ocamlPackages.findlib: detect conflicts of version early

just like pythonCatchConflictHook

doc/languages-frameworks/ocaml.section.md

@@ -129,3 +129,8 @@ packaged libraries may still use the old spelling: maintainers are invited to
 fix this when updating packages. Massive renaming is strongly discouraged as it
 would be challenging to review, difficult to test, and will cause unnecessary
 rebuild.
+
+The build will automatically fail if two distinct versions of the same library
+are added to `buildInputs` (which usually happens transitively because of
+`propagatedBuildInputs`). Set `dontDetectOcamlConflicts` to true to disable this
+behavior.

pkgs/development/tools/ocaml/findlib/default.nix

@@ -44,6 +44,15 @@ stdenv.mkDerivation rec {
           mkdir -p $OCAMLFIND_DESTDIR
         fi
     }
+    detectOcamlConflicts () {
+      local conflict
+      conflict="$(ocamlfind list |& grep "has multiple definitions" || true)"
+      if [[ -n "$conflict" ]]; then
+        echo "Conflicting ocaml packages detected";
+        echo "$conflict"
+        exit 1
+      fi
+    }
 
     # run for every buildInput
     addEnvHooks "$targetOffset" addOCamlPath
@@ -51,6 +60,10 @@ stdenv.mkDerivation rec {
     preInstallHooks+=(createOcamlDestDir)
     # run even in nix-shell, and even without buildInputs
     addEnvHooks "$hostOffset" exportOcamlDestDir
+    # runs after all calls to addOCamlPath
+    if [[ -z "''${dontDetectOcamlConflicts-}" ]]; then
+      postHooks+=("detectOcamlConflicts")
+    fi
   '';
 
   meta = {