C# 7 §13.6.2 ref readonly in local_variable_initializer

[KalleOlaviNiemitalo]

Describe the bug

In the C# 7 draft, §13.6.2 (Statements / Declaration statements / Local variable declarations) specifies the following restriction for "implicitly typed local variable declaration":

The local_variable_initializer shall be an expression, optionally preceded by ref or ref readonly.

However, ref readonly at the start of a local_variable_initializer is not allowed by the grammar and not allowed by Roslyn either. The grammar allows ref readonly in local_variable_declaration in front of local_variable_type, but not in front of an expression.

Example

The text of the draft gives the impression that ref readonly i is valid in the following. It is not.

public class C {
    static readonly int i;
    
    void M()
    {
        ref readonly var huh = ref readonly i;
    }
}

Expected behavior

Say just "optionally preceded by ref" and don't mention ref readonly.

Additional context

Declaring and using ref readonly locals in the C# 7.2 proposal shows the initialization ref readonly var r1 = ref M1(); without using ref readonly in the initializer.

[KalleOlaviNiemitalo]

If the text allows ref readonly in the initializer but the grammar disallows it, and both are normative, I think that means the standard does not actually allow it. Therefore it would be ok to publish C# 7 with this error even though it is misleading to human readers.