diff --git a/src/Lean/Compiler/LCNF/ToLCNF.lean b/src/Lean/Compiler/LCNF/ToLCNF.lean
index 0d1b4937b4..89ce351ec6 100644
--- a/src/Lean/Compiler/LCNF/ToLCNF.lean
+++ b/src/Lean/Compiler/LCNF/ToLCNF.lean
@@ -568,33 +568,6 @@ where
           let result := .fvar auxDecl.fvarId
           mkOverApplication result args casesInfo.arity
 
-  visitCasesImplementedBy (casesInfo : CasesInfo) (f : Expr) (args : Array Expr) : M Arg := do
-    let mut args := args
-    let discr := args[casesInfo.discrPos]!
-    if discr matches .fvar _ then
-      let typeName := casesInfo.declName.getPrefix
-      let .inductInfo indVal ← getConstInfo typeName | unreachable!
-      args ← args.mapIdxM fun i arg => do
-        unless casesInfo.altsRange.start <= i && i < casesInfo.altsRange.stop do return arg
-        let altIdx := i - casesInfo.altsRange.start
-        let numParams := casesInfo.altNumParams[altIdx]!
-        let ctorName := indVal.ctors[altIdx]!
-
-        -- We simplify `casesOn` arguments that simply reconstruct the discriminant and replace
-        -- them with the actual discriminant. This is required for hash consing to work correctly,
-        -- and should eventually be fixed by changing the elaborated term to use the original
-        -- variable.
-        Meta.MetaM.run' <| Meta.lambdaBoundedTelescope arg numParams fun paramExprs body => do
-          let fn := body.getAppFn
-          let args := body.getAppArgs
-          let args := args.map fun arg =>
-            if arg.getAppFn.constName? == some ctorName && arg.getAppArgs == paramExprs then
-              discr
-            else
-              arg
-          Meta.mkLambdaFVars paramExprs (mkAppN fn args)
-    visitAppDefaultConst f args
-
   visitCtor (arity : Nat) (e : Expr) : M Arg :=
     etaIfUnderApplied e arity do
       visitAppDefaultConst e.getAppFn e.getAppArgs
@@ -715,10 +688,7 @@ where
       else if declName == ``False.rec || declName == ``Empty.rec || declName == ``False.casesOn || declName == ``Empty.casesOn then
         visitFalseRec e
       else if let some casesInfo ← getCasesInfo? declName then
-        if (getImplementedBy? (← getEnv) declName).isSome then
-          e.withApp (visitCasesImplementedBy casesInfo)
-        else
-          visitCases casesInfo e
+        visitCases casesInfo e
       else if let some arity ← getCtorArity? declName then
         visitCtor arity e
       else if isNoConfusion (← getEnv) declName then
diff --git a/src/Lean/Compiler/LCNF/ToMono.lean b/src/Lean/Compiler/LCNF/ToMono.lean
index bb8f7013cb..479a7ba69f 100644
--- a/src/Lean/Compiler/LCNF/ToMono.lean
+++ b/src/Lean/Compiler/LCNF/ToMono.lean
@@ -5,6 +5,7 @@ Authors: Leonardo de Moura
 -/
 prelude
 import Lean.Compiler.ExternAttr
+import Lean.Compiler.ImplementedByAttr
 import Lean.Compiler.LCNF.MonoTypes
 import Lean.Compiler.LCNF.InferType
 import Lean.Compiler.NoncomputableAttr
@@ -107,6 +108,24 @@ def LetDecl.toMono (decl : LetDecl) : ToMonoM LetDecl := do
   let value ← decl.value.toMono decl.fvarId
   decl.update type value
 
+def mkFieldParamsForComputedFields (ctorType : Expr) (numParams : Nat) (numNewFields : Nat)
+    (oldFields : Array Param) : ToMonoM (Array Param) := do
+  let mut type := ctorType
+  for _ in [0:numParams] do
+    match type with
+    | .forallE _ _ body _ =>
+      type := body
+    | _ => unreachable!
+  let mut newFields := Array.emptyWithCapacity (oldFields.size + numNewFields)
+  for _ in [0:numNewFields] do
+    match type with
+    | .forallE name fieldType body _ =>
+      let param ← mkParam name (← toMonoType fieldType) false
+      newFields := newFields.push param
+      type := body
+    | _ => unreachable!
+  return newFields ++ oldFields
+
 mutual
 
 partial def FunDecl.toMono (decl : FunDecl) : ToMonoM FunDecl := do
@@ -278,12 +297,30 @@ partial def Code.toMono (code : Code) : ToMonoM Code := do
     else if let some info ← hasTrivialStructure? c.typeName then
       trivialStructToMono info c
     else
-      let type ← toMonoType c.resultType
-      let alts ← c.alts.mapM fun alt =>
-        match alt with
-        | .default k => return alt.updateCode (← k.toMono)
-        | .alt _ ps k => return alt.updateAlt! (← ps.mapM (·.toMono)) (← k.toMono)
-      return code.updateCases! type c.discr alts
+      let resultType ← toMonoType c.resultType
+      let env ← getEnv
+      let some (.inductInfo inductInfo) := env.find? c.typeName | panic! "expected inductive type"
+      let casesOnName := mkCasesOnName inductInfo.name
+      if (getImplementedBy? env casesOnName).isSome then
+        -- TODO: Enforce that this is only used for computed fields.
+        let typeName := c.typeName ++ `_impl
+        let alts ← c.alts.mapM fun alt => do
+          match alt with
+          | .default k => return alt.updateCode (← k.toMono)
+          | .alt ctorName ps k =>
+            let implCtorName := ctorName ++ `_impl
+            let some (.ctorInfo ctorInfo) := env.find? implCtorName | panic! "expected constructor"
+            let numNewFields := ctorInfo.numFields - ps.size
+            let ps ← mkFieldParamsForComputedFields ctorInfo.type ctorInfo.numParams numNewFields ps
+            let k ← k.toMono
+            return .alt implCtorName ps k
+        return .cases { discr := c.discr, resultType, typeName, alts }
+      else
+        let alts ← c.alts.mapM fun alt =>
+          match alt with
+          | .default k => return alt.updateCode (← k.toMono)
+          | .alt _ ps k => return alt.updateAlt! (← ps.mapM (·.toMono)) (← k.toMono)
+        return code.updateCases! resultType c.discr alts
 
 end