dotnet/roslyn

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

Open

#82,552 opened on Feb 26, 2026

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

Repository metrics

Stars
 (20,414 stars)
PR merge metrics
 (Avg merge 7d 9h) (270 merged PRs in 30d)

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

Research direction
Investigate the nullable flow analysis for compound assignment operators with nullable value types, specifically the ` =` operator on `byte?`. Trace how the compiler determines nullable annotations for the result of the compound assignment and why it incorrectly flags a mismatch with `int?`.
Tech stack
csharp
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C#
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70