Allow 'ref readonly struct' for 'readonly ref struct'

[mazhou-msft]

Version Used:
Visual Studio 2017 27428.0.d15.7

Steps to Reproduce:

1. Create a C# project for netcoreapp2.1
2. Write a declaration of a ref struct:

public readonly ref struct S { }

Expected Behavior:
The order of 'ref' and 'readonly' should not matter; I can either use 'readonly ref struct' or 'ref readonly struct' to declare readonly ref-like types.

Actual Behavior:
'ref readonly struct' is not allowed. looks like ref struct is a compound keyword like partial class.

[CyrusNajmabadi]

This is not a compiler issue, but a request for a change in the language. as such, this should be filed over at dotnet/csharplang. Thanks!

[alrz]

this is already championed (dotnet/csharplang#946)

[jcouv]

Thanks for reporting this.
Since this is a known issue, I'll go ahead and close as duplicate. @alrz is close to getting this fix (#23533) into C# 7.3.

[gafter]

This was not championed or fixed. What was fixed was the ordering of ref and partial. There is still an issue with the ordering of ref and readonly.

[gafter]

Issue moved to dotnet/csharplang #1536 via ZenHub

[alrz]

@gafter readonly is already permitted with any order. I believe this is exacly what is discussed earlier (allowing ref readonly struct)

[alrz]

that is, if we relax the order of ref the code in the title would compile fine.