fixes #11098

compiler/ccgexprs.nim

@@ -1306,11 +1306,17 @@ proc genNewSeqOfCap(p: BProc; e: PNode; d: var TLoc) =
   let seqtype = skipTypes(e.typ, abstractVarRange)
   var a: TLoc
   initLocExpr(p, e.sons[1], a)
-  putIntoDest(p, d, e, ropecg(p.module,
-              "($1)#nimNewSeqOfCap($2, $3)", [
-              getTypeDesc(p.module, seqtype),
-              genTypeInfo(p.module, seqtype, e.info), a.rdLoc]))
-  gcUsage(p.config, e)
+  if p.config.selectedGC == gcDestructors:
+    if d.k == locNone: getTemp(p, e.typ, d, needsInit=false)
+    linefmt(p, cpsStmts, "$1.len = 0; $1.p = ($4*) #newSeqPayload($2, sizeof($3));$n",
+      [d.rdLoc, a.rdLoc, getTypeDesc(p.module, seqtype.lastSon),
+      getSeqPayloadType(p.module, seqtype)])
+  else:
+    putIntoDest(p, d, e, ropecg(p.module,
+                "($1)#nimNewSeqOfCap($2, $3)", [
+                getTypeDesc(p.module, seqtype),
+                genTypeInfo(p.module, seqtype, e.info), a.rdLoc]))
+    gcUsage(p.config, e)
 
 proc genConstExpr(p: BProc, n: PNode): Rope
 proc handleConstExpr(p: BProc, n: PNode, d: var TLoc): bool =

lib/system.nim

@@ -997,8 +997,7 @@ proc newSeqOfCap*[T](cap: Natural): seq[T] {.
   ##   assert len(x) == 1
   discard
 
-when not defined(JS) and not defined(gcDestructors):
-  # XXX enable this for --gc:destructors
+when not defined(JS):
   proc newSeqUninitialized*[T: SomeNumber](len: Natural): seq[T] =
     ## Creates a new sequence of type ``seq[T]`` with length ``len``.
     ##
@@ -1011,8 +1010,11 @@ when not defined(JS) and not defined(gcDestructors):
     ##   assert len(x) == 3
     ##   x[0] = 10
     result = newSeqOfCap[T](len)
-    var s = cast[PGenericSeq](result)
-    s.len = len
+    when defined(gcDestructors):
+      cast[ptr int](addr result)[] = len
+    else:
+      var s = cast[PGenericSeq](result)
+      s.len = len
 
 proc len*[TOpenArray: openArray|varargs](x: TOpenArray): int {.
   magic: "LengthOpenArray", noSideEffect.}

tests/destructor/tgcdestructors.nim

@@ -7,7 +7,9 @@ ha
 @[1, 2, 3]
 @["red", "yellow", "orange", "rtrt1", "pink"]
 a: @[4, 2, 3]
-35 35'''
+0
+30
+41 41'''
 """
 
 import allocators
@@ -178,6 +180,16 @@ proc mutConstSeq() =
 
 mutConstSeq()
 
+proc mainSeqOfCap =
+  # bug #11098
+  var s = newSeqOfCap[int](10)
+  echo s.len
+
+  var s2 = newSeqUninitialized[int](30)
+  echo s2.len
+
+mainSeqOfCap()
+
 #echo s
 let (a, d) = allocCounters()
 discard cprintf("%ld %ld\n", a, d)