diff --git a/array.rs b/array.rs
index 48116b6..17be32c 100644
--- a/array.rs
+++ b/array.rs
@@ -40,7 +40,7 @@ pub type CFArrayRef = *__CFArray;
 impl AbstractCFTypeRef for CFArrayRef {
     fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
     #[fixed_stack_segment]
-    fn type_id() -> CFTypeID { unsafe { CFArrayGetTypeID() } }
+    fn type_id(_dummy: Option<CFArrayRef>) -> CFTypeID { unsafe { CFArrayGetTypeID() } }
 }
 
 // FIXME: Should be a newtype struct, but that fails due to a Rust compiler bug.
diff --git a/base.rs b/base.rs
index fe451cf..096e56e 100644
--- a/base.rs
+++ b/base.rs
@@ -16,7 +16,9 @@ use std::libc::c_long;
 // borrow semantics. 
 pub trait AbstractCFTypeRef {
     fn as_type_ref(&self) -> CFTypeRef;
-    fn type_id() -> CFTypeID;
+
+    // FIXME https://github.com/mozilla-servo/rust-core-foundation/issues/17
+    fn type_id(_dummy: Option<Self>) -> CFTypeID;
 }
 
 pub type Boolean = u8;
@@ -48,13 +50,14 @@ pub type CFTypeRef = *__CFType;
 impl AbstractCFTypeRef for CFTypeRef {
     fn as_type_ref(&self) -> CFTypeRef { *self }
     // this can't be used, because CFType is the supertype and has no type id.
-    fn type_id() -> CFTypeID { fail!(); }
+    fn type_id(_dummy: Option<CFTypeRef>) -> CFTypeID { fail!(); }
 }
 
 #[fixed_stack_segment]
 pub fn downcast<T:AbstractCFTypeRef>(r: CFTypeRef) -> T {
     unsafe {
-        assert!(CFGetTypeID(r) == AbstractCFTypeRef::type_id::<T>());
+        let dummy: Option<T> = None;
+        assert!(CFGetTypeID(r) == AbstractCFTypeRef::type_id(dummy));
         cast::transmute::<CFTypeRef, T>(r)
     }
 }
@@ -125,7 +128,8 @@ impl<'self, T:Clone + AbstractCFTypeRef, E1, E2> CFWrapper<T,E1,E2> {
 
     pub fn from_CFType(wrapper: CFType) -> CFWrapper<T,E1,E2> {
         unsafe {
-            assert!(wrapper.type_id() == AbstractCFTypeRef::type_id::<T>());
+            let dummy: Option<T> = None;
+            assert!(wrapper.type_id() == AbstractCFTypeRef::type_id(dummy));
             cast::transmute::<CFType,CFWrapper<T,E1,E2>>(wrapper)
         }
     }
diff --git a/boolean.rs b/boolean.rs
index bdd8a8e..c1b8e59 100644
--- a/boolean.rs
+++ b/boolean.rs
@@ -23,7 +23,7 @@ impl AbstractCFTypeRef for CFBooleanRef {
     fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
 
     #[fixed_stack_segment]
-    fn type_id() -> CFTypeID {
+    fn type_id(_dummy: Option<CFBooleanRef>) -> CFTypeID {
         unsafe { CFBooleanGetTypeID() }
     }
 }
diff --git a/data.rs b/data.rs
index 7ae3a07..bf23bab 100644
--- a/data.rs
+++ b/data.rs
@@ -27,7 +27,7 @@ impl AbstractCFTypeRef for CFDataRef {
     fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
 
     #[fixed_stack_segment]
-    fn type_id() -> CFTypeID {
+    fn type_id(_dummy: Option<CFDataRef>) -> CFTypeID {
         unsafe {
             CFDataGetTypeID()
         }
diff --git a/dictionary.rs b/dictionary.rs
index c1d4746..a0b98af 100644
--- a/dictionary.rs
+++ b/dictionary.rs
@@ -55,7 +55,7 @@ impl AbstractCFTypeRef for CFDictionaryRef {
     fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
 
     #[fixed_stack_segment]
-    fn type_id() -> CFTypeID {
+    fn type_id(_dummy: Option<CFDictionaryRef>) -> CFTypeID {
         unsafe {
             CFDictionaryGetTypeID()
         }
diff --git a/number.rs b/number.rs
index 258f11c..1d2ad67 100644
--- a/number.rs
+++ b/number.rs
@@ -44,7 +44,7 @@ impl AbstractCFTypeRef for CFNumberRef {
     fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
 
     #[fixed_stack_segment]
-    fn type_id() -> CFTypeID {
+    fn type_id(_dummy: Option<CFNumberRef>) -> CFTypeID {
         unsafe {
             CFNumberGetTypeID()
         }
diff --git a/set.rs b/set.rs
index aac9696..efc6fa7 100644
--- a/set.rs
+++ b/set.rs
@@ -43,7 +43,7 @@ impl AbstractCFTypeRef for CFSetRef {
     fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
 
     #[fixed_stack_segment]
-    fn type_id() -> CFTypeID {
+    fn type_id(_dummy: Option<CFSetRef>) -> CFTypeID {
         unsafe {
             CFSetGetTypeID()
         }
diff --git a/string.rs b/string.rs
index 4702f45..3c4d0a0 100644
--- a/string.rs
+++ b/string.rs
@@ -203,7 +203,7 @@ impl AbstractCFTypeRef for CFStringRef {
     fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
 
     #[fixed_stack_segment]
-    fn type_id() -> CFTypeID {
+    fn type_id(_dummy: Option<CFStringRef>) -> CFTypeID {
         unsafe {
             CFStringGetTypeID()
         }
diff --git a/url.rs b/url.rs
index b22a8ee..53b5b54 100644
--- a/url.rs
+++ b/url.rs
@@ -20,7 +20,7 @@ impl AbstractCFTypeRef for CFURLRef {
     fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
 
     #[fixed_stack_segment]
-    fn type_id() -> CFTypeID {
+    fn type_id(_dummy: Option<CFURLRef>) -> CFTypeID {
         unsafe {
             CFURLGetTypeID()
         }