What is f# `const` keyword

Question

What is the const keyword in f#. It highlights in visual studio and the error message also calls it a keyword as shown below:

However I could not find it in MSDN https://msdn.microsoft.com/en-us/library/dd233249.aspx or the official language spec http://fsharp.org/specs/language-spec/3.0/FSharpSpec-3.0-final.pdf

Question: any docs on this reserved keyword?

Answer 1

It's right there in the bottom of the first link you pasted:

https://msdn.microsoft.com/en-us/library/dd233249.aspx

The following tokens are reserved as keywords for future expansion of the F# language:

const

Answer 2

[<Literal>]
let something = 2

Values that are intended to be constants can be marked with the Literal attribute. This attribute has the effect of causing a value to be compiled as a constant. see https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/literals