Constant pattern vs governing type that is unconstrained type parameter

[gafter]

The following in currently rejected:

    void M<T>(T t)
    {
        switch (t)
        {
            // Cannot implicitly converty type 'int' to 'T'
            case 3: break;
            // Cannot convert null to type parameter 'T' because it could be a non-nullable value type. Consider using 'default(T)' instead.
            case null: break;
        }
        // Cannot implicitly converty type 'int' to 'T'
        if (t is 3) { }
        // Cannot convert null to type parameter 'T' because it could be a non-nullable value type. Consider using 'default(T)' instead.
        if (t is null) { }
    }

However, these are perfectly reasonable pattern-matching operations. The spec and implementation should change to permit this.

[gafter]

This will be addressed in C# 8.0 with the change dotnet/csharplang#1284

[gafter]

Fixed in #25995