fix(select-field): eval closure when resolving select

[anmonteiro]

fixes #13299

[rgrinberg]

I'll review this one

[anmonteiro]

I'll review this one

Thank you! Summary for the fix:

• it's not enough to resolve a library name to know whether (select ..) should choose its branch; the library and its transitive closure must also resolve correctly, since that's how the library will be compiled.

[rgrinberg]

LGTM. I did some minor cosmetic tweaks.

With this PR, it is now possible to pick a different select clause depending on the compilation mode? If that's right, we should add a test for this.

Changes entry is in order here as well.

[anmonteiro]

With this PR, it is now possible to pick a different select clause depending on the compilation mode?

Theoretically, yes, but that's not yet wired up to anywhere. It's something I'm tracking as part of the single context universal (#10630) project.

[Alizter]

One thing I still don't understand with the reproduction case is why we need virtual libraries here? I was not able to remove them, but perhaps you could explain why?

[Alizter]

This appears to be a regression (well kindof, it was also broken before):

  $ cat > dune-project <<EOF
  > (lang dune 3.22)
  > EOF

  $ mkdir -p mylib_opt
  $ cat > mylib_opt/dune <<EOF
  > (library
  >  (name mylib_opt)
  >  (libraries external_lib_not_found))
  > EOF

  $ cat > mylib_opt/mylib_opt.ml <<EOF
  > let greeting = "Hello from mylib_opt"
  > EOF

  $ mkdir -p mylib_base
  $ cat > mylib_base/dune <<EOF
  > (library
  >  (name mylib_base)
  >  (libraries
  >   (select mylib_base.ml from
  >    (mylib_opt -> mylib_base.with.ml)
  >    (-> mylib_base.without.ml))))
  > EOF

  $ cat > mylib_base/mylib_base.with.ml <<EOF
  > let msg = Mylib_opt.greeting
  > EOF

  $ cat > mylib_base/mylib_base.without.ml <<EOF
  > let msg = "No mylib_opt"
  > EOF

  $ cat > dune <<EOF
  > (executable
  >  (name main)
  >  (libraries mylib_base))
  > EOF

  $ cat > main.ml <<EOF
  > let () = print_endline Mylib_base.msg
  > EOF

  $ dune build main.exe
  File "mylib_base/mylib_base.with.ml", line 1, characters 10-19:
  Error: Unbound module Mylib_opt
  [1]

On main I get:

  $ dune build main.exe
  File "mylib_opt/dune", line 3, characters 12-34:
  3 |  (libraries external_lib_not_found))
                  ^^^^^^^^^^^^^^^^^^^^^^
  Error: Library "external_lib_not_found" not found.
  -> required by library "mylib_opt" in _build/default/mylib_opt
  -> required by executable main in dune:2
  -> required by _build/default/.main.eobjs/native/dune__exe__Main.cmx
  -> required by _build/default/main.exe
  [1]

The default fallback clause should ideally be triggered here.

[anmonteiro]

This appears to be a regression (well kindof, it was also broken before):

Thank you, this is indeed a regression. I've added your test case in #13404 and have fixed it in this branch, too.

There's a follow-up fix needed for forbidden libraries, which I'll push up in a sec.