fixes #5599 (#5610)

lib/pure/segfaults.nim

@@ -13,10 +13,6 @@
 ##
 ## Tested on these OSes: Linux, Windows, OSX
 
-type
-  NilAccessError* = object of SystemError ## \
-    ## Raised on dereferences of ``nil`` pointers.
-
 # do allocate memory upfront:
 var se: ref NilAccessError
 new(se)

lib/system.nim

@@ -558,6 +558,10 @@ type
     ## Raised if it is attempted to send a message to a dead thread.
     ##
     ## See the full `exception hierarchy <manual.html#exception-handling-exception-hierarchy>`_.
+  NilAccessError* = object of SystemError ## \
+    ## Raised on dereferences of ``nil`` pointers.
+    ##
+    ## This is only raised if the ``segfaults.nim`` module was imported!
 
 {.deprecated: [TObject: RootObj, PObject: RootRef, TEffect: RootEffect,
   FTime: TimeEffect, FIO: IOEffect, FReadIO: ReadIOEffect,
@@ -2467,8 +2471,8 @@ template accumulateResult*(iter: untyped) =
 const NimStackTrace = compileOption("stacktrace")
 
 template coroutinesSupportedPlatform(): bool =
-  when defined(sparc) or defined(ELATE) or compileOption("gc", "v2") or 
-    defined(boehmgc) or defined(gogc) or defined(nogc) or defined(gcStack) or 
+  when defined(sparc) or defined(ELATE) or compileOption("gc", "v2") or
+    defined(boehmgc) or defined(gogc) or defined(nogc) or defined(gcStack) or
     defined(gcMarkAndSweep):
     false
   else:

lib/system/chcks.nim

@@ -50,7 +50,11 @@ proc chckRangeF(x, a, b: float): float =
 
 proc chckNil(p: pointer) =
   if p == nil:
-    sysFatal(ValueError, "attempt to write to a nil address")
+    sysFatal(NilAccessError, "attempt to write to a nil address")
+
+proc chckNilDisp(p: pointer) {.compilerproc.} =
+  if p == nil:
+    sysFatal(NilAccessError, "cannot dispatch; dispatcher is nil")
 
 proc chckObj(obj, subclass: PNimType) {.compilerproc.} =
   # checks if obj is of type subclass:

lib/system/jssys.nim

@@ -642,6 +642,10 @@ proc toU32*(a: int64): int32 {.asmNoStackFrame, compilerproc.} =
 proc nimMin(a, b: int): int {.compilerproc.} = return if a <= b: a else: b
 proc nimMax(a, b: int): int {.compilerproc.} = return if a >= b: a else: b
 
+proc chckNilDisp(p: pointer) {.compilerproc.} =
+  if p == nil:
+    sysFatal(NilAccessError, "cannot dispatch; dispatcher is nil")
+
 type NimString = string # hack for hti.nim
 include "system/hti"