diff --git a/tests/playground/deriving.lean b/tests/playground/deriving.lean
new file mode 100644
index 0000000000..f1907871a5
--- /dev/null
+++ b/tests/playground/deriving.lean
@@ -0,0 +1,273 @@
+import Lean
+
+mutual
+
+inductive Vect (α : Type u) (β : Type v) : Nat → Nat → Type max u v where
+  | nil   : Vect α β 0 0
+  | lst   : List (Array (Vect α β 0 0)) → Vect α β 0 0
+  | cons1 : {n m : Nat} → TreeV α β n → Vect α β n m → Vect α β (n+1) m
+  | cons2 : {n m : Nat} → TreeV α β m → Vect α β n m → Vect α β n (m+1)
+
+inductive TreeV (α : Type u) (β : Type v) : Nat → Type max u v where
+  | diag   : {n : Nat}   → Vect α β n n → TreeV α β n
+  | lower  : {n m : Nat} → n < m → Vect α β n m → TreeV α β m
+
+end
+
+mutual
+
+partial def aux1 {α β m n} [ToString α] [ToString β] (s : Vect α β m n) : String :=
+  let inst1 {m' n' : Nat} : ToString (Vect α β m' n') := ⟨aux1⟩
+  let inst1 {n' : Nat}    : ToString (TreeV α β n')   := ⟨aux2⟩
+  match m, n, s with
+  | _, _, Vect.nil       => "nil"
+  | _, _, Vect.lst xs    => "lst " ++ toString xs
+  | _, _, Vect.cons1 t v => "cons1 " ++ toString t ++ " " ++ toString v
+  | _, _, Vect.cons2 t v => "cons2 " ++ toString t ++ " " ++ toString v
+
+
+partial def aux2 {α β n} [ToString α] [ToString β] (s : TreeV α β n) : String :=
+  let inst1 {m' n' : Nat} : ToString (Vect α β m' n') := ⟨aux1⟩
+  let inst1 {n' : Nat}    : ToString (TreeV α β n')   := ⟨aux2⟩
+  match n, s with
+  | _, TreeV.diag v    => "diag  " ++ toString v
+  | _, TreeV.lower _ v => "lower " ++ "<proof>" ++ " " ++ toString v
+
+end
+
+instance {α β m n} [ToString α] [ToString β] : ToString (Vect α β m n) where
+  toString := aux1
+
+instance {α β n} [ToString α] [ToString β] : ToString (TreeV α β n) where
+  toString := aux2
+
+
+namespace Test
+mutual
+inductive Foo (α : Type) where
+  | mk : List (Bla α) → Foo α
+  | leaf : α → Foo α
+inductive Bla (α : Type) where
+  | nil : Bla α
+  | cons : Foo α → Bla α → Bla α
+end
+
+
+open Lean
+open Lean.Meta
+
+def tst : MetaM Unit := do
+  let info ← getConstInfoInduct `Test.Bla
+  trace[Meta.debug]! "nested: {info.isNested}"
+  pure ()
+
+#eval tst
+
+mutual
+
+partial def fooToString {α : Type} [ToString α] (s : Foo α) : String :=
+  let inst1 : ToString (Bla α) := ⟨blaToString⟩
+  match s with
+  | Foo.mk bs  => "Foo.mk " ++ toString bs
+  | Foo.leaf a => "Foo.leaf " ++ toString a
+
+partial def blaToString {α : Type} [ToString α] (s : Bla α) : String :=
+  let inst1 : ToString (Bla α) := ⟨blaToString⟩
+  let inst2 : ToString (Foo α) := ⟨fooToString⟩
+  match s with
+  | Bla.nil      => "Bla.nil"
+  | Bla.cons h t => "Bla.cons (" ++ toString h ++ ") " ++ toString t
+
+end
+
+instance [ToString α] : ToString (Foo α) where
+  toString := fooToString
+
+instance [ToString α] : ToString (Bla α) where
+  toString := blaToString
+
+#eval Foo.mk [Bla.cons (Foo.leaf 10) Bla.nil]
+
+end Test
+
+
+namespace Lean.Elab
+open Meta
+
+namespace Deriving
+
+/- For type classes such as BEq, ToString, Format -/
+namespace Default
+
+structure AuxFun where
+  name    : Name
+  declSig : Array Syntax := #[]
+  type    : Syntax       := Syntax.missing
+  value   : Syntax       := Syntax.missing
+
+structure State where
+  auxFns  : NameMap AuxFun -- type name to function
+
