fixes #6234

compiler/ccgexprs.nim

@@ -1100,9 +1100,9 @@ proc rawGenNew(p: BProc, a: TLoc, sizeExpr: Rope) =
   if a.s == OnHeap and usesNativeGC():
     # use newObjRC1 as an optimization
     if canFormAcycle(a.t):
-      linefmt(p, cpsStmts, "if ($1) #nimGCunref($1);$n", a.rdLoc)
+      linefmt(p, cpsStmts, "if ($1) { #nimGCunrefRC1($1); $1 = NIM_NIL; }$n", a.rdLoc)
     else:
-      linefmt(p, cpsStmts, "if ($1) #nimGCunrefNoCycle($1);$n", a.rdLoc)
+      linefmt(p, cpsStmts, "if ($1) { #nimGCunrefNoCycle($1); $1 = NIM_NIL; }$n", a.rdLoc)
     b.r = ropecg(p.module, "($1) #newObjRC1($2, $3)", args)
     linefmt(p, cpsStmts, "$1 = $2;$n", a.rdLoc, b.rdLoc)
   else:
@@ -1130,9 +1130,9 @@ proc genNewSeqAux(p: BProc, dest: TLoc, length: Rope) =
   initLoc(call, locExpr, dest.t, OnHeap)
   if dest.s == OnHeap and usesNativeGC():
     if canFormAcycle(dest.t):
-      linefmt(p, cpsStmts, "if ($1) #nimGCunref($1);$n", dest.rdLoc)
+      linefmt(p, cpsStmts, "if ($1) { #nimGCunrefRC1($1); $1 = NIM_NIL; }$n", dest.rdLoc)
     else:
-      linefmt(p, cpsStmts, "if ($1) #nimGCunrefNoCycle($1);$n", dest.rdLoc)
+      linefmt(p, cpsStmts, "if ($1) { #nimGCunrefNoCycle($1); $1 = NIM_NIL; }$n", dest.rdLoc)
     call.r = ropecg(p.module, "($1) #newSeqRC1($2, $3)", args)
     linefmt(p, cpsStmts, "$1 = $2;$n", dest.rdLoc, call.rdLoc)
   else:

lib/system/gc.nim

@@ -259,6 +259,9 @@ proc nimGCunrefNoCycle(p: pointer) {.compilerProc, inline.} =
     sysAssert(allocInv(gch.region), "end nimGCunrefNoCycle 2")
   sysAssert(allocInv(gch.region), "end nimGCunrefNoCycle 5")
 
+proc nimGCunrefRC1(p: pointer) {.compilerProc, inline.} =
+  decRef(usrToCell(p))
+
 proc asgnRef(dest: PPointer, src: pointer) {.compilerProc, inline.} =
   # the code generator calls this proc!
   gcAssert(not isOnStack(dest), "asgnRef")

lib/system/mmdisp.nim

@@ -16,7 +16,7 @@
 const
   debugGC = false # we wish to debug the GC...
   logGC = false
-  traceGC = defined(smokeCycles) # extensive debugging
+  traceGC = false # extensive debugging
   alwaysCycleGC = defined(smokeCycles)
   alwaysGC = defined(fulldebug) # collect after every memory
                                 # allocation (for debugging)

tests/ccgbugs/twrongrefcounting.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''ok'''
+  cmd: "nim c -r --gc:refc -d:useGcAssert -d:useSysAssert -d:fulldebug -d:smokeCycles $file"
+"""
+# bug #6234
+type
+    Foo = ref object
+        s: seq[Bar]
+    Bar = ref object
+        f: Foo
+
+proc test() =
+    var f = Foo.new()
+    for i in 0 .. 5:
+        f.s = @[]
+        for j in 0 .. 5:
+            var b = Bar.new()
+            b.f = f
+            f.s.add(b)
+
+test()
+echo "ok"