try with getmap lookup

src/Reactant.jl

@@ -208,6 +208,10 @@ using Enzyme
     TracedSetPath = 5
 end
 
+@inline getmap(::Val{T}) where T = nothing
+@inline getmap(::Val{T}, a, b, args...) where {T} = getmap(Val(T), args...)
+@inline getmap(::Val{T}, ::Val{T}, ::Val{T2}, args...) where {T, T2} = T2
+
 @inline is_concrete_tuple(x::T2) where {T2} =
     (x <: Tuple) && !(x === Tuple) && !(x isa UnionAll)
 @inline function traced_type(val::Type{T}, seen::ST, ::Val{mode}) where {ST,T,mode}
@@ -391,17 +395,18 @@ end
         return IdDict{iddict_name(T),traced_type(iddict_val(T), seen, Val(mode))}
     end
 
-    if Val(T) ∈ seen
-        return T
+    nextTy = getmap(Val(T), seen...)
+    if nextTy != nothing
+        return nextTy
     end
 
-    seen = (Val(T), seen...)
+    seen2 = (Val(T), Val(T), seen...)
 
     changed = false
     subTys = Type[]
     for f in 1:fieldcount(T)
         subT = fieldtype(T, f)
-        subTT = traced_type(subT, seen, Val(mode))
+        subTT = traced_type(subT, seen2, Val(mode))
         changed |= subT != subTT
         push!(subTys, subTT)
     end
@@ -421,12 +426,13 @@ end
     end
 
     TT2 = Core.apply_type(T.name.wrapper, subParms...)
+    seen3 = (Val(T), Val(TT2), seen...)
     if fieldcount(T) == fieldcount(TT2)
         legal = true
         for f in 1:fieldcount(T)
             subT = fieldtype(T, f)
             subT2 = fieldtype(TT2, f)
-            subTT = traced_type(subT, seen, Val(mode))
+            subTT = traced_type(subT, seen3, Val(mode))
             if subT2 != subTT
                 @show "illegal subs", subT2, subTT
                 legal = false