Prevent crash when accessing the "__kwdefaults__" special attribute of fused functions.

Closes #1470.

Cython/Compiler/ExprNodes.py

@@ -9460,7 +9460,8 @@ class PyCFunctionNode(ExprNode, ModuleNameMixin):
         if self.defaults_kwdict:
             code.putln('__Pyx_CyFunction_SetDefaultsKwDict(%s, %s);' % (
                 self.result(), self.defaults_kwdict.py_result()))
-        if def_node.defaults_getter:
+        if def_node.defaults_getter and not self.specialized_cpdefs:
+            # Fused functions do not support dynamic defaults, only their specialisations can have them for now.
             code.putln('__Pyx_CyFunction_SetDefaultsGetter(%s, %s);' % (
                 self.result(), def_node.defaults_getter.entry.pyfunc_cname))
         if self.annotations_dict:

tests/run/fused_def.pyx

@@ -58,12 +58,21 @@ def opt_func(fused_t obj, cython.floating myf = 1.2, cython.integral myi = 7):
     >>> opt_func("spam", f, i)
     str object double long
     spam 5.60 9 5.60 9
+    >>> opt_func("spam", f, myi=i)
+    str object double long
+    spam 5.60 9 5.60 9
+    >>> opt_func("spam", myf=f, myi=i)
+    str object double long
+    spam 5.60 9 5.60 9
     >>> opt_func[str, float, int]("spam", f, i)
     str object float int
     spam 5.60 9 5.60 9
     >>> opt_func[str, cy.double, cy.long]("spam", f, i)
     str object double long
     spam 5.60 9 5.60 9
+    >>> opt_func[str, cy.double, cy.long]("spam", f, myi=i)
+    str object double long
+    spam 5.60 9 5.60 9
     >>> opt_func[str, float, cy.int]("spam", f, i)
     str object float int
     spam 5.60 9 5.60 9
@@ -129,6 +138,28 @@ def test_opt_func():
     opt_func("ham", f, entry4)
 
 
+def test_opt_func_introspection():
+    """
+    >>> opt_func.__defaults__
+    (1.2, 7)
+    >>> opt_func.__kwdefaults__
+    >>> opt_func.__annotations__
+    {}
+
+    >>> opt_func[str, float, int].__defaults__
+    (1.2, 7)
+    >>> opt_func[str, float, int].__kwdefaults__
+    >>> opt_func[str, float, int].__annotations__
+    {}
+
+    >>> opt_func[str, cy.double, cy.long].__defaults__
+    (1.2, 7)
+    >>> opt_func[str, cy.double, cy.long].__kwdefaults__
+    >>> opt_func[str, cy.double, cy.long].__annotations__
+    {}
+    """
+
+
 def func_with_object(fused_with_object obj, cython.integral myi = 7):
     """
     >>> func_with_object(1)