Fix UnionConverter.CanConvert for boxed multi-case DU

[Copilot]

When a multi-case F# DU is boxed and serialized via JsonSerializer.Serialize(v :> obj), STJ resolves the runtime type (e.g. Test+Test1 nested type) rather than the union base type. UnionConverter<'T>.CanConvert rejected these case types, so STJ fell through to its built-in UnsupportedTypeConverter.

Resolves #128

Root cause

F# multi-case DUs compile to a base class (Test) with nested subclasses per case (Test+Test1, Test+Test2). STJ looks up [JsonConverter] attributes with inherit: false, so the attribute on the base type is invisible for case types. However, when the converter is registered via options.Converters, STJ does call CanConvert with the case type — which previously returned false.

Single-case and struct DUs are unaffected (no nested types generated).

Changes

• UnionConverter.CanConvert: Accept types where t.DeclaringType = typeof<'T> (the DU case nested types), and validate isUnion against typeof<'T> instead of t
• Tests: Three new tests for boxed multi-case DU serialization via options.Converters

Usage

// Register converter via options (required for boxed serialization of multi-case DUs)
let options = JsonSerializerOptions()
options.Converters.Add(UnionConverter<MyUnion>())
JsonSerializer.Serialize(MyCase 42 :> obj, options) // works

// Also works via autoUnionToJsonObject
let options = Options.Create(autoUnionToJsonObject = true)
JsonSerializer.Serialize(MyCase 42 :> obj, options) // works

Known limitation

Attribute-only usage ([<JsonConverter(typeof<UnionConverter<T>>)>] without options.Converters) still fails for boxed multi-case DUs — STJ's inherit: false attribute lookup is not overridable from converter code.

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[bartelink]

@copilot please add a terse changelog.md entry citing #138

[Copilot]

@copilot please add a terse changelog.md entry citing #138

Added in 9ca5615.

[Copilot]

Pull request overview

Fixes System.Text.Json serialization of boxed multi-case F# discriminated unions when UnionConverter<'T> is registered via options.Converters, by allowing the converter to be selected for runtime union case types (nested subclasses like Test+Case1).

Changes:

• Update UnionConverter<'T>.CanConvert to accept union case nested runtime types when 'T is a multi-case DU base type.
• Add System.Text.Json-only tests covering boxed multi-case DU serialization through JsonSerializerOptions.Converters.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/FsCodec.SystemTextJson/UnionConverter.fs	Expands CanConvert to match boxed multi-case DU runtime types so STJ can select the converter.
tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs	Adds #if SYSTEM_TEXT_JSON tests validating boxed multi-case DU serialization using options.Converters.

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