From b415faceca9ea32823ec3701580dc3c1d2530d82 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 12 Jun 2025 20:38:51 +0200
Subject: [PATCH] Don't allow flake inputs to have both a flakeref and a
 follows

Having both doesn't make sense so it's best to disallow it. If this
causes issues we could turn into a warning.
---
 src/libflake/flake.cc                   |  3 +++
 tests/functional/flakes/follow-paths.sh | 13 ++++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/libflake/flake.cc b/src/libflake/flake.cc
index 2b73dbf0b..e59649fec 100644
--- a/src/libflake/flake.cc
+++ b/src/libflake/flake.cc
@@ -170,6 +170,9 @@ static FlakeInput parseFlakeInput(
             input.ref = parseFlakeRef(state.fetchSettings, *url, {}, true, input.isFlake, true);
     }
 
+    if (input.ref && input.follows)
+        throw Error("flake input has both a flake reference and a follows attribute, at %s", state.positions[pos]);
+
     return input;
 }
 
diff --git a/tests/functional/flakes/follow-paths.sh b/tests/functional/flakes/follow-paths.sh
index abc09dfc2..cf27681cb 100755
--- a/tests/functional/flakes/follow-paths.sh
+++ b/tests/functional/flakes/follow-paths.sh
@@ -401,7 +401,6 @@ EOF
 cat <<EOF > $flakeFollowsB/flake.nix
 {
   inputs.C.url = "path:nosuchflake";
-  inputs.D.url = "path:nosuchflake";
   inputs.D.follows = "C/D";
   outputs = _: {};
 }
@@ -419,3 +418,15 @@ EOF
 nix flake lock $flakeFollowsA --recreate-lock-file
 
 [[ $(jq -c .nodes.B.inputs.D $flakeFollowsA/flake.lock) = '["B","C","D"]' ]]
+
+# Check that you can't have both a flakeref and a follows attribute on an input.
+cat <<EOF > $flakeFollowsB/flake.nix
+{
+  inputs.C.url = "path:nosuchflake";
+  inputs.D.url = "path:nosuchflake";
+  inputs.D.follows = "C/D";
+  outputs = _: {};
+}
+EOF
+
+expectStderr 1 nix flake lock $flakeFollowsA --recreate-lock-file | grepQuiet "flake input has both a flake reference and a follows attribute"