Roslyn: Simplifying default expression changes meaning of code

[wilhelmzapiain]

Version Used: Visual Studio Community 2017, C# 7.1

Steps to Reproduce:

Type the following code:

public bool? IsStringCorrect(string expected, string actual) => expected == null ? default(bool?) : expected == actual;

The IDE marks it with the message IDE0034: 'default' expression can be simplified.
When applying the suggested correction the code is this:

public bool? IsStringCorrect(string expected, string actual) => expected == null ? default : expected == actual;

Expected Behavior:

Either the first example should not trigger IDE0034 or the second example (when the suggested correction is applied) should behave the same.

Actual Behavior:

Both examples behave differently. When expected is null the first example returns null and the second example returns false

[jcouv]

FYI @CyrusNajmabadi @Pilchie

[sharwell]

@zaytsev-victor this seems up your alley if you are interested :smile:

[zaytsev-victor]

@sharwell I think that was fixed in PR #20856
I can't reproduce it on VS 2017 15.3.3

[sharwell]

@WilhelmZapiain Can you check which version of Visual Studio 2017 you are using?

[wilhelmzapiain]

Visual Studio Community 2017 15.3.3
Tested on a brand new Desktop Console app, targeting .NET framework 4.6.2 and building on C# latest minor version.

[sharwell]

@jasonmalinowski Do you know when dev15.6 branch became master again? I'm trying to figure out if the assumed fix for this issue went into 15.3 or 15.5.

[jasonmalinowski]

@sharwell If you're trying to figure out which release this went into, better to use git tag --list --contains <sha1> than trying to figure it out via branches.

[dpoeschl]

Using a preview of 15.4 and this has indeed been fixed.

[sharwell]

@WilhelmZapiain so far we've been unable to reproduce this in 15.3.4 and 15.4 Preview. If you are still encountering this, can you post a more complete example, e.g. a complete file which demonstrates this issue?

[wilhelmzapiain]

I am still seeing it and just upgraded Visual Studio.

Using 15.3.5
Steps to reproduce:

1. Create new Console App C#
2. Go to Project Properties/Build/Advanced and change Language version to C# latest minor version (latest)
3. Typing the following code shows up IDE0034

```C#
namespace ConsoleApp2
{
class Program
{
static void Main(string[] args)
{
}

    public static bool? IsStringCorrect(string expected, string actual) => expected == null ? default(bool?) : expected == actual;
}

}
```

[sharwell]

@WilhelmZapiain Thank you, that was sufficient for me to reproduce the issue in both 15.3.4 and 15.4 Preview 2.

[zaytsev-victor]

Can't reproduce on the latest master.

[sharwell]

@zaytsev-victor Awesome 😆

@WilhelmZapiain Let's look at this again after 15.5 Preview 1 comes out. If it's fixed in master today then we can expect it will already be fixed for you starting with that preview.

[sharwell]

For now I'm marking this as blocked on the release of 15.5 Preview 1, unless someone feels like going through code to figure out exactly where it got fixed. 😁

[weitzhandler]

@sharwell
Please read this too: #22484

[jcouv]

Thanks for reporting this issue.
It was fixed by this 15.5 PR: https://github.com/dotnet/roslyn/pull/20856
Closing