[Question] Why the output is CS0165 when using a is Constant pattern.

[InCerryGit]

Steps to Reproduce:

https://sharplab.io/#v2:D4AQTAjAsAUCDMACciDCiDetE+UkALIgLIAUAlJtrjQG4CGATogC4CmAzi4gLyIB2bAO6IAKhQDc1GjgCWAM1LsuiWRzGIAxgHt+tNo1GduPPvPoAbDm3LSZNEAHYpMezhAQAnKR16DRrgA6ADFtbXIXGgBfWBi4fDAxLFdcBGQIAAZEUO1MAHM2FglrIriooA==

Given the following C# example:

using System;
public class C {
    public void M() {
        var test = new T();
        if(test is T converTest == false)
            return;
        Console.WriteLine(converTest.Foo);
    }
}
public class T{
    public string Foo {get;set;}
}

The following result is returned:

CS0165: Use of unassigned local variable 'converTest '

if converTest is null, the method return . why the output is CS0165 when using a is Constant pattern?

[RikkiGibson]

This would require a change to the definite assignment specification. If you want to see this happen, feel free to search for or open an issue in the csharplang repo.

[gafter]

Previously (incorrectly) reported at

• Flow analysis failure on "obj is Type == false" #31469
• Incorrect "Use of unassigned local variable" for (x is T y) == false. #20969
• C#7 pattern matching: if (obj is string s == false) does not work as expected. #17828
• Strange behaviour of pattern matching #39721

The fact that people keep asking this question is perhaps evidence that something should be done. But the suggestion to do something belongs in csharplang. As it stands the compiler is obeying the specification.

[RikkiGibson]

A spec change proposal has been submitted for discussion at dotnet/csharplang#4240.