$define would occasionally not properly evaluate declarations it encountered.

lib/std/collections/hashmap.c3

@@ -1,6 +1,9 @@
 // Copyright (c) 2023 Christoffer Lerno. All rights reserved.
 // Use of this source code is governed by the MIT license
 // a copy of which can be found in the LICENSE_STDLIB file.
+<*
+ @require $defined(Key{}.hash()) `No .hash function found on the key`
+*>
 module std::collections::map(<Key, Value>);
 import std::math;