False positive CS8619 warning using `-=` with `byte?`

[jeremysalwen]

Version Used: 9.0.205

Steps to Reproduce:

Compile a file a file containing

byte? b = 1;
b -= 1;

.NET Lab link: https://lab.razor.fyi/#41Li4gooyk8vSszVSy4WEkmqLEm1V0hSsFUwtOZKUtAF0V5MycVRnByXPn05NTldgCWBsYIRAA

Diagnostic Id:

Nullability of reference types in value of type 'int?' doesn't match target type 'byte?'.CS8619

Expected Behavior:
No warning, since subtracting 1 from a nullable byte is fine.

Actual Behavior:
CS8619 warning appears.

Note: Even if somehow there should be a warning since -= should never be used with byte? types, there still is a bug here in that the warning message is incorrect, since the nullablility of int? does match the nullability of byte?.

[RikkiGibson]

This is a real bug. This kind of false warning tends to get reported because the particular spot in NullableWalker expected the types to only differ in nullable annotations, but instead there is a real type difference.