Support operators in type extensions, and also operators on internal types [RFC FS-1043]

**Submitted by Gustavo Guerra on 3/28/2014 12:00:00 AM**
**80 votes on UserVoice prior to migration**

This is currently not allowed:

``` fsharp
type Foo with
static member (+) (foo1, foo2) = foo1.Bar + foo2.Bar
```

**[Original UserVoice Submission](https://fslang.uservoice.com/forums/245727-f-language/suggestions/5695277)**
[Archived Uservoice Comments](../tree/master/archive/suggestion-5695277-allow-the-addition-of-operators-in-type-extension.md#comments)

Start with the type-extension example in the issue and read archive/suggestion-5695277-allow-the-addition-of-operators-in-type-extension.md#comments. Define the language and internal-type cases that need support; done means the requested operator forms are specified and their acceptance is covered by appropriate compiler tests.

Written by the indexing model from the issue text.