From 96ddf6972d088e0e389840e718da7ca498129142 Mon Sep 17 00:00:00 2001
From: Araq <rumpf_a@web.de>
Date: Thu, 23 Nov 2017 03:12:09 +0100
Subject: [PATCH] fixes #3993

---
 compiler/sempass2.nim          |  3 ++-
 tests/notnil/tmust_compile.nim | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/compiler/sempass2.nim b/compiler/sempass2.nim
index 051f215e0c..2bdfaea3ff 100644
--- a/compiler/sempass2.nim
+++ b/compiler/sempass2.nim
@@ -525,7 +525,8 @@ proc notNilCheck(tracked: PEffects, n: PNode, paramType: PType) =
       # addr(x[]) can't be proven, but addr(x) can:
       if not containsNode(n, {nkDerefExpr, nkHiddenDeref}): return
     elif (n.kind == nkSym and n.sym.kind in routineKinds) or
-         n.kind in procDefs+{nkObjConstr, nkBracket, nkClosure}:
+         (n.kind in procDefs+{nkObjConstr, nkBracket, nkClosure}) or
+         (n.kind in nkCallKinds and n[0].kind == nkSym and n[0].sym.magic == mArrToSeq):
       # 'p' is not nil obviously:
       return
     case impliesNotNil(tracked.guards, n)
diff --git a/tests/notnil/tmust_compile.nim b/tests/notnil/tmust_compile.nim
index 117921a57e..10da154f00 100644
--- a/tests/notnil/tmust_compile.nim
+++ b/tests/notnil/tmust_compile.nim
@@ -62,3 +62,17 @@ proc parse(cts: CTS, jn: JsonNode) =
 
 proc p(x: proc(){.closure.} not nil) = discard
 p(proc(){.closure.} = discard)
+
+# bug #3993
+
+type
+  List[T] = seq[T] not nil
+
+proc `^^`[T](v: T, lst: List[T]): List[T] =
+  result = @[v]
+  result.add(lst)
+
+proc Nil[T](): List[T] = @[]
+
+when isMainModule:
+  let lst = 1 ^^ 2 ^^ Nil[int]()