fixes #3313

compiler/ast.nim

@@ -738,6 +738,8 @@ type
     name*: Rope
     path*: PNode              # can be a string literal!
 
+  CompilesId* = int ## id that is used for the caching logic within
+                    ## ``system.compiles``. See the seminst module.
   TInstantiation* = object
     sym*: PSym
     concreteTypes*: seq[PType]
@@ -745,6 +747,7 @@ type
                               # needed in caas mode for purging the cache
                               # XXX: it's possible to switch to a
                               # simple ref count here
+    compilesId*: CompilesId
 
   PInstantiation* = ref TInstantiation