Warn when creating switched without a default repository

[reynir]

Today I was trying to create a switch, and it failed with to me confusing error messages:

$ opam switch create . --deps-only -t

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml" {>= "4.05.0"}]
[ERROR] Could not determine which packages to install for this switch:
  * Missing dependency:
    - (invariant) -> ocaml
    unknown package

Switch initialisation failed: clean up? ('n' will leave the switch partially installed) [y/n] y
$ opam switch create . 4.14.2 --deps-only -t
[ERROR] No compiler matching `4.14.2' found, use `opam switch list-available' to see what is
        available, or use `--packages' to select packages explicitly.
$ opam switch list-available
# Listing available compilers from repositories: 
# No matches found

In the end it turned out that I had removed the default repository and added my own repository (using opam-mirror). I had used --all but not --set-default when adding the local repository. This then results in new switches to defaulting to no repositories and it makes sense it can't find ocaml when there are no package repositories. Only the last command really gave a hint of the cause, but it is somewhat ambiguous because it's not clear that it's an empty list and not just "here are the compilers I found:" colon.

I think it is very rare that it is desirable to create a switch without repositories - at least when the switch variant isn't [] - so I think it could improve the user experience to tell me that I probably made a mistake.

Thank you!

[rjbou]

Discussed in dev meeting: There is two points to address here:

1. opam permitting to have no repository registered, and create a switch with no repository
   -> We want to continue supporting that, it is useful on tests case (environment setup)
   => one solution would be to advertise when the last repository is removed, in case it wasn't user intention
2. not having a default repository registered
   => When creating a switch with no default repository, we can display a note to highlight that.

reftest for these cases

``` ### opam-version: "2.0" ### opam-version: "2.0" ### opam-version: "2.0" ### opam-version: "2.0" ### opam-version: "2.0" ### opam repo --all # Repository # Url # Switches(rank) default file://${BASEDIR}/REPO ### :-- ### opam repo add snd ./REPOO --all [snd] Initialised ### opam repo --all # Repository # Url # Switches(rank) default file://${BASEDIR}/REPO snd file://${BASEDIR}/REPOO ### :-- ### opam repo remove default --all ### opam repo --all # Repository # Url # Switches(rank) snd file://${BASEDIR}/REPOO ### :-- ### opam repo remove snd --all ### opam repo --all # Repository # Url # Switches(rank) ### :-- ### opam repo --all # Repository # Url # Switches(rank) ### opam switch create no-repo [ERROR] No compiler matching `no-repo' found, use `opam switch list-available' to see what is available, or use `--packages' to select packages explicitly. # Return code 5 # ### opam repo --all # Repository # Url # Switches(rank) ### :-- ### opam repo add snd ./REPOO --all [snd] Initialised ### opam repo --all # Repository # Url # Switches(rank) snd file://${BASEDIR}/REPOO ### opam switch create no-default [ERROR] No compiler matching `no-default' found, use `opam switch list-available' to see what is available, or use `--packages' to select packages explicitly. # Return code 5 # ### opam repo --all # Repository # Url # Switches(rank) snd file://${BASEDIR}/REPOO ### :-- ### opam repo --all # Repository # Url # Switches(rank) snd file://${BASEDIR}/REPOO ### opam switch create no-default --packages foo

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["foo"]
[ERROR] Could not determine which packages to install for this switch:

• Missing dependency:
  • (invariant) -> foo
    unknown package

Switch initialisation failed: clean up? ('n' will leave the switch partially installed) [Y/n] y

Return code 20

opam repo --all

Repository # Url # Switches(rank)

snd file://${BASEDIR}/REPOO

:--

opam repo add default ./REPO --all --set-default

[default] Initialised

opam repo --all

Repository # Url # Switches(rank)

default file://${BASEDIR}/REPO
snd file://${BASEDIR}/REPOO

opam switch create no-compilers

[ERROR] No compiler matching no-compilers' found, use opam switch list-available' to see what is available, or use `--packages' to select packages explicitly.

Return code 5

opam repo --all

Repository # Url # Switches(rank)

default file://${BASEDIR}/REPO
snd file://${BASEDIR}/REPOO

:--

pkg:foo.1

opam-version: "2.0"
flags: compiler

opam repo --all

Repository # Url # Switches(rank)

default file://${BASEDIR}/REPO
snd file://${BASEDIR}/REPOO

opam switch create compiler

[ERROR] No compiler matching compiler' found, use opam switch list-available' to see what is available, or use `--packages' to select packages explicitly.

Return code 5

opam repo --all

Repository # Url # Switches(rank)

default file://${BASEDIR}/REPO
snd file://${BASEDIR}/REPOO

:--

opam repo --all

Repository # Url # Switches(rank)

default file://${BASEDIR}/REPO
snd file://${BASEDIR}/REPOO

opam switch create foo

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["foo"]

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
-> installed foo.1
Done.

opam repo --all

Repository # Url # Switches(rank)

default file://${BASEDIR}/REPO foo
snd file://${BASEDIR}/REPOO

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