fixes #23556; typeinfo.extendSeq generates random values in ORC (#23557)

fixes #23556

It should somehow handle default fields in the future

lib/core/typeinfo.nim

@@ -129,7 +129,9 @@ when not defined(gcDestructors):
 else:
   proc nimNewObj(size, align: int): pointer {.importCompilerProc.}
   proc newSeqPayload(cap, elemSize, elemAlign: int): pointer {.importCompilerProc.}
-  proc prepareSeqAdd(len: int; p: pointer; addlen, elemSize, elemAlign: int): pointer {.
+  proc prepareSeqAddUninit(len: int; p: pointer; addlen, elemSize, elemAlign: int): pointer {.
+    importCompilerProc.}
+  proc zeroNewElements(len: int; p: pointer; addlen, elemSize, elemAlign: int) {.
     importCompilerProc.}
 
 template `+!!`(a, b): untyped = cast[pointer](cast[int](a) + b)
@@ -221,7 +223,8 @@ proc extendSeq*(x: Any) =
     var s = cast[ptr NimSeqV2Reimpl](x.value)
     let elem = x.rawType.base
     if s.p == nil or s.p.cap < s.len+1:
-      s.p = cast[ptr NimSeqPayloadReimpl](prepareSeqAdd(s.len, s.p, 1, elem.size, elem.align))
+      s.p = cast[ptr NimSeqPayloadReimpl](prepareSeqAddUninit(s.len, s.p, 1, elem.size, elem.align))
+    zeroNewElements(s.len, s.p, 1, elem.size, elem.align)
     inc s.len
   else:
     var y = cast[ptr PGenSeq](x.value)[]

lib/system/seqs_v2.nim

@@ -90,6 +90,12 @@ proc prepareSeqAdd(len: int; p: pointer; addlen, elemSize, elemAlign: int): poin
       q.cap = newCap
       result = q
 
+proc zeroNewElements(len: int; q: pointer; addlen, elemSize, elemAlign: int) {.
+    noSideEffect, tags: [], raises: [], compilerRtl.} =
+  {.noSideEffect.}:
+    let headerSize = align(sizeof(NimSeqPayloadBase), elemAlign)
+    zeroMem(q +! headerSize +! len * elemSize, addlen * elemSize)
+
 proc prepareSeqAddUninit(len: int; p: pointer; addlen, elemSize, elemAlign: int): pointer {.
     noSideEffect, tags: [], raises: [], compilerRtl.} =
   {.noSideEffect.}:

tests/stdlib/ttypeinfo.nim

@@ -74,3 +74,20 @@ block:
 
   doAssert getEnumOrdinal(y, "Hello") == 0
   doAssert getEnumOrdinal(y, "hello") == 1
+
+block: # bug #23556
+  proc test =
+    var
+      t: seq[int]
+      aseq = toAny(t)
+
+    invokeNewSeq(aseq, 0)
+
+    # Got random value only when loop 8 times.
+    for i in 1 .. 8:
+      extendSeq(aseq)
+
+    doAssert t == @[0, 0, 0, 0, 0, 0, 0, 0]
+
+  for i in 1 .. 7:
+    test()