Fixes discard bug in cpp codegen (#9747) [backport]

Fixes discard bug in cpp codegen

(cherry picked from commit 9b9f5dee0b)

tests/cpp/tcasts.nim

@@ -1,6 +1,6 @@
 discard """
   cmd: "nim cpp $file"
-  output: ""
+  output: '''{"vas": "kas", "123": "123"}'''
   targets: "cpp"
 """
 
@@ -9,3 +9,13 @@ block: #5979
   var p: pointer = cast[pointer](a)
   var c = cast[char](p)
   doAssert(c == 'a')
+
+
+#----------------------------------------------------
+# bug #9739
+import tables
+
+var t = initTable[string, string]()
+discard t.hasKeyOrPut("123", "123")
+discard t.mgetOrPut("vas", "kas")
+echo t