Revert #138 CanConvert widening; replace with workaround example

CHANGELOG.md

@@ -11,6 +11,9 @@ The `Unreleased` section name is replaced by the expected version of next releas
 ### Added
 ### Changed
 ### Removed
+
+- `SystemTextJson.UnionConverter`: Reverted `CanConvert` widening for boxed DU case types [#144](https://github.com/jet/FsCodec/pull/144)
+
 ### Fixed
 
 <a name="3.1.1"></a>

src/FsCodec.SystemTextJson/UnionConverter.fs

@@ -26,7 +26,7 @@ type UnionConverter<'T>() =
     let converterOptions = UnionConverterOptions.get typeof<'T>
     let info = FsCodec.Union.Info.get typeof<'T>
 
-    override _.CanConvert t = (t = typeof<'T> || t.DeclaringType = typeof<'T>) && FsCodec.Union.isUnion t
+    override _.CanConvert t = t = typeof<'T> && FsCodec.Union.isUnion t
 
     override _.Write(writer: Utf8JsonWriter, value, options: JsonSerializerOptions) =
         let value = box value

tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs

@@ -684,14 +684,23 @@ module IsomorphismUnionEncoder =
 // NOTE: STJ uses inherit: false when looking up [JsonConverter] attributes, so the attribute on the
 // union base type is not found for case-specific nested types. The converter must be registered
 // via options.Converters (or autoUnionToJsonObject) for boxed serialization to work.
+// This is not recommended practice from a System.Text.Json library perspective; if you need it,
+// use a custom JsonConverterFactory as shown here to provide the widened CanConvert logic.
 module ``Boxed DU Serialization`` =
 
+    /// Custom factory that widens CanConvert to also accept the nested case types that STJ resolves
+    /// for boxed multi-case DUs (where the runtime type is e.g. Union+Case1 with DeclaringType = Union).
+    type BoxedUnionConverterFactory<'T>() =
+        inherit Serialization.JsonConverterFactory()
+        override _.CanConvert t = (t = typeof<'T> || t.DeclaringType = typeof<'T>) && FsCodec.Union.isUnion t
+        override _.CreateConverter(_t, _options) = UnionConverter<'T>()
+
     type BoxedMultiCase =
         | Case1 of int
         | Case2 of string
 
     let optsWithConverter = JsonSerializerOptions()
-    optsWithConverter.Converters.Add(UnionConverter<BoxedMultiCase>())
+    optsWithConverter.Converters.Add(BoxedUnionConverterFactory<BoxedMultiCase>())
 
     let [<Fact>] ``Serialize boxed multi-case DU via options does not throw`` () =
         let json = JsonSerializer.Serialize(Case1 42 :> obj, optsWithConverter)