Pattern matching an enum using an enum value following by using "with expression" on a record causes a compilation error

[NicholasMoss]

Version Used:
Visual Studio 17.12.0 Preview 2.0
DotNet 9.0.100-rc.1.24452.12

Steps to Reproduce:
Create a console app with just this snippet

enum E { A, B }
record R(int E) { }

static class TestClass
{
    public static R TestMethod(E testEnum, R record) =>
        testEnum is E.A ? record with { E = 1 } : record with { E = 1 };
}

Diagnostic Id:

Build started at 11:46...
1>------ Build started: Project: ConsoleApp1, Configuration: Debug Any CPU ------
1>C:\Program Files\dotnet\sdk\9.0.100-rc.1.24452.12\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.RuntimeIdentifierInference.targets(326,5): message NETSDK1057: You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
1>...\Program.cs(9,33,9,37): error CS1073: Unexpected token 'with'
1>...\Program.cs(9,48,9,49): error CS1002: ; expected
1>...\Program.cs(9,48,9,49): error CS1519: Invalid token ':' in class, record, struct, or interface member declaration
1>...\Program.cs(9,66,9,67): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
1>Done building project "ConsoleApp1.csproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
========== Build completed at 11:46 and took 01.248 seconds ==========

Expected Behavior:
Code would compile as it does using stable compiler

Actual Behavior:
Errors as shown above

[NicholasMoss]

I have also found issues when using the linq query syntax

enum E { A, B }
record R(int E) { }

static class TestClass
{
    public static IEnumerable<R> TestMethod(E testEnum, R[] records) =>
        testEnum is E.A
            ? from item in records
              select item
            : records;
}

Which produces the following errors

Build started at 14:07...
1>------ Build started: Project: ConsoleApp1, Configuration: Debug Any CPU ------
1>C:\Program Files\dotnet\sdk\9.0.100-rc.1.24452.12\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.RuntimeIdentifierInference.targets(326,5): message NETSDK1057: You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
1>...\Program.cs(10,20,10,24): error CS1002: ; expected
1>...\Program.cs(10,25,10,27): error CS1519: Invalid token 'in' in class, record, struct, or interface member declaration
1>...\Program.cs(11,22,11,26): error CS1002: ; expected
1>...\Program.cs(12,13,12,14): error CS1519: Invalid token ':' in class, record, struct, or interface member declaration
1>...\Program.cs(12,22,12,23): error CS1519: Invalid token ';' in class, record, struct, or interface member declaration
1>Done building project "ConsoleApp1.csproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
========== Build completed at 14:07 and took 01.687 seconds ==========

[KyouyamaKazusa0805]

See #66841 (comment)

In short, because you defined a property named E, which is conflicted with type declaration E. You should append an extra discard token _ to tell compiler that here E is a type with a discard, and not be a field (i.e. use is E _ instead).

C# compiler will make is Type to be recognized as a type, but C# also introduced a pattern type called "constant pattern", which also uses such syntax is ConstantField, which leads to a confliction.

[NicholasMoss]

This is only a snippet to replicate it in a standalone solution and only applies to the preview compiler. In the actual solution where I hit these issues the names do not overlap.

The E part is unrelated which you can prove by wrapping the enum parts in parenthesis (which the analyzers tell you are unnecessary)

enum E { A, B }
record R(int E) { }

static class TestClass
{
    public static IEnumerable<R> TestMethod(E testEnum, R[] records) =>
        testEnum is (E.A)
            ? from item in records
              select item
            : records;
}

This will build successfully.

Note if you create a console app with that code you will still get the error even if you the property from E to something else.

[KyouyamaKazusa0805]

This compiles fine because you don't have a type also called A.

Please note that brackets like (E.A) will be treated as a normal pattern E.A. Therefore you can remove the bracket - it also compiles.

To make an confliction, just declare a constant and a type, and both named same within a same scope.

[DoctorKrolic]

There is nothing to do with semantics here, this is purely a parser regression introduced in #72805. It doesn't matter what additional types or variables you declare: the expression just isn't parsed correctly and produces a syntax error