Add test for strange behavior from #4786

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Author: rgrinberg

test/blackbox-tests/test-cases/optional-executable.t/run.t

@@ -52,3 +52,38 @@ The following command should fail because the executable is not optional:
                   ^^^^^^^^^^^^^^
   Error: Library "does-not-exist" not found.
   [1]
+
+A strange behavior discovered in #4786. Dune would ignore an executable if any
+of its dependencies were optional.
+
+  $ mkdir optional-binary
+  $ cd optional-binary
+  $ cat >dune-project <<EOF
+  > (lang dune 3.0)
+  > (package (name myfoo))
+  > EOF
+
+  $ mkdir exe
+  $ cat >exe/bar.ml <<EOF
+  > print_endline "hello world"
+  > EOF
+  $ cat >exe/dune <<EOF
+  > (executable (public_name dunetestbar) (name bar) (libraries foo))
+  > EOF
+
+  $ mkdir lib
+  $ cat >lib/dune <<EOF
+  > (library (name foo) (libraries xxx-does-not-exist) (optional) (modules ()))
+  > EOF
+
+  $ cat >dune <<EOF
+  > (rule
+  >  (alias run-x)
+  >  (action (echo %{exe:bar.exe})))
+  > EOF
+
+  $ dune build @run-x
+  Error: No rule found for bar.exe
+  -> required by %{exe:bar.exe} at dune:3
+  -> required by alias run-x in dune:1
+  [1]