[I stumbled on this today while looking at how to spec the addition of scoped to a parameter for a future version.]

The relevant grammar for method parameters in §15.6.2.1 is, as follows:

fixed_parameter
    : attributes? parameter_modifier? type identifier default_argument?
    ;

parameter_modifier
    : parameter_mode_modifier
    | 'this'
    ;

parameter_mode_modifier
    : 'ref'
    | 'out'
    | 'in'
    ;

Rule parameter_modifier allows parameter_mode_modifier or this, but not both. However, two combinations of these are permitted, as follows:

public static class E
{
    public static void F1b(this in int p) { }
    public static void F1d(this ref int p) { }
}

This possibility is supported by the text in §15.6.10 Extension methods, which states:

When the first parameter of a method includes the this modifier, that method is said to be an extension method. Extension methods shall only be declared in non-generic, non-nested static classes. The first parameter of an extension method is restricted, as follows:

• It may only be an input parameter if it has a value type
• It may only be a reference parameter if it has a value type or has a generic type constrained to struct
• It shall not be a pointer type.

I propose the following grammar change:

parameter_modifier
    : parameter_mode_modifier
    | 'this' parameter_mode_modifier?
    ;

Now this new grammar also allows this out, which according to my compiler is not permitted [CS8328], and that suggests we also add to the extension bullet list above, the following constraint:

• It shall not be an output parameter.