From b1a230de7547751e612fcfb5cc06baab4119bfa0 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Sat, 20 Dec 2025 02:09:57 +0100
Subject: [PATCH] libcmd: improve --override-input error message clarity

Make the error message more explicit about what went wrong and why
it's invalid. The new message explains that a zero-length path was
passed and clarifies that it would refer to the flake itself.
---
 src/libcmd/installables.cc        | 3 ++-
 tests/functional/flakes/flakes.sh | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 50f904e91..415d63d87 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -137,7 +137,8 @@ MixFlakeOptions::MixFlakeOptions()
             lockFlags.writeLockFile = false;
             auto path = flake::NonEmptyInputAttrPath::parse(inputAttrPath);
             if (!path)
-                throw UsageError("--override-input: input path cannot be empty");
+                throw UsageError(
+                    "--override-input was passed a zero-length input path, which would refer to the flake itself, not an input");
             lockFlags.inputOverrides.insert_or_assign(
                 std::move(*path), parseFlakeRef(fetchSettings, flakeRef, absPath(getCommandBaseDir()).string(), true));
         }},
diff --git a/tests/functional/flakes/flakes.sh b/tests/functional/flakes/flakes.sh
index 85357bf2a..1d5546d1b 100755
--- a/tests/functional/flakes/flakes.sh
+++ b/tests/functional/flakes/flakes.sh
@@ -396,7 +396,7 @@ nix flake lock "$flake3Dir" --override-input flake2/flake1 flake1/master/"$hash1
 [[ $(jq -r .nodes.flake1_2.locked.rev "$flake3Dir/flake.lock") =~ $hash1 ]]
 
 # Test that --override-input with empty input path is rejected (issue #14816).
-expectStderr 1 nix flake lock "$flake3Dir" --override-input '' . | grepQuiet "input path cannot be empty"
+expectStderr 1 nix flake lock "$flake3Dir" --override-input '' . | grepQuiet -- "--override-input was passed a zero-length input path, which would refer to the flake itself, not an input"
 
 # Test that deprecated --update-input with empty input path is rejected.
 expectStderr 1 nix flake lock "$flake3Dir" --update-input '' | grepQuiet -- "--update-input was passed a zero-length input path, which would refer to the flake itself, not an input"