fixes #3993

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)

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]()