CS0508 not triggering in net 5 preview-8, which allows runtime TypeLoadException errors

[ChadJessup]

Sorry if this is the incorrect repo, hard to know where this bug report should go..

Description

In net 5 preview 7 CS0508 - 'Type 1': return type must be 'Type 2' to match overridden member 'Member Name' would prevent compilation. But in preview 8 this builds just fine, and during runtime a TypeLoadException is thrown that is very difficult to troubleshoot.

This top-level program demonstrates the problem:

new Base().CrashyCrashInCallingScope();

class Base
{
    public GenericDerived<string> CrashyCrashInCallingScope() => new GenericDerived<string>();
    public virtual Base NeededToCrash() => this;
}

class GenericDerived<TInputKey> : Base
{
    public override GenericDerived<TInputKey> NeededToCrash() => this;
}

There seems to be some special interaction happening between CrashyCrashInCallingScope() and NeededToCrash().

Compilation and running is fine if NeededToCrash() is called, even though that's where I would expect the compilation error, but if the valid CrashyCrashInCallingScope() is referenced, the whole thing blows up on startup.

Oddly enough, both methods have to exist, and one of them overridden in a derived class to trigger the issue.

Configuration

1. dotnet run

Regression?

Yes - 5.0.100-preview.7.20366.6 would prevent compilation:

Program.cs(11,47): error CS0508: 'GenericDerived<TInputKey>.NeededToCrash()': return type must be 'Base' to match overridden member 'Base.NeededToCrash()' [C:\Projects\TypeLoadTrigger\TypeLoadTrigger.csproj]

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

In 5.0.100-preview.8.20417.9

Unhandled exception. System.TypeLoadException: Could not load type 'GenericDerived`1' from assembly 'TypeLoadTrigger, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
   at <Program>$.<Main>$(String[] args)

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[am11]

Simpler repro without generic type:

class Program
{
    static void Main() => new Base().CrashyCrashInCallingScope();
}
class Base
{
    public Derived CrashyCrashInCallingScope() => null;
    public virtual Base NeededToCrash() => null;
}
class Derived : Base
{
    public override Derived NeededToCrash() => null;
}

Perhaps, it is related to dotnet/roslyn#44207 (covariant return diagnostics).

[huoyaoyuan]

.NET 5 is supposed to support covariant return, so the right behavior is not causing TypeLoadException and run well.

[am11]

Yes. Problem here is with the non-virtual field/property/method returning derived type in base, in the presence of covariant overrides is failing to load the derived type (directly or indirectly). A more stripped repro with direct load:

class Program
{
    static void Main() => new D();
}
class B
{
    public D X = null;
    public virtual B M() => null;
}
class D : B
{
    public override D M() => null;
}

Unhandled exception. System.TypeLoadException: Could not load type 'D' from assembly 'gh-41557, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

cc @janvorli, maybe it is something tracked by #37265?

[am11]

I think this belongs to runtime repo @jeffschwMSFT.

[gafter]

This appears to be a runtime issue. The program looks correct but the runtime blows up.

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mangod9]

@janvorli @davidwrighton assume this is something we need to fix in 5?

[janvorli]

@mangod9 It seems so. I am going to look into it today.