Allow the use of colon in all custom operators

[roboz0r]

I propose we Allow the use of colon : in all custom operators.

Since the inclusion of dotnet/fsharp#15923 the : operator has been permitted only when the first non-ignored operator character is >.

I discovered this while working on updating the spec document resulting in this discussion. fsharp/fslang-spec#68

Pros and Cons

The advantages of making this adjustment to F# are it will simplify the lexing rules to allow : anywhere.

Currently the rules allow >: ?.>: or ><+-*/=~%.&|@^!?: but not <: or +:.

Existing uses of : specialized as keyword operators like : :?> :? :> :: := would be unaffected. A slight modification to this proposal would keep : as the first character of an operator reserved for type-related or other special operations in the compiler since that's what it tends to be used for today.

The disadvantages of making this adjustment to F# are operators containing : won't be able to be used for special purposes in the compiler in the future but there are no active plans to do this dotnet/fsharp#15923 (comment)

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[T-Gro]

Is the A slight modification to this proposal about the first position only, or did you have other restrictions in mind?

i.e. first position : would still be a compiler-reserved usage for any future additions to the language, and anything else would be allowed by user code?

[roboz0r]

first position : would still be a compiler-reserved usage for any future additions to the language, and anything else would be allowed by user code

That's exactly what I had in mind