From c7e7b8b0125ec1f4ae4d681ecf76c34d1c28624e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bastian=20K=C3=B6cher?= <bkchr@users.noreply.github.com>
Date: Sat, 6 Jul 2019 13:56:55 +0200
Subject: [PATCH] Document limitations of `decl_storage!` with instancing
 (#3044)

---
 substrate/srml/support/procedural/src/lib.rs | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/substrate/srml/support/procedural/src/lib.rs b/substrate/srml/support/procedural/src/lib.rs
index 2d35a237c6..b8bcb15de1 100644
--- a/substrate/srml/support/procedural/src/lib.rs
+++ b/substrate/srml/support/procedural/src/lib.rs
@@ -147,6 +147,25 @@ use proc_macro::TokenStream;
 /// ```nocompile
 /// trait Store for Module<T: Trait> as Example where T::AccountId: std::fmt::Display {}
 /// ```
+///
+/// ## Limitations
+///
+/// # Instancing and generic `GenesisConfig`
+///
+/// If your module supports instancing and you see an error like `parameter `I` is never used` for
+/// your `decl_storage!`, you are hitting a limitation of the current implementation. You probably
+/// try to use an associated type of a non-instantiable trait. To solve this, add the following to
+/// your macro call:
+///
+/// ```nocompile
+/// add_extra_genesis {
+/// 	config(phantom): std::marker::PhantomData<I>,
+/// }
+/// ...
+///
+/// This adds a field to your `GenesisConfig` with the name `phantom` that you can initialize with
+/// `Default::default()`.
+///
 #[proc_macro]
 pub fn decl_storage(input: TokenStream) -> TokenStream {
 	storage::transformation::decl_storage_impl(input)