Mirror Attributes with out keyword on generics type confuses IL2CPP

[Reimirno]

Describe the bug
IL2CPP build failure when you have a function that has both

• one of the attributes from [Server], [Client], [ServerCallback] or [ClientCallback].
• an out parameter on a generic type
  Error message varies, but generally you will see some Bee Compiler error like this, indicating syntax error in generated cpp code:
  

Additional info

• out parameter on array, or ordinary types works fine with the Mirror Attributes in Unity 2021.
• in and ref parameter works fine in Unity 2021.
• The Bee Compiler is new to Unity in 2021 version. I am not sure about the behaviour on lower versions.

How can we reproduce the issue, step by step:
In a Unity 2021 or above project, include a script containing a function as described above. Then, build the project with IL2CPP backend.
I have included a sample script at the end.

Expected behavior
IL2CPP should compile and build without problem.

Environment

• OS: Windows Education 18362
• Build target: Windows
• Unity version: 2021.3.3f1
• Mirror branch: v78.4.3

Additional context

• I have looked at the generated cpp code. If I use out HashSet<int> in function A, then IL2CPP seems to believe that there are two separate out HashSet<int> types. It gives you HashSet_1_t3EAA698F222717A950BC3D871BB65D51F83F6B70 in A's signature, but inside A it refers to HashSet<int> as HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735 (mismatch).
• James on the Discord server claims that they have worked out a fix (message ID: 1105571751498371072)

Sample Script

// Problematic.cs
using System.Collections.Generic;
using Mirror;
using UnityEngine;

public class Problematic : MonoBehaviour
{
    // You can uncomment a method at a time to see the error individually

    // Server Attribute + out generic parameter = error
    [Server]
    public void ServerAndOutGenericParameter(out HashSet<int> dummySet)
    {
        dummySet = new HashSet<int>();
    }

    // // Client Attribute + out generic parameter = error
    // [Client]
    // public void ClientAndOutGenericParameter(out HashSet<int> dummySet)
    // {
    //     dummySet = new HashSet<int>();
    // }

    // // ServerCallback Attribute + out generic parameter = error
    // [ServerCallback]
    // public void ServerCallbackAndOutGenericParameter(out HashSet<int> dummySet)
    // {
    //     dummySet = new HashSet<int>();
    // }

    // // ClientCallback Attribute + out generic parameter = error
    // [ClientCallback]
    // public void ClientCallbackAndOutGenericParameter(out HashSet<int> dummySet)
    // {
    //     dummySet = new HashSet<int>();
    // }
}

[miwarnec]

ok, can reproduce it by adding the above function to Tanks/Player.cs and building with IL2CPP with unity 2021 LTS

[miwarnec]

generated IL:

	// Methods
	.method public hidebysig 
		instance void ServerAndOutGenericParameter (
			[out] class [netstandard]System.Collections.Generic.HashSet`1<int32>& dummySet
		) cil managed 
	{
		.custom instance void [Mirror]Mirror.ServerAttribute::.ctor() = (
			01 00 00 00
		)
		// Method begins at RVA 0x2100
		// Header size: 12
		// Code size: 50 (0x32)
		.maxstack 2
		.locals init (
			[0] class [netstandard]System.Collections.Generic.HashSet`1
		)

		// if (!NetworkServer.active)
		IL_0000: call bool [Mirror]Mirror.NetworkServer::get_active()
		IL_0005: brtrue IL_002a

		// Debug.LogWarning((object)"[Server] function 'System.Void Mirror.Examples.Tanks.Tank::ServerAndOutGenericParameter(System.Collections.Generic.HashSet`1<System.Int32>&)' called when server was not active");
		IL_000a: ldstr "[Server] function 'System.Void Mirror.Examples.Tanks.Tank::ServerAndOutGenericParameter(System.Collections.Generic.HashSet`1<System.Int32>&)' called when server was not active"
		IL_000f: call void [UnityEngine.CoreModule]UnityEngine.Debug::LogWarning(object)
		// dummySet = null;
		IL_0014: ldarg dummySet
		IL_0018: ldloca.s 0
		IL_001a: initobj [netstandard]System.Collections.Generic.HashSet`1
		IL_0020: ldloc 0
		IL_0024: stobj [netstandard]System.Collections.Generic.HashSet`1
		// }
		IL_0029: ret

