fixes #1286; object case transitions are now sound

changelog.md

@@ -115,6 +115,9 @@
   ints and floats to string have been deprecated.
   Use `string.addInt(int)` and `string.addFloat(float)` instead.
 
+- ``case object`` branch transitions via ``system.reset`` are deprecated.
+  Compile your code with ``-d:nimOldCaseObjects`` for a transition period.
+
 
 #### Breaking changes in the compiler
 

compiler/nim.cfg

@@ -12,6 +12,7 @@ define:nimcore
 @end
 
 define:useStdoutAsStdmsg
+define:nimOldCaseObjects
 
 #define:useNodeIds
 #gc:markAndSweep

doc/manual.rst

@@ -1538,10 +1538,23 @@ the ``case`` statement: The branches in a ``case`` section may be indented too.
 
 In the example the ``kind`` field is called the `discriminator`:idx:\: For
 safety its address cannot be taken and assignments to it are restricted: The
-new value must not lead to a change of the active object branch. For an object
+new value must not lead to a change of the active object branch. Also, when the
-branch switch ``system.reset`` has to be used. Also, when the fields of a
+fields of a particular branch are specified during object construction, the
-particular branch are specified during object construction, the corresponding
+corresponding discriminator value must be specified as a constant expression.
-discriminator value must be specified as a constant expression.
+
+Instead of changing the active object branch, replace the old object in memory
+with a new one completely:
+
+.. code-block:: nim
+
+  var x = Node(kind: nkAdd, leftOp: Node(kind: nkInt, intVal: 4),
+                            rightOp: Node(kind: nkInt, intVal: 2))
+  # change the node's contents:
+  x[] = NodeObj(kind: nkString, strVal: "abc")
+
+
+Starting with version 0.20 ``system.reset`` cannot be used anymore to support
+object branch changes as this never was completely memory safe.
 
 As a special rule, the discriminator kind can also be bounded using a ``case``
 statement. If possible values of the discriminator variable in a

lib/pure/asyncnet.nim

@@ -115,20 +115,16 @@ type
   AsyncSocketDesc = object
     fd: SocketHandle
     closed: bool ## determines whether this socket has been closed
-    case isBuffered: bool ## determines whether this socket is buffered.
+    isBuffered: bool ## determines whether this socket is buffered.
-    of true:
+    buffer: array[0..BufferSize, char]
-      buffer: array[0..BufferSize, char]
+    currPos: int # current index in buffer
-      currPos: int # current index in buffer
+    bufLen: int # current length of buffer
-      bufLen: int # current length of buffer
+    isSsl: bool
-    of false: nil
+    when defineSsl:
-    case isSsl: bool
+      sslHandle: SslPtr
-    of true:
+      sslContext: SslContext
-      when defineSsl:
+      bioIn: BIO
-        sslHandle: SslPtr
+      bioOut: BIO
-        sslContext: SslContext
-        bioIn: BIO
-        bioOut: BIO
-    of false: nil
     domain: Domain
     sockType: SockType
     protocol: Protocol

lib/pure/collections/critbits.nim

@@ -57,9 +57,7 @@ proc hasKey*[T](c: CritBitTree[T], key: string): bool {.inline.} =
 
 proc rawInsert[T](c: var CritBitTree[T], key: string): Node[T] =
   if c.root == nil:
-    new c.root
+    c.root = Node[T](isleaf: true, key: key)
-    c.root.isleaf = true
-    c.root.key = key
     result = c.root
   else:
     var it = c.root
@@ -89,9 +87,7 @@ proc rawInsert[T](c: var CritBitTree[T], key: string): Node[T] =
 
     var inner: Node[T]
     new inner
-    new result
+    result = Node[T](isLeaf: true, key: key)
-    result.isLeaf = true
-    result.key = key
     inner.otherBits = chr(newOtherBits)
     inner.byte = newByte
     inner.child[1 - dir] = result

lib/pure/json.nim

@@ -188,48 +188,35 @@ type
 
 proc newJString*(s: string): JsonNode =
   ## Creates a new `JString JsonNode`.
-  new(result)
+  result = JsonNode(kind: JString, str: s)
-  result.kind = JString
-  result.str = s
 
