Hide nonce implementation details in metadata (#6562)

See https://github.com/polkadot-fellows/runtimes/issues/248 : using
`TypeWithDefault` having derived `TypeInfo` for `Nonce` causes a
breaking change in metadata for nonce type because it's no longer `u64`.

Adding a default implementation of `TypeInfo` for `TypeWithDefault` to
restore the original type info in metadata.

---------

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

prdoc/pr_6562.prdoc

+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Hide nonce implementation details in metadata
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Use custom implementation of TypeInfo for TypeWithDefault to show inner value's type info.
+      This should bring back nonce to u64 in metadata.
+
+crates:
+- name: sp-runtime
+  bump: minor
\ No newline at end of file

substrate/primitives/runtime/src/type_with_default.rs

@@ -31,7 +31,7 @@ use num_traits::{
 	CheckedAdd, CheckedDiv, CheckedMul, CheckedNeg, CheckedRem, CheckedShl, CheckedShr, CheckedSub,
 	Num, NumCast, PrimInt, Saturating, ToPrimitive,
 };
-use scale_info::TypeInfo;
+use scale_info::{StaticTypeInfo, TypeInfo};
 use sp_core::Get;
 
 #[cfg(feature = "serde")]
@@ -40,7 +40,8 @@ use serde::{Deserialize, Serialize};
 /// A type that wraps another type and provides a default value.
 ///
 /// Passes through arithmetical and many other operations to the inner value.
-#[derive(Encode, Decode, TypeInfo, Debug, MaxEncodedLen)]
+/// Type information for metadata is the same as the inner value's type.
+#[derive(Encode, Decode, Debug, MaxEncodedLen)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct TypeWithDefault<T, D: Get<T>>(T, PhantomData<D>);
 
@@ -50,6 +51,17 @@ impl<T, D: Get<T>> TypeWithDefault<T, D> {
 	}
 }
 
+// Hides implementation details from the outside (for metadata type information).
+//
+// The type info showed in metadata is the one of the inner value's type.
+impl<T: StaticTypeInfo, D: Get<T> + 'static> TypeInfo for TypeWithDefault<T, D> {
+	type Identity = Self;
+
+	fn type_info() -> scale_info::Type {
+		T::type_info()
+	}
+}
+
 impl<T: Clone, D: Get<T>> Clone for TypeWithDefault<T, D> {
 	fn clone(&self) -> Self {
 		Self(self.0.clone(), PhantomData)
@@ -511,6 +523,7 @@ impl<T: HasCompact, D: Get<T>> CompactAs for TypeWithDefault<T, D> {
 #[cfg(test)]
 mod tests {
 	use super::TypeWithDefault;
+	use scale_info::TypeInfo;
 	use sp_arithmetic::traits::{AtLeast16Bit, AtLeast32Bit, AtLeast8Bit};
 	use sp_core::Get;
 
@@ -565,5 +578,11 @@ mod tests {
 		}
 		type U128WithDefault = TypeWithDefault<u128, Getu128>;
 		impl WrapAtLeast32Bit for U128WithDefault {}
+
+		assert_eq!(U8WithDefault::type_info(), <u8 as TypeInfo>::type_info());
+		assert_eq!(U16WithDefault::type_info(), <u16 as TypeInfo>::type_info());
+		assert_eq!(U32WithDefault::type_info(), <u32 as TypeInfo>::type_info());
+		assert_eq!(U64WithDefault::type_info(), <u64 as TypeInfo>::type_info());
+		assert_eq!(U128WithDefault::type_info(), <u128 as TypeInfo>::type_info());
 	}
 }