Roslyn: C# 9 Records: Explicit property declaration breaks primary constructor

[albahari]

Version Used: 3.8.0-3.20458.6

Steps to Reproduce:

Run the following code:

Console.WriteLine (new Test(1).X == 1);

record Test (int X)
{
    int _x;
    public int X { get => _x; init => _x = value; }
}

Expected Behavior:

This should print "True"

Actual Behavior:

It prints "False"

The auto-generated constructor fails to set the X property.

[alrz]

When you redefine the property you're responsible for the assignment as well.

record Test (int X)
{
    int _x = X; // <--
    public int X { get => _x; init => _x = value; }
}

[canton7]

Can I suggest that Roslyn reports a warning when a record's positional parameter is unused? That would catch cases where the user assumes that a positional parameter's value will be used to initialise some member, but this isn't actually the case.

[alrz]

Can I suggest that Roslyn reports a warning when a record's positional parameter is unused?

That's similar to https://github.com/dotnet/roslyn/issues/47677, both a good candidate for warning waves.

[canton7]

Why would this be a candidate for a warning wave? Records haven't been released yet, so there's surely no backwards compat concern with adding a warning to records now?

[Youssef1313]

I personally agree with @canton7. This should be something to address as a regular warning before the final release.

[jaredpar]

A warning on unused ctor arguments seems reasonable.

[jcouv]

Discussion from closed PR https://github.com/dotnet/roslyn/pull/47675 may be useful for reference.