 proc newJStringMove(s: string): JsonNode =
-  new(result)
+  result = JsonNode(kind: JString)
-  result.kind = JString
   shallowCopy(result.str, s)
 
 proc newJInt*(n: BiggestInt): JsonNode =
   ## Creates a new `JInt JsonNode`.
-  new(result)
+  result = JsonNode(kind: JInt, num: n)
-  result.kind = JInt
-  result.num  = n
 
 proc newJFloat*(n: float): JsonNode =
   ## Creates a new `JFloat JsonNode`.
-  new(result)
+  result = JsonNode(kind: JFloat, fnum: n)
-  result.kind = JFloat
-  result.fnum  = n
 
 proc newJBool*(b: bool): JsonNode =
   ## Creates a new `JBool JsonNode`.
-  new(result)
+  result = JsonNode(kind: JBool, bval: b)
-  result.kind = JBool
-  result.bval = b
 
 proc newJNull*(): JsonNode =
   ## Creates a new `JNull JsonNode`.
-  new(result)
+  result = JsonNode(kind: JNull)
 
 proc newJObject*(): JsonNode =
   ## Creates a new `JObject JsonNode`
-  new(result)
+  result = JsonNode(kind: JObject, fields: initOrderedTable[string, JsonNode](4))
-  result.kind = JObject
-  result.fields = initOrderedTable[string, JsonNode](4)
 
 proc newJArray*(): JsonNode =
   ## Creates a new `JArray JsonNode`
-  new(result)
+  result = JsonNode(kind: JArray, elems: @[])
-  result.kind = JArray
-  result.elems = @[]
 
 proc getStr*(n: JsonNode, default: string = ""): string =
   ## Retrieves the string value of a `JString JsonNode`.
@@ -309,45 +296,31 @@ proc add*(obj: JsonNode, key: string, val: JsonNode) =
 
 proc `%`*(s: string): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JString JsonNode`.
-  new(result)
+  result = JsonNode(kind: JString, str: s)
-  result.kind = JString
-  result.str = s
 
 proc `%`*(n: uint): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JInt JsonNode`.
-  new(result)
+  result = JsonNode(kind: JInt, num: BiggestInt(n))
-  result.kind = JInt
-  result.num  = BiggestInt(n)
 
 proc `%`*(n: int): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JInt JsonNode`.
-  new(result)
+  result = JsonNode(kind: JInt, num: n)
-  result.kind = JInt
-  result.num  = n
 
 proc `%`*(n: BiggestUInt): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JInt JsonNode`.
-  new(result)
+  result = JsonNode(kind: JInt, num: BiggestInt(n))
-  result.kind = JInt
-  result.num  = BiggestInt(n)
 
 proc `%`*(n: BiggestInt): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JInt JsonNode`.
-  new(result)
+  result = JsonNode(kind: JInt, num: n)
-  result.kind = JInt
-  result.num  = n
 
 proc `%`*(n: float): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JFloat JsonNode`.
-  new(result)
+  result = JsonNode(kind: JFloat, fnum: n)
-  result.kind = JFloat
-  result.fnum  = n
 
 proc `%`*(b: bool): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JBool JsonNode`.
-  new(result)
+  result = JsonNode(kind: JBool, bval: b)
-  result.kind = JBool
-  result.bval = b
 
 proc `%`*(keyVals: openArray[tuple[key: string, val: JsonNode]]): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JObject JsonNode`

lib/pure/net.nim

@@ -117,21 +117,17 @@ const
 type
   SocketImpl* = object ## socket type
     fd: SocketHandle
-    case isBuffered: bool # determines whether this socket is buffered.
+    isBuffered: bool # determines whether this socket is buffered.
-    of true:
+    buffer: array[0..BufferSize, char]
-      buffer: array[0..BufferSize, char]
+    currPos: int # current index in buffer
-      currPos: int # current index in buffer
+    bufLen: int # current length of buffer
-      bufLen: int # current length of buffer
-    of false: nil
     when defineSsl:
-      case isSsl: bool
+      isSsl: bool
-      of true:
+      sslHandle: SSLPtr
-        sslHandle: SSLPtr
+      sslContext: SSLContext
-        sslContext: SSLContext
+      sslNoHandshake: bool # True if needs handshake.
-        sslNoHandshake: bool # True if needs handshake.
+      sslHasPeekChar: bool
-        sslHasPeekChar: bool
+      sslPeekChar: char
-        sslPeekChar: char
-      of false: nil
     lastError: OSErrorCode ## stores the last error on this socket
     domain: Domain
     sockType: SockType
@@ -235,7 +231,7 @@ proc parseIPv4Address(addressStr: string): IpAddress =
     currentByte:uint16 = 0
     separatorValid = false
 
