Revert "fix #15623 (#16009)"

This reverts commit 0f7f159a35.

compiler/semfold.nim

@@ -674,7 +674,7 @@ proc getConstExpr(m: PSym, n: PNode; idgen: IdGenerator; g: ModuleGraph): PNode
   of nkCast:
     var a = getConstExpr(m, n[1], idgen, g)
     if a == nil: return
-    if n.typ != nil and n.typ.kind in NilableTypes and a.kind != nkNilLit:
+    if n.typ != nil and n.typ.kind in NilableTypes:
       # we allow compile-time 'cast' for pointer types:
       result = a
       result.typ = n.typ

lib/posix/posix_linux_amd64_consts.nim

@@ -365,7 +365,7 @@ const SCHED_RR* = cint(2)
 const SCHED_OTHER* = cint(0)
 
 # <semaphore.h>
-const SEM_FAILED* = nil
+const SEM_FAILED* = cast[pointer]((nil))
 
 # <signal.h>
 const SIGEV_NONE* = cint(1)

tests/ccgbugs/t15623.nim

@@ -1,11 +0,0 @@
-discard """
-  action: "compile"
-"""
-
-# bug #15623
-block:
-  echo cast[ptr int](nil)[]
-
-block:
-  var x: ref int = nil
-  echo cast[ptr int](x)[]

tests/ccgbugs/t15623_2.nim

@@ -1,13 +0,0 @@
-discard """
-  output: '''0
-0
-'''
-"""
-
-# bug #15623
-block:
-  echo cast[int](cast[ptr int](nil))
-
-block:
-  var x: ref int = nil
-  echo cast[int](cast[ptr int](x))