fixes #15717

(cherry picked from commit 558115fa29)

compiler/vmgen.nim

@@ -683,6 +683,8 @@ proc genNewSeq(c: PCtx; n: PNode) =
 
 proc genNewSeqOfCap(c: PCtx; n: PNode; dest: var TDest) =
   let t = n.typ
+  if dest < 0:
+    dest = c.getTemp(n.typ)
   let tmp = c.getTemp(n[1].typ)
   c.gABx(n, opcLdNull, dest, c.genType(t))
   c.gABx(n, opcLdImmInt, tmp, 0)

tests/vm/tnewseqofcap.nim

@@ -0,0 +1,19 @@
+discard """
+  output: '''@["aaa", "bbb", "ccc"]'''
+"""
+
+
+const
+  foo = @["aaa", "bbb", "ccc"]
+
+proc myTuple: tuple[n: int, bar: seq[string]] =
+  result.n = 42
+  result.bar = newSeqOfCap[string](foo.len)
+  for f in foo:
+    result.bar.add(f)
+
+# It works if you change the below `const` to `let`
+const
+  (n, bar) = myTuple()
+
+echo bar