fixes #3730

compiler/ccgexprs.nim

@@ -1847,6 +1847,8 @@ proc genClosure(p: BProc, n: PNode, d: var TLoc) =
     var tmp, a, b: TLoc
     initLocExpr(p, n.sons[0], a)
     initLocExpr(p, n.sons[1], b)
+    if n.sons[0].skipConv.kind == nkClosure:
+      internalError(n.info, "closure to closure created")
     getTemp(p, n.typ, tmp)
     linefmt(p, cpsStmts, "$1.ClPrc = $2; $1.ClEnv = $3;$n",
             tmp.rdLoc, a.rdLoc, b.rdLoc)

compiler/lambdalifting.nim

@@ -213,7 +213,7 @@ proc makeClosure*(prc: PSym; env: PNode; info: TLineInfo): PNode =
   if env == nil:
     result.add(newNodeIT(nkNilLit, info, getSysType(tyNil)))
   else:
-    if env.kind == nkClosure:
+    if env.skipConv.kind == nkClosure:
       localError(info, "internal error: taking closure of closure")
     result.add(env)
 
@@ -711,7 +711,11 @@ proc liftCapturedVars(n: PNode; owner: PSym; d: DetectionPass;
   of nkClosure:
     if n[1].kind == nkNilLit:
       n.sons[0] = liftCapturedVars(n[0], owner, d, c)
-      #if n.sons[0].kind == nkClosure: result = n.sons[0]
+      let x = n.sons[0].skipConv
+      if x.kind == nkClosure:
+        #localError(n.info, "internal error: closure to closure created")
+        # now we know better, so patch it:
+        n.sons[0] = x.sons[0]
   of nkLambdaKinds, nkIteratorDef:
     if n.typ != nil and n[namePos].kind == nkSym:
       let m = newSymNode(n[namePos].sym)

compiler/transf.nim

@@ -383,9 +383,11 @@ proc generateThunk(prc: PNode, dest: PType): PNode =
   # (see internal documentation):
   if gCmd == cmdCompileToJS: return prc
   result = newNodeIT(nkClosure, prc.info, dest)
-  var conv = newNodeIT(nkHiddenStdConv, prc.info, dest)
+  var conv = newNodeIT(nkHiddenSubConv, prc.info, dest)
   conv.add(emptyNode)
   conv.add(prc)
+  if prc.kind == nkClosure:
+    internalError(prc.info, "closure to closure created")
   result.add(conv)
   result.add(newNodeIT(nkNilLit, prc.info, getSysType(tyNil)))