-  result.family = IpAddressFamily.IPv4
+  result = IpAddress(family: IpAddressFamily.IPv4)
 
   for i in 0 .. high(addressStr):
     if addressStr[i] in strutils.Digits: # Character is a number
@@ -264,7 +260,7 @@ proc parseIPv4Address(addressStr: string): IpAddress =
 proc parseIPv6Address(addressStr: string): IpAddress =
   ## Parses IPv6 adresses
   ## Raises ValueError on errors
-  result.family = IpAddressFamily.IPv6
+  result = IpAddress(family: IpAddressFamily.IPv6)
   if addressStr.len < 2:
     raise newException(ValueError, "Invalid IP Address")
 

lib/pure/parsecfg.nim

@@ -391,29 +391,27 @@ proc ignoreMsg*(c: CfgParser, e: CfgEvent): string {.rtl, extern: "npc$1".} =
 
 proc getKeyValPair(c: var CfgParser, kind: CfgEventKind): CfgEvent =
   if c.tok.kind == tkSymbol:
+    result = CfgEvent(kind: cfgKeyValuePair, key: c.tok.literal, value: "")
     result.kind = kind
-    result.key = c.tok.literal
-    result.value = ""
     rawGetTok(c, c.tok)
     if c.tok.kind in {tkEquals, tkColon}:
       rawGetTok(c, c.tok)
       if c.tok.kind == tkSymbol:
         result.value = c.tok.literal
       else:
-        reset result
+        result = CfgEvent(kind: cfgError,
-        result.kind = cfgError
+          msg: errorStr(c, "symbol expected, but found: " & c.tok.literal))
-        result.msg = errorStr(c, "symbol expected, but found: " & c.tok.literal)
       rawGetTok(c, c.tok)
   else:
-    result.kind = cfgError
+    result = CfgEvent(kind: cfgError,
-    result.msg = errorStr(c, "symbol expected, but found: " & c.tok.literal)
+      msg: errorStr(c, "symbol expected, but found: " & c.tok.literal))
     rawGetTok(c, c.tok)
 
 proc next*(c: var CfgParser): CfgEvent {.rtl, extern: "npc$1".} =
   ## retrieves the first/next event. This controls the parser.
   case c.tok.kind
   of tkEof:
-    result.kind = cfgEof
+    result = CfgEvent(kind: cfgEof)
   of tkDashDash:
     rawGetTok(c, c.tok)
     result = getKeyValPair(c, cfgOption)
@@ -422,21 +420,19 @@ proc next*(c: var CfgParser): CfgEvent {.rtl, extern: "npc$1".} =
   of tkBracketLe:
     rawGetTok(c, c.tok)
     if c.tok.kind == tkSymbol:
-      result.kind = cfgSectionStart
+      result = CfgEvent(kind: cfgSectionStart, section: c.tok.literal)
-      result.section = c.tok.literal
     else:
-      result.kind = cfgError
+      result = CfgEvent(kind: cfgError,
-      result.msg = errorStr(c, "symbol expected, but found: " & c.tok.literal)
+        msg: errorStr(c, "symbol expected, but found: " & c.tok.literal))
     rawGetTok(c, c.tok)
     if c.tok.kind == tkBracketRi:
       rawGetTok(c, c.tok)
     else:
-      reset(result)
+      result = CfgEvent(kind: cfgError,
-      result.kind = cfgError
+        msg: errorStr(c, "']' expected, but found: " & c.tok.literal))
-      result.msg = errorStr(c, "']' expected, but found: " & c.tok.literal)
   of tkInvalid, tkEquals, tkColon, tkBracketRi:
-    result.kind = cfgError
+    result = CfgEvent(kind: cfgError,
-    result.msg = errorStr(c, "invalid token: " & c.tok.literal)
+      msg: errorStr(c, "invalid token: " & c.tok.literal))
     rawGetTok(c, c.tok)
 
