From 69e9a47d92ff8599c270d77a6ce2917c97059d8c Mon Sep 17 00:00:00 2001
From: MinerSebas <scherthan_sebastian@web.de>
Date: Thu, 3 Feb 2022 03:32:02 +0000
Subject: [PATCH] SystemParam Derive fixes (#2838)

# Objective

A user on Discord couldn't derive SystemParam for this Struct:

```rs
#[derive(SystemParam)]
pub struct SpatialQuery<'w, 's, Q: WorldQuery + Send + Sync + 'static, F: WorldQuery + Send + Sync + 'static = ()>
where
    F::Fetch: FilterFetch,
{
    query: Query<'w, 's, (C, &'static Transform), F>,
}
```

## Solution

1. The `where`-clause is now also copied to the `SystemParamFetch` impl Block.
2. The `SystemParamState` impl Block no longer gets any defaults for generics


Co-authored-by: MinerSebas <66798382+MinerSebas@users.noreply.github.com>
---
 crates/bevy_ecs/macros/src/lib.rs          | 14 +++++++++----
 crates/bevy_ecs/src/system/system_param.rs | 24 ++++++++++++++++++++++
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/crates/bevy_ecs/macros/src/lib.rs b/crates/bevy_ecs/macros/src/lib.rs
index 548f2310a162f..ca5a6b17bff59 100644
--- a/crates/bevy_ecs/macros/src/lib.rs
+++ b/crates/bevy_ecs/macros/src/lib.rs
@@ -11,7 +11,7 @@ use syn::{
     parse_macro_input,
     punctuated::Punctuated,
     token::Comma,
-    DeriveInput, Field, GenericParam, Ident, Index, LitInt, Result, Token,
+    DeriveInput, Field, GenericParam, Ident, Index, LitInt, Result, Token, TypeParam,
 };
 
 struct AllTuples {
@@ -357,12 +357,18 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
         .collect();
 
     let mut punctuated_generics = Punctuated::<_, Token![,]>::new();
-    punctuated_generics.extend(lifetimeless_generics.iter());
+    punctuated_generics.extend(lifetimeless_generics.iter().map(|g| match g {
+        GenericParam::Type(g) => GenericParam::Type(TypeParam {
+            default: None,
+            ..g.clone()
+        }),
+        _ => unreachable!(),
+    }));
 
     let mut punctuated_generic_idents = Punctuated::<_, Token![,]>::new();
     punctuated_generic_idents.extend(lifetimeless_generics.iter().map(|g| match g {
         GenericParam::Type(g) => &g.ident,
-        _ => panic!(),
+        _ => unreachable!(),
     }));
 
     let struct_name = &ast.ident;
@@ -402,7 +408,7 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
             }
         }
 
-        impl #impl_generics #path::system::SystemParamFetch<'w, 's> for #fetch_struct_name <(#(<#field_types as #path::system::SystemParam>::Fetch,)*), #punctuated_generic_idents> {
+        impl #impl_generics #path::system::SystemParamFetch<'w, 's> for #fetch_struct_name <(#(<#field_types as #path::system::SystemParam>::Fetch,)*), #punctuated_generic_idents> #where_clause {
             type Item = #struct_name #ty_generics;
             unsafe fn get_param(
                 state: &'s mut Self,
diff --git a/crates/bevy_ecs/src/system/system_param.rs b/crates/bevy_ecs/src/system/system_param.rs
index d919f2788052c..56900decf213e 100644
--- a/crates/bevy_ecs/src/system/system_param.rs
+++ b/crates/bevy_ecs/src/system/system_param.rs
@@ -1253,3 +1253,27 @@ pub mod lifetimeless {
     pub type SResMut<T> = super::ResMut<'static, T>;
     pub type SCommands = crate::system::Commands<'static, 'static>;
 }
+
+#[cfg(test)]
+mod tests {
+    use super::SystemParam;
+    use crate::{
+        self as bevy_ecs, // Necessary for the `SystemParam` Derive when used inside `bevy_ecs`.
+        query::{FilterFetch, WorldQuery},
+        system::Query,
+    };
+
+    // Compile test for #2838
+    #[derive(SystemParam)]
+    pub struct SpecialQuery<
+        'w,
+        's,
+        Q: WorldQuery + Send + Sync + 'static,
+        F: WorldQuery + Send + Sync + 'static = (),
+    >
+    where
+        F::Fetch: FilterFetch,
+    {
+        _query: Query<'w, 's, Q, F>,
+    }
+}