Declare a default constructor only when there is no other one inherited

(if the class redefines a constructor with a compatible signature,
it will get overridden, so it effectively prevents the default
constructor from spawning when there may be an ambiguity)

Cython/Compiler/Symtab.py

@@ -651,6 +651,7 @@ class Scope(object):
             this_type = PyrexTypes.CPtrType(entry.type) if not cypclass else entry.type
             scope.declare_var(name="this", cname="this", type=this_type, pos=entry.pos)
             if cypclass:
+                if not scope.lookup_here("<init>"):
                     # Declare a shadow default constructor
                     wrapper_type = PyrexTypes.CFuncType(entry.type, [], nogil=1)
                     wrapper_cname = "%s__constructor__%s" % (Naming.func_prefix, name)