Add System.Text.Json support for F# discriminated unions

src/libraries/System.Text.Json/src/Resources/Strings.resx

@@ -641,8 +641,17 @@
   <data name="MissingFSharpCoreMember" xml:space="preserve">
     <value>Could not locate required member '{0}' from FSharp.Core. This might happen because your application has enabled member-level trimming.</value>
   </data>
-  <data name="FSharpDiscriminatedUnionsNotSupported" xml:space="preserve">
-    <value>F# discriminated union serialization is not supported. Consider authoring a custom converter for the type.</value>
+  <data name="FSharpUnionDuplicateCaseName" xml:space="preserve">
+    <value>The F# discriminated union type '{0}' has duplicate case discriminator name '{1}'.</value>
+  </data>
+  <data name="FSharpUnionDuplicateFieldName" xml:space="preserve">
+    <value>The F# discriminated union type '{0}' case '{1}' has duplicate field name '{2}'.</value>
+  </data>
+  <data name="FSharpUnionFieldConflictsWithDiscriminator" xml:space="preserve">
+    <value>The F# discriminated union type '{0}' has a case field name '{1}' that conflicts with the type discriminator property name '{2}'.</value>
+  </data>
+  <data name="FSharpUnionMissingDiscriminatorProperty" xml:space="preserve">
+    <value>Required type discriminator property '{0}' not found when deserializing F# discriminated union type '{1}'.</value>
   </data>
   <data name="Polymorphism_BaseConverterDoesNotSupportMetadata" xml:space="preserve">
     <value>The converter for polymorphic type '{0}' does not support metadata writes or reads.</value>

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -204,6 +204,7 @@ The System.Text.Json library is built-in as part of the shared framework in .NET
     <Compile Include="System\Text\Json\Serialization\Converters\FSharp\FSharpMapConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\FSharp\FSharpSetConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\FSharp\FSharpTypeConverterFactory.cs" />
+    <Compile Include="System\Text\Json\Serialization\Converters\FSharp\FSharpUnionConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\FSharp\FSharpOptionConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\FSharp\FSharpValueOptionConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\Node\JsonArrayConverter.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/FSharp/FSharpTypeConverterFactory.cs

@@ -3,6 +3,7 @@
 
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
 using System.Text.Json.Serialization.Metadata;
 using FSharpKind = System.Text.Json.Serialization.Metadata.FSharpCoreReflectionProxy.FSharpKind;
 
@@ -66,13 +67,40 @@ public override bool CanConvert(Type typeToConvert) =>
                     Debug.Assert(objectFactory.CanConvert(typeToConvert));
                     return objectFactory.CreateConverter(typeToConvert, options);
                 case FSharpKind.Union:
-                    return UnsupportedTypeConverterFactory.CreateUnsupportedConverterForType(typeToConvert, SR.FSharpDiscriminatedUnionsNotSupported);
+                    return CreateFSharpUnionConverter(typeToConvert, options);
                 default:
                     Debug.Fail("Unrecognized F# type.");
                     throw new Exception();
             }
 
             return (JsonConverter)Activator.CreateInstance(converterFactoryType, constructorArguments)!;
         }
+
+        [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
+            Justification = "The ctor is marked RequiresUnreferencedCode.")]
+        private static JsonConverter CreateFSharpUnionConverter(Type typeToConvert, JsonSerializerOptions options)
+        {
+            FSharpCoreReflectionProxy proxy = FSharpCoreReflectionProxy.Instance;
+            FSharpCoreReflectionProxy.FSharpUnionCaseInfo[] caseInfos = proxy.GetUnionCaseInfos(typeToConvert);
+            Func<object, int> tagReader = proxy.CreateUnionTagReader(typeToConvert);
+
+            // Read [JsonPolymorphic] for TypeDiscriminatorPropertyName customization.
+            string typeDiscriminatorPropertyName = typeToConvert
+                .GetCustomAttribute<JsonPolymorphicAttribute>(inherit: false)?.TypeDiscriminatorPropertyName
+                ?? JsonSerializer.TypePropertyName;
+
+            // Validate the discriminator property name doesn't conflict with reserved metadata properties ($id, $ref, $values).
+            if (!typeDiscriminatorPropertyName.Equals(JsonSerializer.TypePropertyName, StringComparison.Ordinal))
+            {
+                byte[] utf8EncodedName = System.Text.Encoding.UTF8.GetBytes(typeDiscriminatorPropertyName);
+                if ((JsonSerializer.GetMetadataPropertyName(utf8EncodedName, resolver: null) & ~MetadataPropertyName.Type) != 0)
+                {
+                    ThrowHelper.ThrowInvalidOperationException_InvalidCustomTypeDiscriminatorPropertyName();
+                }
+            }
+
+            Type converterType = typeof(FSharpUnionConverter<>).MakeGenericType(typeToConvert);
+            return (JsonConverter)Activator.CreateInstance(converterType, new object[] { caseInfos, tagReader, options, typeDiscriminatorPropertyName })!;
+        }
     }
 }