pkg: test of how the solver chooses compiler packages (ocaml#10595)

Demonstrates several problems where dune chooses unstable compiler
packages by default due to the fact that it ignores the avoid-version
flag.

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

Author: gridbugs

test/blackbox-tests/test-cases/pkg/solve-compiler-dependency.t

@@ -0,0 +1,70 @@
+Repro for https://github.com/ocaml/dune/issues/10592. Creates some packages
+that simulate some of the ocaml compiler packages and test solving a project
+that depends on "ocaml".
+
+  $ . ./helpers.sh
+  $ mkrepo
+
+  $ CURRENT=5.2.0
+  $ NEXT=5.3.0
+  $ NEXT_NEXT=5.3.1
+
+The vanilla compiler package, intended to be the default compiler package.
+  $ mkpkg ocaml-base-compiler $CURRENT
+
+A configurable compiler. It's marked as avoid-version with the intention that
+packages explicitly opt into using it.
+  $ mkpkg ocaml-variants $CURRENT+trunk << EOF
+  > flags: [avoid-version]
+  > EOF
+
+A meta package which depends on a disjunction of different compiler
+implementations.
+  $ mkpkg ocaml $CURRENT << EOF
+  > depends: [
+  >   "ocaml-base-compiler" {>= "$CURRENT~" & < "$NEXT~" } |
+  >   "ocaml-variants" {>= "$CURRENT~" & < "$NEXT~" }
+  > ]
+  > EOF
+
+When the latest version of all packages is the same, the ocaml-base-compiler
+package is chosen, which is what we want.
+  $ solve ocaml
+  Solution for dune.lock:
+  - ocaml.5.2.0
+  - ocaml-base-compiler.5.2.0
+
+Now pretend that there was an alpha release of the current version of the
+compiler also in the repo.
+  $ mkpkg ocaml-base-compiler $CURRENT+alpha1 << EOF
+  > flags: [avoid-version]
+  > EOF
+
+The alpha version of the compiler was chosen instead of the stable version.
+This would ideally be avoided due to the avoid-version flag, but this flag is
+ignored by dune.
+  $ solve ocaml
+  Solution for dune.lock:
+  - ocaml.5.2.0
+  - ocaml-base-compiler.5.2.0+alpha1
+
+Now release a new version of ocaml-variants and a new version of ocaml that
+uses it. The dependency specification for ocaml is based on how the package is
+organized in the wild.
+  $ mkpkg ocaml-variants $NEXT+trunk << EOF
+  > flags: [avoid-version]
+  > EOF
+  $ mkpkg ocaml $NEXT << EOF
+  > depends: [
+  >   "ocaml-base-compiler" {= "$NEXT" } |
+  >   "ocaml-variants" {>= "$NEXT~" & < "$NEXT_NEXT~" }
+  > ]
+  > EOF
+
+Here ocaml-variants is chosen despite its avoid-version flag. This is because
+dune currently ignores this flag. This is a problem because the chosen compiler
+is not officially released and possibly unstable.
+  $ solve ocaml
+  Solution for dune.lock:
+  - ocaml.5.3.0
+  - ocaml-variants.5.3.0+trunk