Fixes #6604. Rejects unnamed tuples with error.

tests/stdlib/tjsonmacro.nim

@@ -275,4 +275,24 @@ when isMainModule:
     let parsed = to(j, Thing)
     doAssert parsed.animal.fur
     doAssert parsed.animal.legs == 6
-    doAssert parsed.color == Red
+    doAssert parsed.color == Red
+
+  block:
+    type
+      Car = object
+        engine: tuple[name: string, capacity: float]
+        model: string
+
+    let j = """
+      {"engine": {"name": "V8", "capacity": 5.5}, "model": "Skyline"}
+    """
+
+    var i = 0
+    proc mulTest: JsonNode =
+      i.inc()
+      return parseJson(j)
+
+    let parsed = mulTest().to(Car)
+    doAssert parsed.engine.name == "V8"
+
+    doAssert i == 1

tests/stdlib/tjsonmacro_reject.nim

@@ -0,0 +1,18 @@
+discard """
+  file: "tjsonmacro_reject.nim"
+  line: 11
+  errormsg: "Use a named tuple instead of: (string, float)"
+"""
+
+import json
+
+type
+  Car = object
+    engine: (string, float)
+    model: string
+
+let j = """
+  {"engine": {"name": "V8", "capacity": 5.5}, model: "Skyline"}
+"""
+let parsed = parseJson(j)
+echo(to(parsed, Car))