Cannot recognize C type aliases

Here is a snippet of the C header file that I want to create a binding for:

``` c
#include

typedef struct _my_string_utf8_t {
char* str;
size_t length;
} my_string_utf8_t;

typedef my_string_utf8_t my_string_t;

void my_string_free(my_string_t* value);
void my_string_utf8_free(my_string_utf8_t* value);
```

`my_string_t` is a type alias for `my_string_utf8_t`.

``` haskell
{-# LANGUAGE ForeignFunctionInterface #-}

#include "foo.h"

module Foo where

import Foreign.C.Types
import Foreign.Marshal
import Foreign.Ptr
import Foreign.Storable

data MyStringUtf8 = MyStringUtf8
{ str'MyStringUtf8 :: Ptr CChar
, length'MyStringUtf8 :: CSize
}
{#pointer *my_string_utf8_t as MyStringUtf8Ptr -> MyStringUtf8#}

instance Storable MyStringUtf8 where
sizeOf _ = {#sizeof my_string_utf8_t#}
alignment _ = {#alignof my_string_utf8_t#}
peek p = MyStringUtf8
<$> {#get my_string_utf8_t->str#} p
<*> {#get my_string_utf8_t->length#} p
poke p x = do
{#set my_string_utf8_t->str#} p (str'MyStringUtf8 x)
{#set my_string_utf8_t->length#} p (length'MyStringUtf8 x)

type MyString = MyStringUtf8
{#pointer *my_string_t as MyStringPtr -> MyString#}

{#fun unsafe my_string_free as ^ {id `MyStringPtr'} -> `()' #}
{#fun unsafe my_string_utf8_free as ^ {id `MyStringUtf8Ptr'} -> `()' #}
```

I created a Haskell type alias `MyString` and used its pointer type, `MyStringPtr` in defining the function hook for `my_string_free`. However, the binding for `my_string_free` fails to recognize `my_string_t` type and used `Ptr ()` instead of `MyStringPtr`. It seems c2hs cannot recognize the C type alias `typedef my_string_utf8_t my_string_t;`.

``` haskell
foreign import ccall unsafe "Foo.chs.h my_string_free"
myStringFree'_ :: ((C2HSImp.Ptr ()) -> (IO ()))
```

The binding `for my_string_utf8_free` is created as I expected:

``` haskell
foreign import ccall unsafe "Foo.chs.h my_string_utf8_free"
myStringUtf8Free'_ :: ((MyStringPtr) -> (IO ()))
```

I am not sure why c2hs used `MyStringPtr` instead of `MyStringUtf8Ptr` though.