+structure Context where
+  classInfo   : ConstantInfo
+  typeInfos   : Array InductiveVal
+  auxFunNames : Array Name
+  usePartial  : Bool
+  resultType  : Syntax
+
+def mkContext (className : Name) (typeName : Name) (resultType : Syntax) : TermElabM Context := do
+  let indVal ← getConstInfoInduct typeName
+  let mut typeInfos := #[]
+  for typeName in indVal.all do
+    typeInfos ← typeInfos.push (← getConstInfoInduct typeName)
+  let classInfo ← getConstInfo className
+  let mut auxFunNames := #[]
+  for typeName in indVal.all do
+    match className.eraseMacroScopes, typeName.eraseMacroScopes with
+    | Name.str _ c _, Name.str _ t _ => auxFunNames := auxFunNames.push (← mkFreshUserName <| Name.mkSimple <| c.decapitalize ++ t)
+    | _, _ => auxFunNames := auxFunNames.push (← mkFreshUserName `instFn)
+  trace[Meta.debug]! "{auxFunNames}"
+  let usePartial := indVal.isNested || typeInfos.size > 1
+  return {
+    classInfo   := classInfo
+    typeInfos   := typeInfos
+    auxFunNames := auxFunNames
+    usePartial  := usePartial
+    resultType  := resultType
+  }
+
+def mkInductArgNames (indVal : InductiveVal) : TermElabM (Array Name) := do
+  forallTelescopeReducing indVal.type fun xs _ => do
+    let mut argNames := #[]
+    for x in xs do
+      let localDecl ← getLocalDecl x.fvarId!
+      let paramName ← mkFreshUserName localDecl.userName.eraseMacroScopes
+      argNames := argNames.push paramName
+    pure argNames
+
+def mkInductiveApp (indVal : InductiveVal) (argNames : Array Name) : TermElabM Syntax :=
+  let f    := mkIdent indVal.name
+  let args := argNames.map mkIdent
+  `(@$f $args*)
+
+def implicitBinderF := Parser.Term.implicitBinder
+def instBinderF     := Parser.Term.instBinder
+def explicitBinderF := Parser.Term.explicitBinder
+
+def mkImplicitBinders (argNames : Array Name) : TermElabM (Array Syntax) :=
+  argNames.mapM fun argName =>
+    `(implicitBinderF| { $(mkIdent argName) })
+
+def mkInstImplicitBinders (ctx : Context) (indVal : InductiveVal) (argNames : Array Name) : TermElabM (Array Syntax) :=
+  forallBoundedTelescope indVal.type indVal.nparams fun xs _ => do
+    let mut binders := #[]
+    for i in [:xs.size] do
+      try
+        let x := xs[i]
+        let clsName := ctx.classInfo.name
+        let c ← mkAppM clsName #[x]
+        if (← isTypeCorrect c) then
+          let argName := argNames[i]
+          let binder ← `(instBinderF| [ $(mkIdent clsName):ident $(mkIdent argName):ident ])
+          binders := binders.push binder
+      catch _ =>
+        pure ()
+    return binders
+
+structure Header where
+  binders    : Array Syntax
+  argNames   : Array Name
+  targetName : Name
+
+def mkHeader (ctx : Context) (indVal : InductiveVal) : TermElabM Header := do
+  let argNames     ← mkInductArgNames indVal
+  let binders      ← mkImplicitBinders argNames
+  let targetType   ← mkInductiveApp indVal argNames
+  let targetName   ← mkFreshUserName `x
+  let targetBinder ← `(explicitBinderF| ($(mkIdent targetName) : $targetType))
+  let binders      := binders ++ (← mkInstImplicitBinders ctx indVal argNames)
+  let binders      := binders.push targetBinder
+  return {
+    binders    := binders
+    argNames   := argNames
+    targetName := targetName
+  }
+
