Suggestion: restrict "private" for items in namespaces to mean "private to the namespace declaration group"

[KevinRansom]

Opened at CodePlex by ovatsus

This should give a compile time error, but doesn't:
[File1.fs]

namespace MyApp

module private Module1 =
    let f x = x

[Program.fs]

namespace MyApp

module Module2 =
    open Module1
    let y = f 1

    do
        printfn "%O" y

When evaluating in fsx the error is correctly issued

[latkin]

@dsyme - related to https://visualfsharp.codeplex.com/workitem/29 and/or #27 ?

[dsyme]

For F# 4.0 (and F# 3.x) it seems best to resolve this as by design. In F#, "private" means "private to the enclosing module or type". However, the meaning of "private" for a construct in a namespace needs clarification.

The F# Language Spec for F# 2.x-4.0 will be adjusted to clarify that "private" on a type or module in a namespace declaration group "namespace N = " means "accessible from that namespace declaration group, or any namespace declaration group in the same assembly contributing to either N of a sub-namespace of N." This means that a private construct in namespace N is accessible from all other namespace declaration groups in the same assembly contributing to N.

I understand that this is a weaker form of "private" than simply "private to the encloding namespace declaration group" or "private to the file". However, it is as F# 2.0-3.x has always implemented. Any adjustment to restrict this is surprisingly non-trivial, partly because we must give a warning for existing uses, rather than an error. This needs a lot of care beyond what I'd like to see for F# 4.0 at this stage.

I've added an F# Language User Voice suggestion suggesting we consider restricting this in future versions of F#.

(BTW the only other modification that looks reasonable here for F# 4.0 would be to simply give a warning to recommend that people use "internal" rather than "private" for types and modules in namespaces)

[dsyme]

Tracked by the user voice request