Docs: No info about ref extension methods

[taquion87]

There is no info here about the new feature regarding ref extension methods for structs

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Document Details

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• ID: 0f86ebf5-b5c0-324a-43df-c17015729d01
• Version Independent ID: 8ce4ad4a-3dbd-42dc-bae3-230900097299
• Content: What's new in C# 7.2
• Content Source: docs/csharp/whats-new/csharp-7-2.md
• Product: dotnet-csharp
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

According to my tests, this feature is available in C# 7.0 (It's not mentioned in that article either, so scheduling this for October).

[svick]

@BillWagner When I compile the following code with <LangVersion>7.0</LangVersion>:

```c#
struct S {}

static class Extensions
{
static void M(ref this S s) {}
}
```

I get the error:

error CS8107: Feature 'ref extension methods' is not available in C# 7.0. Please use language version 7.2 or greater.

So I don't think it's a C# 7.0 feature.

[BillWagner]

@svick Thanks for checking. I tried the same thing on a preview build and it worked. I'll check again.

Update: I had the prototype as follows (which compiled in 7.0):

static void M(this ref S s) {}

[taquion87]

@BillWagner, it compiles in 7.0 if you change the order of the method parameter modifiers. But if you want to consume the extension method you get a compile error:

error CS8107: Feature 'ref extension methods' is not available in C# 7.0. Please use language version 7.2 or greater.

So, the only way to use the extension method is as a static helper method:

var s = new S();
Extensions.M(ref s);

[svick]

I thought this odd behavior was worth reporting to Roslyn, so I did so: https://github.com/dotnet/roslyn/issues/38486.