dotnet/roslyn

Avoid checking HasValue in nullable comparison to non-default constants

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#52,629 opened on Apr 14, 2021

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Description

@canton7 mentioned the following in https://github.com/dotnet/roslyn/issues/44109#issuecomment-664292439

It occurs to me that, provided one side of the == is a constant which has a non-default value, we can skip the & x.HasValue check altogether.

It feels like it would be a nice little win if, when comparing a System.Nullable<T> to a constant with a non-default value, we could avoid generating code to check HasValue. For example: SharpLab

void M(int? x)
{
    if (x == 42)
    {
        System.Console.Write(1);
    }
}

Could be lowered to:

void M(int? x)
{
    if (x.GetValueOrDefault() == 42)
    {
        System.Console.Write(1);
    }
}

Currently we generate:

void M(Nullable<int> x)
{
    Nullable<int> num = x;
    int num2 = 42;
    if ((num.GetValueOrDefault() == num2) & num.HasValue)
    {
        Console.Write(1);
    }
}

Contributor guide

Research direction
Investigate the Roslyn compiler's lowering logic for nullable comparisons, specifically the code generation for `Nullable<T>.Equals` or `==` operator. Identify where the `HasValue` check is emitted and modify it to skip the check when comparing to a constant with a non default value (e.g., 42 for int?). The goal is to replace `x.HasValue && x.Value == constant` with just `x.Value == constant` when constant is non default, because if the value equals a non default constant, HasValue must be true.
Tech stack
csharp
Domain
backend
Issue type
Feature
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#Nullable value types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50