+def mkLocalInstanceLetDecls (ctx : Context) (argNames : Array Name) : TermElabM (Array Syntax) := do
+  let mut letDecls := #[]
+  for i in [:ctx.typeInfos.size] do
+    let indVal       := ctx.typeInfos[i]
+    let auxFunName   := ctx.auxFunNames[i]
+    let currArgNames ← mkInductArgNames indVal
+    let numParams    := indVal.nparams
+    let currIndices  := currArgNames[numParams:]
+    let binders      ← mkImplicitBinders currIndices
+    let argNamesNew  := argNames[:numParams] ++ currIndices
+    let indType      ← mkInductiveApp indVal argNamesNew
+    let type         ← `($(mkIdent ctx.classInfo.name) $indType)
+    let val          ← `(⟨$(mkIdent auxFunName)⟩)
+    let instName     ← mkFreshUserName `localinst
+    let letDecl      ← `(Parser.Term.letDecl| $(mkIdent instName):ident $binders:implicitBinder* : $type := $val)
+    letDecls := letDecls.push letDecl
+  return letDecls
+
+def mkLet (letDecls : Array Syntax) (body : Syntax) : TermElabM Syntax :=
+  letDecls.foldrM (init := body) fun letDecl body =>
+    `(let $letDecl:letDecl; $body)
+
+def mkAuxFunction (ctx : Context) (i : Nat) : TermElabM Syntax := do
+  let auxFunName ← ctx.auxFunNames[i]
+  let indVal     ← ctx.typeInfos[i]
+  let header     ← mkHeader ctx indVal
+  let mut body   ← `("testing") -- TODO
+  if ctx.usePartial then
+    let letDecls ← mkLocalInstanceLetDecls ctx header.argNames
+    body ← mkLet letDecls body
+  let binders    := header.binders
+  if ctx.usePartial then
+    `(private partial def $(mkIdent auxFunName):ident $binders:explicitBinder* : $ctx.resultType:term := $body:term)
+  else
+    `(private def $(mkIdent auxFunName):ident $binders:explicitBinder* : $ctx.resultType:term := $body:term)
+
+def mkMutualBlock (ctx : Context) : TermElabM Syntax := do
+  let mut auxDefs := #[]
+  for i in [:ctx.typeInfos.size] do
+    auxDefs := auxDefs.push (← mkAuxFunction ctx i)
+  `(mutual $auxDefs:command* end)
+
+def mkInstanceCmds (ctx : Context) : TermElabM (Array Syntax) := do
+  let mut instances := #[]
+  for i in [:ctx.typeInfos.size] do
+    let indVal       := ctx.typeInfos[i]
+    let auxFunName   := ctx.auxFunNames[i]
+    let argNames     ← mkInductArgNames indVal
+    let binders      ← mkImplicitBinders argNames
+    let binders      := binders ++ (← mkInstImplicitBinders ctx indVal argNames)
+    let indType      ← mkInductiveApp indVal argNames
+    let type         ← `($(mkIdent ctx.classInfo.name) $indType)
+    let val          ← `(⟨$(mkIdent auxFunName)⟩)
+    let instCmd ← `(instance $binders:implicitBinder* : $type := $val)
+    trace[Meta.debug]! "\n{instCmd}"
+    instances := instances.push instCmd
+  return instances
+
+open Command in
+def tst (className : Name) (typeName : Name) (resultTypeName : Name) : CommandElabM Unit := do
+  let cmds ← liftTermElabM none do
+    let resultType := mkIdent resultTypeName
+    let ctx ← mkContext className typeName resultType
+    let block ← mkMutualBlock ctx
+    trace[Meta.debug]! "\n{block}"
+    return #[block] ++ (← mkInstanceCmds ctx)
+  cmds.forM elabCommand
+
+syntax[runTstKind] "runTst" : command
+
+open Command in
+@[commandElab runTstKind] def elabTst : CommandElab := fun stx =>
+  tst `ToString `Vect `String
+
+set_option trace.Meta.debug true
+runTst
+
+#eval (Vect.nil : Vect Nat Nat _ _)