 # ---------------- Configuration file related operations ----------------

lib/pure/parsesql.nim

@@ -554,17 +554,14 @@ type
     tok: Token
 
 proc newNode*(k: SqlNodeKind): SqlNode =
-  new(result)
+  result = SqlNode(kind: k)
-  result.kind = k
 
 proc newNode*(k: SqlNodeKind, s: string): SqlNode =
-  new(result)
+  result = SqlNode(kind: k)
-  result.kind = k
   result.strVal = s
 
 proc newNode*(k: SqlNodeKind, sons: seq[SqlNode]): SqlNode =
-  new(result)
+  result = SqlNode(kind: k)
-  result.kind = k
   result.sons = sons
 
 proc len*(n: SqlNode): int =

lib/pure/pegs.nim

@@ -142,34 +142,29 @@ proc rule*(nt: NonTerminal): Peg = nt.rule
 proc term*(t: string): Peg {.nosideEffect, rtl, extern: "npegs$1Str".} =
   ## constructs a PEG from a terminal string
   if t.len != 1:
-    result.kind = pkTerminal
+    result = Peg(kind: pkTerminal, term: t)
-    result.term = t
   else:
-    result.kind = pkChar
+    result = Peg(kind: pkChar, ch: t[0])
-    result.ch = t[0]
 
 proc termIgnoreCase*(t: string): Peg {.
   nosideEffect, rtl, extern: "npegs$1".} =
   ## constructs a PEG from a terminal string; ignore case for matching
-  result.kind = pkTerminalIgnoreCase
+  result = Peg(kind: pkTerminalIgnoreCase, term: t)
-  result.term = t
 
 proc termIgnoreStyle*(t: string): Peg {.
   nosideEffect, rtl, extern: "npegs$1".} =
   ## constructs a PEG from a terminal string; ignore style for matching
-  result.kind = pkTerminalIgnoreStyle
+  result = Peg(kind: pkTerminalIgnoreStyle, term: t)
-  result.term = t
 
 proc term*(t: char): Peg {.nosideEffect, rtl, extern: "npegs$1Char".} =
   ## constructs a PEG from a terminal char
   assert t != '\0'
-  result.kind = pkChar
+  result = Peg(kind: pkChar, ch: t)
-  result.ch = t
 
 proc charSet*(s: set[char]): Peg {.nosideEffect, rtl, extern: "npegs$1".} =
   ## constructs a PEG from a character set `s`
   assert '\0' notin s
-  result.kind = pkCharChoice
+  result = Peg(kind: pkCharChoice)
   new(result.charChoice)
   result.charChoice[] = s
 
@@ -189,8 +184,7 @@ proc addChoice(dest: var Peg, elem: Peg) =
   else: add(dest, elem)
 
 template multipleOp(k: PegKind, localOpt: untyped) =
-  result.kind = k
+  result = Peg(kind: k, sons: @[])
-  result.sons = @[]
   for x in items(a):
     if x.kind == k:
       for y in items(x.sons):
@@ -230,8 +224,7 @@ proc `?`*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsOptional".} =
     # a? ?  --> a?
     result = a
   else:
-    result.kind = pkOption
+    result = Peg(kind: pkOption, sons: @[a])
-    result.sons = @[a]
 
 proc `*`*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsGreedyRep".} =
   ## constructs a "greedy repetition" for the PEG `a`
@@ -240,27 +233,22 @@ proc `*`*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsGreedyRep".} =
     assert false
     # produces endless loop!
   of pkChar:
-    result.kind = pkGreedyRepChar
+    result = Peg(kind: pkGreedyRepChar, ch: a.ch)
-    result.ch = a.ch
   of pkCharChoice:
-    result.kind = pkGreedyRepSet
+    result = Peg(kind: pkGreedyRepSet, charChoice: a.charChoice)
-    result.charChoice = a.charChoice # copying a reference suffices!
   of pkAny, pkAnyRune:
-    result.kind = pkGreedyAny
+    result = Peg(kind: pkGreedyAny)
   else:
-    result.kind = pkGreedyRep
+    result = Peg(kind: pkGreedyRep, sons: @[a])
-    result.sons = @[a]
 
