Improve error message on where clause on pallet error (#7821)

* improve error message on where clause on pallet error

* Revert "improve error message on where clause on pallet error"

This reverts commit 5a3cc38976813fccef3357833553ce30f5b988ea.

* Revert "Revert "improve error message on where clause on pallet error""

This reverts commit e3b3fca6bc4fa89816f80dbcb82dc4536a9b2549.

substrate/frame/support/procedural/src/pallet/parse/error.rs

@@ -53,7 +53,7 @@ impl ErrorDef {
 		instances.push(helper::check_type_def_gen_no_bounds(&item.generics, item.ident.span())?);
 
 		if item.generics.where_clause.is_some() {
-			let msg = "Invalid pallet::error, unexpected where clause";
+			let msg = "Invalid pallet::error, where clause is not allowed on pallet error item";
 			return Err(syn::Error::new(item.generics.where_clause.as_ref().unwrap().span(), msg));
 		}
 

substrate/frame/support/src/lib.rs

@@ -1256,6 +1256,8 @@ pub mod pallet_prelude {
 /// }
 /// ```
 /// I.e. a regular rust enum named `Error`, with generic `T` and fieldless variants.
+/// The generic `T` mustn't bound anything and where clause is not allowed. But bounds and where
+/// clause shouldn't be needed for any usecase.
 ///
 /// ### Macro expansion
 ///

substrate/frame/support/test/tests/pallet_ui/error_where_clause.rs

@@ -0,0 +1,23 @@
+#[frame_support::pallet]
+mod pallet {
+	use frame_support::pallet_prelude::Hooks;
+	use frame_system::pallet_prelude::BlockNumberFor;
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(core::marker::PhantomData<T>);
+
+	#[pallet::hooks]
+	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
+
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {}
+
+	#[pallet::error]
+	pub enum Error<T> where u32: From<u8> {}
+}
+
+fn main() {
+}

substrate/frame/support/test/tests/pallet_ui/error_where_clause.stderr

@@ -0,0 +1,5 @@
+error: Invalid pallet::error, where clause is not allowed on pallet error item
+  --> $DIR/error_where_clause.rs:19:20
+   |
+19 |     pub enum Error<T> where u32: From<u8> {}
+   |                       ^^^^^