From 6043e3af4ec34a42d1f45620527eb3081e5b564a Mon Sep 17 00:00:00 2001
From: Guillaume Girol <symphorien+git@xlumurb.eu>
Date: Sun, 11 Dec 2022 12:00:00 +0000
Subject: [PATCH] ocamlPackages.findlib: detect conflicts of version early

just like pythonCatchConflictHook
---
 doc/languages-frameworks/ocaml.section.md        |  5 +++++
 pkgs/development/tools/ocaml/findlib/default.nix | 13 +++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/doc/languages-frameworks/ocaml.section.md b/doc/languages-frameworks/ocaml.section.md
index 347b2ea2eca..1e5a5cb4536 100644
--- a/doc/languages-frameworks/ocaml.section.md
+++ b/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.
diff --git a/pkgs/development/tools/ocaml/findlib/default.nix b/pkgs/development/tools/ocaml/findlib/default.nix
index 51df81cea5e..f48ce5abf3f 100644
--- a/pkgs/development/tools/ocaml/findlib/default.nix
+++ b/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 = {