 proc `!*`*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsSearch".} =
   ## constructs a "search" for the PEG `a`
-  result.kind = pkSearch
+  result = Peg(kind: pkSearch, sons: @[a])
-  result.sons = @[a]
 
 proc `!*\`*(a: Peg): Peg {.noSideEffect, rtl,
                              extern: "npgegsCapturedSearch".} =
   ## constructs a "captured search" for the PEG `a`
-  result.kind = pkCapturedSearch
+  result = Peg(kind: pkCapturedSearch, sons: @[a])
-  result.sons = @[a]
 
 proc `+`*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsGreedyPosRep".} =
   ## constructs a "greedy positive repetition" with the PEG `a`
@@ -268,50 +256,48 @@ proc `+`*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsGreedyPosRep".} =
 
 proc `&`*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsAndPredicate".} =
   ## constructs an "and predicate" with the PEG `a`
-  result.kind = pkAndPredicate
+  result = Peg(kind: pkAndPredicate, sons: @[a])
-  result.sons = @[a]
 
 proc `!`*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsNotPredicate".} =
   ## constructs a "not predicate" with the PEG `a`
-  result.kind = pkNotPredicate
+  result = Peg(kind: pkNotPredicate, sons: @[a])
-  result.sons = @[a]
 
 proc any*: Peg {.inline.} =
   ## constructs the PEG `any character`:idx: (``.``)
-  result.kind = pkAny
+  result = Peg(kind: pkAny)
 
 proc anyRune*: Peg {.inline.} =
   ## constructs the PEG `any rune`:idx: (``_``)
-  result.kind = pkAnyRune
+  result = Peg(kind: pkAnyRune)
 
 proc newLine*: Peg {.inline.} =
   ## constructs the PEG `newline`:idx: (``\n``)
-  result.kind = pkNewLine
+  result = Peg(kind: pkNewLine)
 
 proc unicodeLetter*: Peg {.inline.} =
   ## constructs the PEG ``\letter`` which matches any Unicode letter.
-  result.kind = pkLetter
+  result = Peg(kind: pkLetter)
 
 proc unicodeLower*: Peg {.inline.} =
   ## constructs the PEG ``\lower`` which matches any Unicode lowercase letter.
-  result.kind = pkLower
+  result = Peg(kind: pkLower)
 
 proc unicodeUpper*: Peg {.inline.} =
   ## constructs the PEG ``\upper`` which matches any Unicode uppercase letter.
-  result.kind = pkUpper
+  result = Peg(kind: pkUpper)
 
 proc unicodeTitle*: Peg {.inline.} =
   ## constructs the PEG ``\title`` which matches any Unicode title letter.
-  result.kind = pkTitle
+  result = Peg(kind: pkTitle)
 
 proc unicodeWhitespace*: Peg {.inline.} =
   ## constructs the PEG ``\white`` which matches any Unicode
   ## whitespace character.
-  result.kind = pkWhitespace
+  result = Peg(kind: pkWhitespace)
 
 proc startAnchor*: Peg {.inline.} =
   ## constructs the PEG ``^`` which matches the start of the input.
-  result.kind = pkStartAnchor
+  result = Peg(kind: pkStartAnchor)
 
 proc endAnchor*: Peg {.inline.} =
   ## constructs the PEG ``$`` which matches the end of the input.
@@ -319,29 +305,25 @@ proc endAnchor*: Peg {.inline.} =
 
 proc capture*(a: Peg): Peg {.nosideEffect, rtl, extern: "npegsCapture".} =
   ## constructs a capture with the PEG `a`
-  result.kind = pkCapture
+  result = Peg(kind: pkCapture, sons: @[a])
-  result.sons = @[a]
 
 proc backref*(index: range[1..MaxSubpatterns]): Peg {.
   nosideEffect, rtl, extern: "npegs$1".} =
   ## constructs a back reference of the given `index`. `index` starts counting
   ## from 1.
-  result.kind = pkBackRef
+  result = Peg(kind: pkBackRef, index: index-1)
-  result.index = index-1
 
