From 5b65230d9da69bbc4d2d4db7f6478e57d861fba2 Mon Sep 17 00:00:00 2001
From: Stephan Schroevers <stephan.schroevers@teampicnic.com>
Date: Tue, 2 May 2023 12:51:54 +0200
Subject: [PATCH] Suggestion

---
 .../errorprone/refasterrules/ImmutableSetRules.java | 13 +++++++++++++
 .../refasterrules/ImmutableSetRulesTestInput.java   |  5 +++++
 .../refasterrules/ImmutableSetRulesTestOutput.java  |  4 ++++
 3 files changed, 22 insertions(+)

diff --git a/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ImmutableSetRules.java b/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ImmutableSetRules.java
index a5af1a00e42..628bb1cf6b9 100644
--- a/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ImmutableSetRules.java
+++ b/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ImmutableSetRules.java
@@ -314,4 +314,17 @@ ImmutableSet<T> after(Set<T> set, Multimap<K, V> multimap) {
       return Sets.intersection(set, multimap.keySet()).immutableCopy();
     }
   }
+
+  /** Prefer an immutable copy of {@link Sets#union(Set, Set)} over more contrived alternatives. */
+  static final class SetsUnion<S, T extends S, U extends S> {
+    @BeforeTemplate
+    ImmutableSet<S> before(Set<T> set1, Set<U> set2) {
+      return Stream.concat(set1.stream(), set2.stream()).collect(toImmutableSet());
+    }
+
+    @AfterTemplate
+    ImmutableSet<S> after(Set<T> set1, Set<U> set2) {
+      return Sets.union(set1, set2).immutableCopy();
+    }
+  }
 }
diff --git a/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ImmutableSetRulesTestInput.java b/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ImmutableSetRulesTestInput.java
index 09fdae535bd..53e7c41556d 100644
--- a/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ImmutableSetRulesTestInput.java
+++ b/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ImmutableSetRulesTestInput.java
@@ -123,4 +123,9 @@ ImmutableSet<Integer> testSetsIntersectionMultimap() {
         .filter(ImmutableSetMultimap.of(2, 3)::containsKey)
         .collect(toImmutableSet());
   }
+
+  ImmutableSet<Integer> testSetsUnion() {
+    return Stream.concat(ImmutableSet.of(1).stream(), ImmutableSet.of(2).stream())
+        .collect(toImmutableSet());
+  }
 }
diff --git a/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ImmutableSetRulesTestOutput.java b/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ImmutableSetRulesTestOutput.java
index f60aa49842e..74a6c9d67e0 100644
--- a/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ImmutableSetRulesTestOutput.java
+++ b/error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/ImmutableSetRulesTestOutput.java
@@ -102,4 +102,8 @@ ImmutableSet<Integer> testSetsIntersectionMultimap() {
     return Sets.intersection(ImmutableSet.of(1), ImmutableSetMultimap.of(2, 3).keySet())
         .immutableCopy();
   }
+
+  ImmutableSet<Integer> testSetsUnion() {
+    return Sets.union(ImmutableSet.of(1), ImmutableSet.of(2)).immutableCopy();
+  }
 }