Type pattern in switch expression involving generic type with non-keyword type parameters causes syntax error

[PathogenDavid]

Version Used: Compiler version: '3.8.0-4.20460.4 (64f2392)'. Language version: preview.

Steps to Reproduce:

Compile this code: (SharpLab)

using System;

public class Test
{
    public string M(object o)
        => o switch
        {
            string => "String",
            Action => "Action",
            Action<int> => "Aciton<int>",
            Action<object> => "Action<object>",
            Action<Int16> => "Action<Int16>", // <-- Syntax Errors
            Action<Test> => "Action<Test>", // <-- Syntax Errors
            _ => "Unknown"
        };
}

Expected Behavior: Compiles without errors

Actual Behavior: The lines marked have errors associated with them:

Line 12: error CS1001: Identifier expected
Line 12: error CS1003: Syntax error, '=>' expected
Line 12: error CS1525: Invalid expression term ','
Line 13: error CS1001: Identifier expected
Line 13: error CS1003: Syntax error, '=>' expected
Line 13: error CS1525: Invalid expression term ','

---

This does not happen with the equivalent switch statement: (SharpLab)

using System;

public class Test
{
    public string M(object o)
    {
        switch (o)
        {
            case string:
                return "String";
            case Action:
                return "Action";
            case Action<int>:
                return "Aciton<int>";
            case Action<object>:
                return "Aciton<object>";
            case Action<Int16>:
                return "Aciton<Int16>";
            case Action<Test>:
                return "Aciton<Test>";
            default:
                return "Unknown";
        }
    }
}

[PathogenDavid]

I almost forgot to mention, this does not happen with declaration patterns with/without a discard: (SharpLab)

using System;

public class Test
{
    public string M(object o)
        => o switch
        {
            string => "String",
            Action => "Action",
            Action<int> => "Aciton<int>",
            Action<object> => "Action<object>",
            Action<Int16> _ => "Action<Int16>",
            Action<Test> actionTest => "Action<Test>",
            _ => "Unknown"
        };
}

Although a well-meaning but unhelpful (and somewhat misleadingly labeled) code fix offers to remove the variable declaration:

[gafter]

See https://github.com/dotnet/csharplang/blob/master/proposals/csharp-9.0/patterns3.md#change-to-7542-grammar-ambiguities for the spec change needed to support this.

[PathogenDavid]

Thanks for the fix @gafter, works great!