Avoid concatenating superfluous empty list for aggregate semantics

conformance/BUILD.bazel

@@ -24,7 +24,10 @@ _TEST_DIRS = [
     "aggregate",
     "aggregate_explicit_list_output",
     "aggregate_explicit_optional_none",
+    "aggregate_nested_all_rules_fall_through_to_empty",
     "aggregate_nested_explicit_list_double_wrapping",
+    "aggregate_nested_multi_branch_match",
+    "aggregate_nested_single_conditional_match",
     "aggregate_shadowed_variables",
     "context_pb",
     "first_match_nested_aggregate",

conformance/testdata/aggregate_nested_all_rules_fall_through_to_empty/config.yaml

@@ -0,0 +1,18 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: "aggregate_nested_all_rules_fall_through_to_empty"
+variables:
+  - name: "x"
+    type_name: "int"

conformance/testdata/aggregate_nested_all_rules_fall_through_to_empty/policy.yaml

@@ -0,0 +1,27 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: aggregate_nested_all_rules_fall_through_to_empty
+rule:
+  aggregate:
+    - rule:
+        match:
+          - condition: "x == 1"
+            output: "'ONE'"
+          - condition: "x == 2"
+            output: "'TWO'"
+    - rule:
+        match:
+          - condition: "x == 3"
+            output: "'THREE'"

conformance/testdata/aggregate_nested_all_rules_fall_through_to_empty/tests.yaml

@@ -0,0 +1,42 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+description: Conformance test for multiple nested match sub-rules under aggregate falling through to empty list
+section:
+  - name: "basic"
+    tests:
+      - name: "first_rule_match_one"
+        input:
+          x:
+            value: 1
+        output:
+          value: ["ONE"]
+      - name: "first_rule_match_two"
+        input:
+          x:
+            value: 2
+        output:
+          value: ["TWO"]
+      - name: "second_rule_match_three"
+        input:
+          x:
+            value: 3
+        output:
+          value: ["THREE"]
+      - name: "all_rules_fall_through_to_empty"
+        input:
+          x:
+            value: 99
+        output:
+          value: []

conformance/testdata/aggregate_nested_multi_branch_match/config.yaml

@@ -0,0 +1,20 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: "aggregate_nested_multi_branch_match"
+variables:
+  - name: "cond"
+    type_name: "bool"
+  - name: "x"
+    type_name: "int"

conformance/testdata/aggregate_nested_multi_branch_match/policy.yaml

@@ -0,0 +1,26 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: aggregate_nested_multi_branch_match
+rule:
+  aggregate:
+    - condition: "cond"
+      rule:
+        match:
+          - condition: "x > 10"
+            output: "'HIGH'"
+          - condition: "x > 0"
+            output: "'LOW'"
+    - condition: "true"
+      output: "'DEFAULT'"

conformance/testdata/aggregate_nested_multi_branch_match/tests.yaml

@@ -0,0 +1,50 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+description: Conformance test for multi-branch nested match rule with optional fallback under aggregate
+section:
+  - name: "basic"
+    tests:
+      - name: "first_branch_match"
+        input:
+          cond:
+            value: true
+          x:
+            value: 15
+        output:
+          value: ["HIGH", "DEFAULT"]
+      - name: "second_branch_match"
+        input:
+          cond:
+            value: true
+          x:
+            value: 5
+        output:
+          value: ["LOW", "DEFAULT"]
+      - name: "nested_fallback_to_empty"
+        input:
+          cond:
+            value: true
+          x:
+            value: -1
+        output:
+          value: ["DEFAULT"]
+      - name: "outer_condition_false"
+        input:
+          cond:
+            value: false
+          x:
+            value: 15
+        output:
+          value: ["DEFAULT"]

conformance/testdata/aggregate_nested_single_conditional_match/config.yaml

@@ -0,0 +1,18 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: "aggregate_nested_single_conditional_match"
+variables:
+  - name: "x"
+    type_name: "int"

conformance/testdata/aggregate_nested_single_conditional_match/policy.yaml

@@ -0,0 +1,22 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+name: aggregate_nested_single_conditional_match
+rule:
+  aggregate:
+    - condition: "true"
+      rule:
+        match:
+          - condition: "x > 10"
+            output: "'GT10'"

conformance/testdata/aggregate_nested_single_conditional_match/tests.yaml

@@ -0,0 +1,30 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+description: Conformance test for single conditional match nested inside aggregate
+section:
+  - name: "basic"
+    tests:
+      - name: "condition_true"
+        input:
+          x:
+            value: 15
+        output:
+          value: ["GT10"]
+      - name: "condition_false"
+        input:
+          x:
+            value: 5
+        output:
+          value: []