dotnet/roslyn

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

Open

#82552 opened on Feb 26, 2026

View on GitHub
 (1 comment) (0 reactions) (0 assignees)C# (20,414 stars) (4,257 forks)batch import
Area-CompilersBugFeature - Nullable Reference Typeshelp wanted

Description

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?.

Contributor guide

Tech stack
csharp
Domain
developer experiencetooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# nullable reference typesRoslyn compiler basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45
Research direction
The false positive occurs for compound assignment (` =`) with nullable byte. The issue likely lies in the nullable flow analysis in the Roslyn compiler, specifically in the handling of compound assignment operators where the operand type is nullable. Examine the `NullableWalker` or related analysis classes that infer nullability after binary operations. The reproduction code is provided and can be tested locally. Focus on how the compiler determines the nullability of the result after `b = 1` when `b` is `byte?`. A good starting point is the method that handles compound assignments in `NullableWalker.cs`.