Make TypeInfo for SkipCheckIfFeeless transparent, too (#2449)

The `SkipCheckifFeeless::IDENTIFIER` became transparent (ie was whatever
the inner signed ext was). This PR just makes the `TypeInfo` transparent
too, so that libraries that use said info to decode the data (ie subxt)
can behave identically whether or not the `SkipCheckifFeeless` wrapper
is used or not.

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/transaction-payment/skip-feeless-payment/src/lib.rs

@@ -40,7 +40,7 @@ use frame_support::{
 	dispatch::{CheckIfFeeless, DispatchResult},
 	traits::{IsType, OriginTrait},
 };
-use scale_info::TypeInfo;
+use scale_info::{StaticTypeInfo, TypeInfo};
 use sp_runtime::{
 	traits::{DispatchInfoOf, PostDispatchInfoOf, SignedExtension},
 	transaction_validity::TransactionValidityError,
@@ -75,10 +75,17 @@ pub mod pallet {
 }
 
 /// A [`SignedExtension`] that skips the wrapped extension if the dispatchable is feeless.
-#[derive(Encode, Decode, Clone, Eq, PartialEq, TypeInfo)]
-#[scale_info(skip_type_params(T))]
+#[derive(Encode, Decode, Clone, Eq, PartialEq)]
 pub struct SkipCheckIfFeeless<T, S>(pub S, sp_std::marker::PhantomData<T>);
 
+// Make this extension "invisible" from the outside (ie metadata type information)
+impl<T, S: StaticTypeInfo> TypeInfo for SkipCheckIfFeeless<T, S> {
+	type Identity = S;
+	fn type_info() -> scale_info::Type {
+		S::type_info()
+	}
+}
+
 impl<T, S: Encode> sp_std::fmt::Debug for SkipCheckIfFeeless<T, S> {
 	#[cfg(feature = "std")]
 	fn fmt(&self, f: &mut sp_std::fmt::Formatter) -> sp_std::fmt::Result {