Fixes #6604. Rejects unnamed tuples with error.

lib/pure/json.nim

@@ -1701,6 +1701,10 @@ proc createConstructor(typeSym, jsonNode: NimNode): NimNode =
       result = processType(typeSym, obj, jsonNode, false)
   of nnkTupleTy:
     result = processType(typeSym, typeSym, jsonNode, false)
+  of nnkPar:
+    # TODO: The fact that `jsonNode` here works to give a good line number
+    # is weird. Specifying typeSym should work but doesn't.
+    error("Use a named tuple instead of: " & $toStrLit(typeSym), jsonNode)
   else:
     doAssert false, "Unable to create constructor for: " & $typeSym.kind
 
@@ -1828,9 +1832,16 @@ macro to*(node: JsonNode, T: typedesc): untyped =
   expectKind(typeNode, nnkBracketExpr)
   doAssert(($typeNode[0]).normalize == "typedesc")
 
-  result = createConstructor(typeNode[1], node)
+  # Create `temp` variable to store the result in case the user calls this
+  # on `parseJson` (see bug #6604).
+  result = newNimNode(nnkStmtListExpr)
+  let temp = genSym(nskLet, "temp")
+  result.add quote do:
+    let `temp` = `node`
+
+  let constructor = createConstructor(typeNode[1], temp)
   # TODO: Rename postProcessValue and move it (?)
-  result = postProcessValue(result)
+  result.add(postProcessValue(constructor))
 
   # echo(treeRepr(result))
   # echo(toStrLit(result))

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))