Add compile fail tests for custom where

crates/bevy_reflect_compile_fail_tests/tests/reflect_derive/custom_where.fail.rs

@@ -0,0 +1,31 @@
+use bevy_reflect::{Reflect, FromType};
+use std::marker::PhantomData;
+
+#[derive(Clone)]
+struct ReflectMyTrait;
+
+impl<T> FromType<T> for ReflectMyTrait {
+    fn from_type() -> Self {
+        Self
+    }
+}
+
+// Reason: where clause cannot be used with #[reflect(MyTrait)]
+#[derive(Reflect)]
+#[reflect(MyTrait, where)]
+pub struct Foo<T> {
+    value: String,
+    #[reflect(ignore)]
+    _marker: PhantomData<T>,
+}
+
+// Reason: where clause cannot be used with #[reflect(MyTrait)]
+#[derive(Reflect)]
+#[reflect(where, MyTrait)]
+pub struct Bar<T> {
+    value: String,
+    #[reflect(ignore)]
+    _marker: PhantomData<T>,
+}
+
+fn main() {}

crates/bevy_reflect_compile_fail_tests/tests/reflect_derive/custom_where.fail.stderr

@@ -0,0 +1,11 @@
+error: expected identifier, found keyword `where`
+  --> tests/reflect_derive/custom_where.fail.rs:15:20
+   |
+15 | #[reflect(MyTrait, where)]
+   |                    ^^^^^
+
+error: unexpected token
+  --> tests/reflect_derive/custom_where.fail.rs:24:16
+   |
+24 | #[reflect(where, MyTrait)]
+   |                ^

crates/bevy_reflect_compile_fail_tests/tests/reflect_derive/custom_where.pass.rs

@@ -0,0 +1,31 @@
+use bevy_reflect::{Reflect, FromType};
+use std::marker::PhantomData;
+
+#[derive(Clone)]
+struct ReflectMyTrait;
+
+impl<T> FromType<T> for ReflectMyTrait {
+    fn from_type() -> Self {
+        Self
+    }
+}
+
+#[derive(Reflect)]
+#[reflect(MyTrait)]
+#[reflect(where)]
+pub struct Foo<T> {
+    value: String,
+    #[reflect(ignore)]
+    _marker: PhantomData<T>,
+}
+
+#[derive(Reflect)]
+#[reflect(where)]
+#[reflect(MyTrait)]
+pub struct Bar<T> {
+    value: String,
+    #[reflect(ignore)]
+    _marker: PhantomData<T>,
+}
+
+fn main() {}