Offer uninstalled compiler as fallback with warning

[mosteo]

This one may be controversial.

Currently, we will not try to satisfy a gnat dependency with an uninstalled compiler (specific dependencies like gnat_arm_elf do not fall into this case, only the abstract gnat ones), instead reporting a missing dependency:

$ alr with gnat^12
Requested changes:

   ✓ gnat ^12 (add)
                                                                         
Changes to dependency solution:

   New solution is incomplete.
   +↪ gnat ^12 (new,external)

Do you want to proceed?
[Y] Yes  [N] No  (default is No) 

This doesn't seem very intuitive, as nothing says that installing the proper compiler version would fix the issue. With this PR an installed compiler is still chosen first for the dependency but, when none fits, this will happen:

$ alr with gnat^12
Requested changes:

   ✓ gnat ^12 (add)
                                                                         
Changes to dependency solution:

   +📦 gnat 12.2.1 (new,gnat_native,binary)
   
   ❗ The solution requires the download of
      a toolchain not currently installed.

Do you want to proceed?
[Y] Yes  [N] No  (default is Yes) 

[Fabien-Chouteau]

Hi @mosteo ,

❗ The solution requires the download of
a toolchain not currently installed.

I am not sure about this error message because it doesn't provide a way forward.
In which situations a user would get this message?

[mosteo]

Hi @mosteo ,

❗ The solution requires the download of
a toolchain not currently installed.

I am not sure about this error message because it doesn't provide a way forward. In which situations a user would get this message?

The way forward is simply accepting the solution, and the dependency will be installed normally as a toolchain (as happens currently for explicit compiler dependencies like gnat_native. The message could be rephrased, of course.

This situation happens when a solution (alr with, alr get) requires necessarily a compiler that isn't already installed.

[Fabien-Chouteau]

The way forward is simply accepting the solution, and the dependency will be installed normally as a toolchain (as happens currently for explicit compiler dependencies like gnat_native.

Ok so right now Alire would just show an incomplete solution, but not download the compiler even if it knows how to.
And with this change Alire will download the compiler?

The message could be rephrased, of course.

To me this message is telling the user that something went wrong and there's nothing Alire can do about it.

So maybe we should remove the exclamation mark and the message could be The solution requires a toolchain that is not installed yet. Alire with now proceed to download and install this toolchain.

[mosteo]

Yeah, that's the gist of it.

[mosteo]

New message:

Changes to dependency solution:

   +📦 gnat 12.2.1 (new,gnat_native,binary)
   
   🛈  The solution requires a toolchain that is
      not yet installed. Accepting the solution
      will download and install this toolchain.