Fix misleading error message for non-dune dependency

CHANGES.md

@@ -36,6 +36,10 @@
   commands, as some non-virtual packages might only have install commands.
   (#376 @Leonidas-from-XIV, @gridbugs)
 
+- Fix bug where a misleading error message would be displayed in some cases
+  where a dependency doesn't build with dune (#???,
+  fixes #385, @gridbugs)
+
 ### Removed
 
 ### Security

lib/opam_solve.ml

@@ -299,7 +299,13 @@ let mk_request ~allow_compiler_variants packages =
        released yet but that exists in opam with variants such as
        ocaml-variants.x+trunk *)
     let base_compiler = OpamPackage.Name.of_string "ocaml-base-compiler" in
-    base_compiler :: package_names
+    (* It is important that the compiler package appears after the user's
+       package names. Otherwise in cases where no solution can be found, the
+       solver seems to report errors relating to the ocaml compiler rather than
+       the user-controlled dependencies (see
+       https://github.com/tarides/opam-monorepo/issues/385 for an example of
+       this problem). *)
+    package_names @ [ base_compiler ]
 
 let depend_on_compiler_variants local_packages =
   OpamPackage.Name.Map.exists

test/bin/github385.t/run.t

@@ -1,20 +1,24 @@
-This is a reproduction of a problem where depending on a specific version of the
-ocaml compiler at the same time as a package that doesn't build with dune leads
-to an error message that doesn't mention the fact that the problem is due to the
-non-dune dependency, despite the fact that removing that dependency or adding
-an overlay containing a suitable dune port for the package causes the problem to
-go away.
+This test asserts that the issue at
+https://github.com/tarides/opam-monorepo/issues/385 is fixed.
+
+The problem was that depending on a specific version of the ocaml compiler at
+the same time as a package that doesn't build with dune leads to an error
+message that doesn't mention the fact that the problem is due to the non-dune
+dependency, despite the fact that removing that dependency or adding an overlay
+containing a suitable dune port for the package causes the problem to go away.
 
   $ gen-minimal-repo
 
   $ opam-monorepo lock
   ==> Using 1 locally scanned package as the target.
-  opam-monorepo: [ERROR] Can't find all required versions.
-  Selected: base-bigarray.base base-threads.base base-unix.base ocaml.4.13.1
-            ocaml-base-compiler.4.13.1 ocaml-config.2 ocaml-options-vanilla.1
-            ocaml-base-compiler&foo ocaml-base-compiler ocaml-base-compiler
-            ocaml-base-compiler
-  - foo -> (problem)
-      Rejected candidates:
-        foo.zdev: Requires ocaml = 4.14.1
+  opam-monorepo: [ERROR] Some dependencies cannot be built with dune!
+
+  opam-monorepo requires that all dependencies use dune as their build system.
+
+  These dependencies (possibly transitive) don't use dune as their build system:
+  - without-dune
+  
+  The dune-universe opam repository (git+https://github.com/dune-universe/opam-overlays.git) contains dune ports of some popular packages to help build more packages with dune. It doesn't appear to be set up on this switch. Adding it to this switch may fix this issue. Add the dune-universe opam repository to this switch by running the command:
+
+  opam repository add dune-universe git+https://github.com/dune-universe/opam-overlays.git
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