`Arg.Any<T>()` behaves strangely when it's stored in a variable instead of being passed directly

[urnie]

Describe the bug The effect of Arg.Any<T>() depends on whether it's stored in a variable or passed directly.

To Reproduce

    public interface INumberChecker
    {
        public bool Exists(int num);
    }

    public class MyApp
    {
        private readonly INumberChecker _numberChecker;
        public MyApp(INumberChecker myInterfaceImpl)
        {
            _numberChecker = myInterfaceImpl;
        }
        public void CheckIfNumberExists(int num)
        {
            _numberChecker.Exists(num);
        }
    }

    [Fact]
    public void MinimalReproducibleExample()
    {
        var sub = Substitute.For<INumberChecker>();

        var anyInt = Arg.Any<int>(); //storing this as a variable here..

        sub.Exists(anyInt).Returns(true); //..and using it here

        var app = new MyApp(sub);
        app.CheckIfNumberExists(1);

        sub.DidNotReceive().Exists(anyInt); //..and here
    }

Expected behaviour With the given code, I'd expect the test to fail, since sub does receive a call to its Exists(). However, the test passes.

However, if I pass Arg.Any<int>() directly to the methods it's used in, I get expected behaviour (i.e. an error saying that Exists() was called):

    [Fact]
    public void MinimalReproducibleExample()
    {
        var sub = Substitute.For<INumberChecker>();

        sub.Exists(Arg.Any<int>()).Returns(true); //return value doesn't matter here

        var app = new MyApp(sub);
        app.CheckIfNumberExists(1);

        sub.DidNotReceive().Exists(Arg.Any<int>());
    }

Environment:

• NSubstitute version: [5.1.0]
• NSubstitute.Analyzers version: [CSharp 1.0.16]
• Platform: [dotnet 6.0 on Windows, XUnit]

Additional context

• Didn't find this covered in https://nsubstitute.github.io/help/argument-matchers/#how_not_to_use_argument_matchers
• Analyzer didn't pick up any problems.