ return false;
+ }
+
  internal virtual bool IsSetDescriptor(CodeContext context, PythonType owner) {
  return false;
  }

Src/IronPython/Runtime/Types/PythonTypeUserDescriptorSlot.cs

@@ -77,6 +77,12 @@ internal override bool IsSetDescriptor(CodeContext context, PythonType owner) {
  return pt.TryResolveSlot(context, "__set__", out _);
  }
 
+ internal override bool IsGetDescriptor(CodeContext context, PythonType owner) {
+ PythonType pt = DynamicHelpers.GetPythonType(Value);
+ if (pt.IsSystemType) return false;
+ return pt.TryResolveSlot(context, "__get__", out _);
+ }
+
  internal object Value { get; set; }
  }
 }