dotnet/roslyn

class? constraint combines poorly with non-nullable interface constraint

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#49,571 opened on Nov 23, 2020

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 (8 comments) (0 reactions) (1 assignee)C# (4,257 forks)batch import
Area-CompilersBugFeature - Nullable Reference Typeshelp wanted

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Description

Version Used: dc6f316a5e4f8eb3a0d36ab356c5c12686db00a9

Steps to Reproduce:

#nullable enable
using System;
public class C {
    public void M<T>(T t) where T : class?, IEquatable<T> {
        t.ToString(); // no warning?
    }
}

Expected Behavior: Variable of type T has a maybe-null initial state

Actual Behavior: Variable has not-null initial state

Remarks: This may be fine and by-design, however the combination of these particular constraints seems to produce a contradictory/unintuitive result, so maybe there should be a warning to change class? to class, or to change IEquatable<T> to IEquatable<T>?.

We should report a warning under the following condition:

If the collection of interface, base class and the class constraints on a type parameter contain a single one which is "non-nullable" then we should warn if any of the others are "nullable".

Contributor guide

Research direction
Examine the nullable analysis of type parameter constraints and implement a warning when nullable and non nullable constraints conflict.
Tech stack
csharp
Domain
compilers
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C#Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70