From b6044b88620a3201ae487aadf20d8c798604b8fb Mon Sep 17 00:00:00 2001
From: Stephan Renatus <stephan.renatus@gmail.com>
Date: Fri, 28 May 2021 13:45:59 +0200
Subject: [PATCH] topdown: move arg count below ir.Empty()

...to avoid a panic at run time.

Fixes #3501.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>
---
 .../functions/test-functions-unused-arg.yaml         | 12 ++++++++++++
 topdown/eval.go                                      |  4 ++--
 2 files changed, 14 insertions(+), 2 deletions(-)
 create mode 100644 test/cases/testdata/functions/test-functions-unused-arg.yaml

diff --git a/test/cases/testdata/functions/test-functions-unused-arg.yaml b/test/cases/testdata/functions/test-functions-unused-arg.yaml
new file mode 100644
index 0000000000..2d28050574
--- /dev/null
+++ b/test/cases/testdata/functions/test-functions-unused-arg.yaml
@@ -0,0 +1,12 @@
+cases:
+- data:
+  modules:
+  - |
+    package p
+    
+    f(x) {
+        r = input.that_is_not_there
+    }
+  note: basic call
+  query: data.p.f(1)
+  want_result: []
diff --git a/topdown/eval.go b/topdown/eval.go
index 3c7bca3f4b..54791c4f6c 100644
--- a/topdown/eval.go
+++ b/topdown/eval.go
@@ -1506,12 +1506,12 @@ func (e evalFunc) eval(iter unifyIterator) error {
 		return err
 	}
 
-	argCount := len(ir.Rules[0].Head.Args)
-
 	if ir.Empty() {
 		return nil
 	}
 
+	argCount := len(ir.Rules[0].Head.Args)
+
 	if len(ir.Else) > 0 && e.e.unknown(e.e.query[e.e.index], e.e.bindings) {
 		// Partial evaluation of ordered rules is not supported currently. Save the
 		// expression and continue. This could be revisited in the future.