Missing Enum constraint on JsonNumberEnumConverter<TEnum> in reference assembly

[KalleOlaviNiemitalo]

Description

In the source code, System.Text.Json.Serialization.JsonNumberEnumConverter<TEnum> has a type constraint where TEnum : struct, Enum. In the reference assembly however, the constraint is just where TEnum : struct and omits Enum. Thus, if TEnum is a non-enum value type, the compiler doesn't notice any problem but it fails at run time.

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonNumberEnumConverter.cs

Lines 17 to 18 in 3eb129f

	public sealed class JsonNumberEnumConverter<TEnum> : JsonConverterFactory
	where TEnum : struct, Enum

runtime/src/libraries/System.Text.Json/ref/System.Text.Json.cs

Line 981 in 3eb129f

public sealed partial class JsonNumberEnumConverter<TEnum> : System.Text.Json.Serialization.JsonConverterFactory where TEnum : struct

Reproduction Steps

NonEnum.csproj:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

Program.cs:

_ = new System.Text.Json.Serialization.JsonNumberEnumConverter<int>();

dotnet run

Expected behavior

The compiler should report an error:

Program.cs(1,64): error CS0315: The type 'int' cannot be used as type parameter 'TEnum' in the generic type or method 'JsonNumberEnumConverter<TEnum>'. There is no boxing conversion from 'int' to 'System.Enum'.

The build failed. Fix the build errors and run again.

Actual behavior

The build succeeds without any warnings, but the program fails at run time:

Unhandled exception. System.TypeLoadException: GenericArguments[0], 'System.Int32', on 'System.Text.Json.Serialization.JsonNumberEnumConverter`1[TEnum]' violates the constraint of type parameter 'TEnum'.
at Program.<Main>$(String[] args)

Regression?

Not a regression. Both the src and ref changes came from #88984, so they have never been consistent in any release.

Known Workarounds

No response

Configuration

.NET SDK 8.0.202
.NET 8.0.3
Windows 10 22H2 x64

Other information

This bug might be considered part of #26187.

The System.Enum type constraint is missing from the documentation page https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonnumberenumconverter-1?view=net-8.0 as well.

The System.Text.Json 8.0.3 NuGet package does not carry any reference assemblies. Thus, if you reference that and target .NET Framework, then you get the expected error CS0315 at compile time.

[ShreyasJejurkar]

Strange, why APICompact didn't catch this. Looks like bug or missing usecase to check in it.

cc @ViktorHofer

[KalleOlaviNiemitalo]

If I understand correctly, ApiCompat would have to compare ITypeParameterSymbol.ConstraintTypes. However, git grep -e ConstraintTypes v8.0.200 in https://github.com/dotnet/sdk/ doesn't find anything.

[eiriktsarpalis]

@KalleOlaviNiemitalo what branch is this? I see the constraint is present in both main:

runtime/src/libraries/System.Text.Json/ref/System.Text.Json.cs

Line 1095 in c352ef6

public partial class JsonStringEnumConverter<TEnum> : System.Text.Json.Serialization.JsonConverterFactory where TEnum : struct, System.Enum

and release/8.0:

runtime/src/libraries/System.Text.Json/ref/System.Text.Json.cs

Line 1086 in fd8f5b5

public partial class JsonStringEnumConverter<TEnum> : System.Text.Json.Serialization.JsonConverterFactory where TEnum : struct, System.Enum

I also can't reproduce the crash you're seeing:

[KalleOlaviNiemitalo]

@eiriktsarpalis, you're looking at JsonStringEnumConverter<TEnum>, but this issue is for JsonNumberEnumConverter<TEnum>.

[eiriktsarpalis]

D'oh! Didn't even notice it was for a different type. Thanks.

[eiriktsarpalis]

FYI @carlossanlop @ViktorHofer @ericstj this will require a .NET 8 backport that updates the reference source.

[KalleOlaviNiemitalo]

Does this truly deserve a backport? It seems unlikely that the bug would be hurting any production applications.

I originally noticed the missing constraint in the documentation and was going to file an issue there, but filed this one here instead.

[ericstj]

Not sure we need the backport here. Please file an issue in dotnet/sdk that APICompat didn't catch this.

[KalleOlaviNiemitalo]

@ShreyasJejurkar, do you intend to file the APICompat issue? I'm not really familiar with the tool.

[KalleOlaviNiemitalo]

@eiriktsarpalis, should this issue be moved to the 9.0.0 milestone because it won't be serviced in 8.0.x?