Fix an issue trying list.index indexing in FusedNode

In some Python versions, generating the error message when .index
fails to find an index leads to a compiler crash.

Fix this by not relying on type being fully set up while generating
__str__.

Fixes cython#5894 and cython#5588

Cython/Compiler/PyrexTypes.py

@@ -3294,7 +3294,12 @@ def declaration_code(self, entity_code,
         if self.is_overridable:
             arg_decl_list.append("int %s" % Naming.skip_dispatch_cname)
         if self.optional_arg_count:
-            arg_decl_list.append(self.op_arg_struct.declaration_code(Naming.optional_args_cname))
+            if self.op_arg_struct:
+                arg_decl_list.append(self.op_arg_struct.declaration_code(Naming.optional_args_cname))
+            else:
+                # op_arg_struct may not be initialized at this point if this class is being used
+                # to prepare a Python error message or similar.  In this case, just omit the args.
+                assert for_display
         if self.has_varargs:
             arg_decl_list.append("...")
         arg_decl_code = ", ".join(arg_decl_list)

tests/run/fused_cpdef.pxd

@@ -0,0 +1,4 @@
+cimport cython
+
+cdef class C:
+    cpdef object has_default_struct(self, cython.floating x, a=?)

tests/run/fused_cpdef.pyx

@@ -205,3 +205,16 @@ def test_defaults():
     >>> mutable_default(3,[])
     [3]
     """
+
+cdef class C:
+    cpdef object has_default_struct(self, cython.floating x, a=None):
+        return x, a
+
+# https://github.com/cython/cython/issues/5588
+# On some Python versions this was causing a compiler crash
+def test_call_has_default_struct(C c, double x):
+    """
+    >>> test_call_has_default_struct(C(), 5.)
+    (5.0, None)
+    """
+    return c.has_default_struct(x)