 proc backrefIgnoreCase*(index: range[1..MaxSubpatterns]): Peg {.
   nosideEffect, rtl, extern: "npegs$1".} =
   ## constructs a back reference of the given `index`. `index` starts counting
   ## from 1. Ignores case for matching.
-  result.kind = pkBackRefIgnoreCase
+  result = Peg(kind: pkBackRefIgnoreCase, index: index-1)
-  result.index = index-1
 
 proc backrefIgnoreStyle*(index: range[1..MaxSubpatterns]): Peg {.
   nosideEffect, rtl, extern: "npegs$1".}=
   ## constructs a back reference of the given `index`. `index` starts counting
   ## from 1. Ignores style for matching.
-  result.kind = pkBackRefIgnoreStyle
+  result = Peg(kind: pkBackRefIgnoreStyle, index: index-1)
-  result.index = index-1
 
 proc spaceCost(n: Peg): int =
   case n.kind
@@ -366,16 +348,12 @@ proc nonterminal*(n: NonTerminal): Peg {.
     when false: echo "inlining symbol: ", n.name
     result = n.rule # inlining of rule enables better optimizations
   else:
-    result.kind = pkNonTerminal
+    result = Peg(kind: pkNonTerminal, nt: n)
-    result.nt = n
 
 proc newNonTerminal*(name: string, line, column: int): NonTerminal {.
   nosideEffect, rtl, extern: "npegs$1".} =
   ## constructs a nonterminal symbol
-  new(result)
+  result = NonTerminal(name: name, line: line, col: column)
-  result.name = name
-  result.line = line
-  result.col = column
 
 template letters*: Peg =
   ## expands to ``charset({'A'..'Z', 'a'..'z'})``
@@ -585,8 +563,14 @@ template matchOrParse(mopProc: untyped) =
       if p.index >= c.ml: return -1
       var (a, b) = c.matches[p.index]
       var n: Peg
-      n.kind = succ(pkTerminal, ord(p.kind)-ord(pkBackRef))
+      case p.kind
-      n.term = s.substr(a, b)
+      of pkBackRef:
+        n = Peg(kind: pkTerminal, term: s.substr(a, b))
+      of pkBackRefIgnoreStyle:
+        n = Peg(kind: pkTerminalIgnoreStyle, term: s.substr(a, b))
+      of pkBackRefIgnoreCase:
+        n = Peg(kind: pkTerminalIgnoreCase, term: s.substr(a, b))
+      else: assert(false, "impossible case")
       mopProc(s, n, start, c)
 
     case p.kind

lib/pure/xmltree.nim

@@ -70,8 +70,7 @@ const
 
 proc newXmlNode(kind: XmlNodeKind): XmlNode =
   ## Creates a new ``XmlNode``.
-  new(result)
+  result = XmlNode(k: kind)
-  result.k = kind
 
 proc newElement*(tag: string): XmlNode =
   ## Creates a new ``XmlNode`` of kind ``xnElement`` with the given `tag`.

lib/system.nim

@@ -266,8 +266,6 @@ proc new*[T](a: var ref T, finalizer: proc (x: ref T) {.nimcall.}) {.
 
 proc reset*[T](obj: var T) {.magic: "Reset", noSideEffect.}
   ## Resets an object `obj` to its initial (binary zero) value.
-  ##
-  ## This needs to be called before any possible `object branch transition`:idx:.
 
 proc wasMoved*[T](obj: var T) {.magic: "WasMoved", noSideEffect.} =
   ## Resets an object `obj` to its initial (binary zero) value to signify

lib/system/assign.nim

@@ -232,5 +232,9 @@ proc FieldDiscriminantCheck(oldDiscVal, newDiscVal: int,
                             L: int) {.compilerProc.} =
   var oldBranch = selectBranch(oldDiscVal, L, a)
   var newBranch = selectBranch(newDiscVal, L, a)
-  if newBranch != oldBranch and oldDiscVal != 0:
+  when defined(nimOldCaseObjects):
-    sysFatal(FieldError, "assignment to discriminant changes object branch")
+    if newBranch != oldBranch and oldDiscVal != 0:
+      sysFatal(FieldError, "assignment to discriminant changes object branch")
+  else:
+    if newBranch != oldBranch:
+      sysFatal(FieldError, "assignment to discriminant changes object branch")

tests/compilerapi/tcompilerapi.nim.cfg

@@ -0,0 +1 @@
+define:nimOldCaseObjects