topdown: capture prev function result in the "false" case, too (#5286)

Fixes #5272.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

test/cases/testdata/functionerrors/test-conflicts.yaml

@@ -0,0 +1,21 @@
+cases:
+- modules:
+  - |
+    package test
+    o = ["1", "2"]
+    f(x) := o[_] == x
+    p { f("1") }
+  note: 'functionerrors/conflict: plain false and true result, first round'
+  query: data.test.p = x
+  want_error: functions must not produce multiple outputs for same inputs
+  want_error_code: eval_conflict_error
+- modules:
+  - |
+    package test
+    o = ["1", "2"]
+    f(x) := o[_] == x
+    p { f("2") }
+  note: 'functionerrors/conflict: plain false and true result, second round'
+  query: data.test.p = x
+  want_error: functions must not produce multiple outputs for same inputs
+  want_error_code: eval_conflict_error

topdown/eval.go

@@ -1931,6 +1931,7 @@ func (e evalFunc) evalOneRule(iter unifyIterator, rule *ast.Rule, cacheKey ast.R
 					if prev != nil && ast.Compare(prev, result) != 0 {
 						return functionConflictErr(rule.Location)
 					}
+					prev = result
 					return nil
 				}
 			}