dotnet/roslyn

Invalid nullability mismatch warning on generic notnull ref parameter discard

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#82919 opened on Mar 24, 2026

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Area-CompilersFeature - Nullable Reference Typeshelp wanted

Description

Version Used: 5.6.0-2.26161.119

Steps to Reproduce:

#nullable enable

class Gen<T> where T : notnull
{
    public static void Func(ref T t)
    {
        _ = ref t; // Diagnostic CS8619 here.
    }
}

A minimal repro, with source-code provided, is ideal. Most compiler/language issues can be distilled into a snippet that can be pasted into .NET Lab.

Diagnostic Id: CS8619

Expected Behavior: No diagnostic

Actual Behavior: CS8619: Nullability of reference types in value of type 'T' doesn't match target type 'T?'

Contributor guide

Tech stack
csharp
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
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.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# genericsnullable reference types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
The issue involves a false positive CS8619 in the nullable flow analysis when discarding a ref parameter of a generic type constrained to 'notnull'. The fix is likely in the NullableWalker or related flow analysis code. The issue is assigned, so coordinate with the assignee. Look at the test suite for similar nullable mismatch tests.