fixes #5599 (#5610)

compiler/cgmeth.nim

@@ -229,6 +229,7 @@ proc genDispatcher(methods: TSymSeq, relevantCols: IntSet): PSym =
   var base = lastSon(methods[0].ast).sym
   result = base
   var paramLen = sonsLen(base.typ)
+  var nilchecks = newNodeI(nkStmtList, base.info)
   var disp = newNodeI(nkIfStmt, base.info)
   var ands = getSysSym("and")
   var iss = getSysSym("of")
@@ -239,7 +240,11 @@ proc genDispatcher(methods: TSymSeq, relevantCols: IntSet): PSym =
       if contains(relevantCols, col):
         var isn = newNodeIT(nkCall, base.info, getSysType(tyBool))
         addSon(isn, newSymNode(iss))
-        addSon(isn, newSymNode(base.typ.n.sons[col].sym))
+        let param = base.typ.n.sons[col].sym
+        addSon(isn, newSymNode(param))
+        if param.typ.skipTypes(abstractInst).kind in {tyRef, tyPtr}:
+          addSon(nilchecks, newTree(nkCall,
+                newSymNode(getCompilerProc"chckNilDisp"), newSymNode(param)))
         addSon(isn, newNodeIT(nkType, base.info, curr.typ.sons[col]))
         if cond != nil:
           var a = newNodeIT(nkCall, base.info, getSysType(tyBool))
@@ -270,7 +275,8 @@ proc genDispatcher(methods: TSymSeq, relevantCols: IntSet): PSym =
       addSon(disp, a)
     else:
       disp = ret
-  result.ast.sons[bodyPos] = disp
+  nilchecks.add disp
+  result.ast.sons[bodyPos] = nilchecks
 
 proc generateMethodDispatchers*(g: ModuleGraph): PNode =
   result = newNode(nkStmtList)

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,

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"
 

web/news/e031_version_0_16_2.rst

@@ -42,6 +42,9 @@ Changes affecting backwards compatibility
   AST that is the same as what is used to define an enum.  Previously the AST
   returned had a repeated ``EnumTy`` node and was missing the initial pragma
   node (which is currently empty for an enum).
+- If the dispatcher parameter's value used in multi method is ``nil``,
+  a ``NilError`` exception is raised. The old behavior was that the method
+  would be a ``nop`` then.
 
 
 Library Additions