fixes #3995

compiler/ccgexprs.nim

@@ -1856,10 +1856,16 @@ proc genClosure(p: BProc, n: PNode, d: var TLoc) =
     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)
-    putLocIntoDest(p, d, tmp)
+    # tasyncawait.nim breaks with this optimization:
+    when false:
+      if d.k != locNone:
+        linefmt(p, cpsStmts, "$1.ClPrc = $2; $1.ClEnv = $3;$n",
+                d.rdLoc, a.rdLoc, b.rdLoc)
+    else:
+      getTemp(p, n.typ, tmp)
+      linefmt(p, cpsStmts, "$1.ClPrc = $2; $1.ClEnv = $3;$n",
+              tmp.rdLoc, a.rdLoc, b.rdLoc)
+      putLocIntoDest(p, d, tmp)
 
 proc genArrayConstr(p: BProc, n: PNode, d: var TLoc) =
   var arr: TLoc

compiler/lambdalifting.nim

@@ -721,6 +721,10 @@ proc liftCapturedVars(n: PNode; owner: PSym; d: DetectionPass;
       let m = newSymNode(n[namePos].sym)
       m.typ = n.typ
       result = liftCapturedVars(m, owner, d, c)
+  of nkHiddenStdConv:
+    if n.len == 2:
+      n.sons[1] = liftCapturedVars(n[1], owner, d, c)
+      if n[1].kind == nkClosure: result = n[1]
   else:
     if owner.isIterator:
       if n.kind == nkYieldStmt:

tests/closure/tflatmap.nim

@@ -0,0 +1,24 @@
+
+# bug #3995
+
+import future
+
+type
+  RNG* = tuple[]
+  Rand*[A] = (RNG) -> (A, RNG)
+
+proc nextInt*(r: RNG): (int, RNG) =
+  (1, ())
+
+proc flatMap[A,B](f: Rand[A], g: A -> Rand[B]): Rand[B] =
+  (rng: RNG) => (
+    let (a, rng2) = f(rng);
+    let g1 = g(a);
+    g1(rng2)
+  )
+
+proc map[A,B](s: Rand[A], f: A -> B): Rand[B] =
+  let g: A -> Rand[B] = (a: A) => ((rng: RNG) => (f(a), rng))
+  flatMap(s, g)
+
+let f = nextInt.map(i => i - i mod 2)