		// dummySet = new HashSet<int>();
		// sequence point: (line 30, col 13) to (line 30, col 43) in /Users/qwerty/x/dev/project_Mirror/Repository/Assets/Mirror/Examples/Tanks/Scripts/Tank.cs
		IL_002a: ldarg.1
		IL_002b: newobj instance void class [netstandard]System.Collections.Generic.HashSet`1<int32>::.ctor()
		IL_0030: stind.ref
		// (no C# code)
		// sequence point: (line 31, col 9) to (line 31, col 10) in /Users/qwerty/x/dev/project_Mirror/Repository/Assets/Mirror/Examples/Tanks/Scripts/Tank.cs
		IL_0031: ret
	} // end of method Tank::ServerAndOutGenericParameter

[miwarnec]

detailed IL2CPP error:

Building Library/Bee/artifacts/MacStandalonePlayerBuildProgram/4t7v3/tfd0_r.Examples.o failed with output:
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:8785:2: error: unknown type name 'HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735'
        HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735* V_0 = NULL;
        ^
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:8799:41: error: use of undeclared identifier 'HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735'
                il2cpp_codegen_initobj((&V_0), sizeof(HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735*));
                                                      ^
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:8799:93: error: expected expression
                il2cpp_codegen_initobj((&V_0), sizeof(HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735*));
                                                                                                          ^
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:8800:3: error: unknown type name 'HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735'
                HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735* L_2 = V_0;
                ^
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:8801:5: error: use of undeclared identifier 'HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735'
                *(HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735**)L_1 = L_2;
                  ^
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:8801:58: error: expected expression
                *(HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735**)L_1 = L_2;
                                                                       ^
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:8802:38: error: use of undeclared identifier 'HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735'
                Il2CppCodeGenWriteBarrier((void**)(HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735**)L_1, (void*)L_2);
                                                   ^
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:8802:91: error: expected expression
                Il2CppCodeGenWriteBarrier((void**)(HashSet_1_tA1368DD5977A2271895CEEC3D6282C68C9675735**)L_1, (void*)L_2);
                                                                                                        ^
In file included from /Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:1:
In file included from /Applications/Unity/Hub/Editor/2021.3.23f1/Unity.app/Contents/il2cpp/libil2cpp/pch/pch-cpp.hpp:6:
In file included from /Applications/Unity/Hub/Editor/2021.3.23f1/Unity.app/Contents/il2cpp/libil2cpp/codegen/il2cpp-codegen.h:10:
In file included from /Applications/Unity/Hub/Editor/2021.3.23f1/Unity.app/Contents/il2cpp/libil2cpp/codegen/il2cpp-codegen-il2cpp.h:3:
/Applications/Unity/Hub/Editor/2021.3.23f1/Unity.app/Contents/il2cpp/libil2cpp/codegen/il2cpp-codegen-common-small.h:35:20: warning: implicit conversion from 'long long' to 'int' changes value from -9223372036854775808 to 0 [-Wconstant-conversion]
            return INT64_MIN;
            ~~~~~~ ^~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk/usr/include/stdint.h:105:37: note: expanded from macro 'INT64_MIN'
#define INT64_MIN        (-INT64_MAX-1)
                          ~~~~~~~~~~^~
/Users/qwerty/x/dev/project_Mirror/Repository/Library/Bee/artifacts/MacStandalonePlayerBuildProgram/il2cppOutput/cpp/Mirror.Examples.cpp:32284:9: note: in instantiation of function template specialization 'il2cpp_codegen_cast_double_to_int<int>' requested here
                V_0 = il2cpp_codegen_cast_double_to_int<int32_t>(L_1);
                      ^
1 warning and 8 errors generated.
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&) (at /Users/bokken/build/output/unity/unity/Modules/IMGUI/GUIUtility.cs:189)