Invalid backing object for unused nullable static value types

[isadorasophia]

It seems that the CLR is not correctly handling initialization of nullable static variables. The repro steps below should make the problem clearer.

Repro steps:

1. Start debugging the following app, either on x86 or x64, for .NET version 3.1.401 or 5.0.100-preview.8.20362.3:

class NullableTest
{
    static int? i;

    public static void Main()
    {
        System.Diagnostics.Debugger.Break();

        if (i.HasValue)
        {
            System.Console.WriteLine("Value is {0}!", i.Value);
        }
    }
}

2. Stop at breakpoint;
3. Evaluate i at the Watch Window. The following error message will show up: "The debugger is unable to evaluate this expression."
4. Stop at breakpoint and evaluate i = 2 at the Immediate Window;
5. Evaluate i again, it will now have a value of 2;
6. Step over twice;
7. The condition will be skipped and i value is now null.

Regression?

This is a regression when upgrading from 3.1.302 -> 3.1.401. It has also been observed on 5.0.100-preview.3.20216.6 -> 5.0.100-preview.8.20362.3.

Other information

• The CLR is directly returning a null slot for the variable until that value is actually used, which results in the error message shown at step 3. I verified that ICorDebugReferenceValue::GetValue returns 0 at that point.
• After line 9 is executed, ICorDebugReferenceValue::GetValue returns a valid address, which also overwrites any data that was there before (from step 4).

[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.

Tagging subscribers to this area: @tommcdon
See info in area-owners.md if you want to be subscribed.

[tommcdon]

@hoyosjs

[yyjdelete]

#39993
And seems this also happened for all structs except primitive types(int, short, byte, ...) .