Module and partially qualified namespace type shadowing interaction

[NinoFloris]

We shadow quite some things in FSharp.Core, either to poison them (compiler message to point to an alternative) or replace them with our own implementations. This is not really uncommon and a good open source example is the Prelude in Dark https://github.com/darklang/dark/blob/main/fsharp-backend/src/Prelude/Prelude.fs

In our case, probably less common, we're also aliasing Option to ValueOption, and precisely around shadowed types and aliases is where things break down a bit.

Reading the following example I would expect only the inner module 'Core' to be opened. Instead F# also opens the namespace Core (from Microsoft.FSharp.Core) and loads its types and aliases again :/

namespace Repro

module Common = 
    type Option<'T> = ValueOption<'T>

module SomeModule = 

    open Common

    module Core = 
        let maybeTest (x: Option<string>) = ()

    open Core
    // Have to re-open Common which Rider doesn't even see as a used import.
    //open Common

    // This expression was expected to have type 'Repro.Common.Option<string>' but here has type 'Microsoft.FSharp.Core.Option<string>'
    let maybeSomething (x: Option<string>) = maybeTest x

https://sharplab.io/#v2:DYLgZgzgNAJiDUAfAdgQwLYFMIAdUGNMACAJUxwCcB7AWACh70qYBXYYgYSvSeSIF4i9IiKIAXAJ45iAeRxiAllWQAeAOQAVAHwCiANVTAWmOYuXrt9RszbEAyt0wBZG+11W6oolWl8uPZQ8vJlY3LgpiQWEvUXYxInRUCQAjTA1seIAKAA8QIlMlVQgxCgVkAHMtAEpdTKqg0R9MPyoI6JEAeg6iAAlUADdiMSoiCIBaJpaAvgB3AAsFfDmiTBgFYYoIIhhlNXjMQb4ITGJULdQiFmOYIgV0HFaxADp2oi7Jon9eBs7ujQWtphspRsBBCkQZmcVsDMPgxKtxCM5gMhlJiGoyJQqE8vsongVzMVShUtGoiMkWPE5pgIkRkVtJNIiGonItqBAqGBngAxOzIig4HGtTD4+SFFREsqVNSvOIJJKpBxYMQLCpEHJ5AlFEpS6q6RIpNIZIjZIA===

Interestingly, this behavior only seems to be triggered without error if there is a shadowing module in scope:

namespace Sample

module SomeModule = 

    // error FS0893: This declaration opens the namespace or module 'Microsoft.FSharp.Core'
    // through a partially qualified path. Adjust this code to use the full path of the namespace.
    // This change will make your code more robust as new constructs are added to the F# and CLI libraries.
    open Core

    module Core  = 
        let test() = ()

    // Allowed because new module Core is in scope but additionally opens Microsoft.FSharp.Core through a partially qualified path without error.
    open Core

Given that partially qualified opening of namespaces:

• Has produced an error since F# 1.0.
• Can only be triggered via an edge case.
• The implications of an open like this also doesn't flow through FCS or other tooling properly, given this re-import is seen as unused in Rider (can somebody try the repro in VS?).

Could we remove the behavior of re-opening namespaces via a partially qualified path entirely @dsyme?

[dsyme]

Interesting problem, thanks. Yes we should at least emit a warning when this happens, and consider how we can allow explicit qualification so FSharp.Core doesn't come into scope.