fix jsonutils macro with generic case object (#24429)

split from #24425

The added test did not work previously. The result of `getTypeImpl` is
the uninstantiated AST of the original type symbol, and the macro
attempts to use this type for the result. To fix the issue, the provided
`typedesc` argument is used instead.

lib/std/jsonutils.nim

@@ -112,7 +112,7 @@ macro initCaseObject(T: typedesc, fun: untyped): untyped =
   else: raiseAssert $t.kind # xxx `nnkPtrTy` could be handled too
   doAssert t2.kind == nnkRecList
   result = newTree(nnkObjConstr)
-  result.add sym
+  result.add T
   for ti in t2:
     if ti.kind == nnkRecCase:
       let key = ti[0][0]

tests/stdlib/tjsonutils.nim

@@ -206,6 +206,19 @@ template fn() =
     # sanity check: nesting inside a tuple
     testRoundtrip((Foo[int](x0: 1.5, t1: false, z2: 6), "foo")): """[{"x0":1.5,"t1":false,"z2":6,"x1":""},"foo"]"""
 
+  block: # generic case object using generic type
+    type Foo[T] = ref object
+      x0: float
+      case t1: bool
+      of true: z1: int8
+      of false: z2: uint16
+      x1: string
+      x2: T
+    testRoundtrip(Foo[float](t1: true, z1: 5, x1: "bar", x2: 2.5)): """{"x0":0.0,"t1":true,"z1":5,"x1":"bar","x2":2.5}"""
+    testRoundtrip(Foo[int](x0: 1.5, t1: false, z2: 6, x2: 2)): """{"x0":1.5,"t1":false,"z2":6,"x1":"","x2":2}"""
+    # sanity check: nesting inside a tuple
+    testRoundtrip((Foo[int](x0: 1.5, t1: false, z2: 6, x2: 2), "foo")): """[{"x0":1.5,"t1":false,"z2":6,"x1":"","x2":2},"foo"]"""
+
   block: # case object: 2 discriminants, `when` branch, range discriminant
     type Foo[T] = object
       case t1: bool