From 9be95f24ddcd15d1cfd12fa5cb0da9fec0a0dd5c Mon Sep 17 00:00:00 2001
From: kevin9327 <5299031+kevin9327@users.noreply.github.com>
Date: Mon, 5 Oct 2026 10:11:59 +0900
Subject: [PATCH 1/2] [TypeDeclaration] Skip redundant intersection types in
 union return types

---
 .../redundant_intersection.php.inc            | 49 ++++++++++++++
 .../TypeMapper/UnionTypeMapper.php            | 67 +++++++++++++++++++
 2 files changed, 116 insertions(+)
 create mode 100644 rules-tests/TypeDeclaration/Rector/ClassMethod/ReturnUnionTypeRector/FixtureUnionIntersection/redundant_intersection.php.inc

diff --git a/rules-tests/TypeDeclaration/Rector/ClassMethod/ReturnUnionTypeRector/FixtureUnionIntersection/redundant_intersection.php.inc b/rules-tests/TypeDeclaration/Rector/ClassMethod/ReturnUnionTypeRector/FixtureUnionIntersection/redundant_intersection.php.inc
new file mode 100644
index 00000000000..0154fd51214
--- /dev/null
+++ b/rules-tests/TypeDeclaration/Rector/ClassMethod/ReturnUnionTypeRector/FixtureUnionIntersection/redundant_intersection.php.inc
@@ -0,0 +1,49 @@
+<?php
+
+namespace Rector\Tests\TypeDeclaration\Rector\ClassMethod\ReturnUnionTypeRector\FixtureUnionIntersection;
+
+/**
+ * "\Iterator|(\Iterator&\Countable)" is a compile error in PHP: the intersection is redundant
+ */
+final class RedundantIntersection
+{
+    public function run(\Iterator $obj)
+    {
+        if (! $obj instanceof \Countable) {
+            return $obj;
+        }
+
+        if (rand(0, 1)) {
+            return $obj;
+        }
+
+        return '10';
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\Tests\TypeDeclaration\Rector\ClassMethod\ReturnUnionTypeRector\FixtureUnionIntersection;
+
+/**
+ * "\Iterator|(\Iterator&\Countable)" is a compile error in PHP: the intersection is redundant
+ */
+final class RedundantIntersection
+{
+    public function run(\Iterator $obj): \Iterator|string
+    {
+        if (! $obj instanceof \Countable) {
+            return $obj;
+        }
+
+        if (rand(0, 1)) {
+            return $obj;
+        }
+
+        return '10';
+    }
+}
+
+?>
diff --git a/src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php b/src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php
index a610e4a441b..89d9d0c752b 100644
--- a/src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php
+++ b/src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php
@@ -225,6 +225,7 @@ private function matchPhpParserUnionType(UnionType $unionType, string $typeKind)
 
         /** @var Identifier[]|Name[] $phpParserUnionedTypes */
         $phpParserUnionedTypes = array_unique($phpParserUnionedTypes, SORT_REGULAR);
+        $phpParserUnionedTypes = $this->removeRedundantIntersectionTypes($phpParserUnionedTypes);
 
         $countPhpParserUnionedTypes = count($phpParserUnionedTypes);
         if ($countPhpParserUnionedTypes === 1) {
@@ -234,6 +235,72 @@ private function matchPhpParserUnionType(UnionType $unionType, string $typeKind)
         return $this->resolveTypeWithNullablePHPParserUnionType(new PhpParserUnionType($phpParserUnionedTypes));
     }
 
+    /**
+     * PHP rejects e.g. "A|(A&B)" with "Type A&B is redundant as it is more restrictive than type A",
+     * so drop intersections that contain all parts of another member of the union
+     *
+     * @param array<Identifier|Name|PHPParserNodeIntersectionType> $phpParserUnionedTypes
+     * @return list<Identifier|Name|PHPParserNodeIntersectionType>
+     */
+    private function removeRedundantIntersectionTypes(array $phpParserUnionedTypes): array
+    {
+        $phpParserUnionedTypes = array_values($phpParserUnionedTypes);
+
+        $typeNames = [];
+        foreach ($phpParserUnionedTypes as $key => $phpParserUnionedType) {
+            $typeNames[$key] = $this->resolveIntersectionPartNames($phpParserUnionedType);
+        }
+
+        foreach ($phpParserUnionedTypes as $key => $phpParserUnionedType) {
+            if (! $phpParserUnionedType instanceof PHPParserNodeIntersectionType) {
+                continue;
+            }
+
+            foreach ($typeNames as $otherKey => $otherTypeNames) {
+                if ($otherKey === $key || $otherTypeNames === []) {
+                    continue;
+                }
+
+                // already removed
+                if (! isset($phpParserUnionedTypes[$otherKey])) {
+                    continue;
+                }
+
+                if (array_diff($otherTypeNames, $typeNames[$key]) === []) {
+                    unset($phpParserUnionedTypes[$key]);
+                    continue 2;
+                }
+            }
+        }
+
+        return array_values($phpParserUnionedTypes);
+    }
+
+    /**
+     * @return string[]
+     */
+    private function resolveIntersectionPartNames(Node $node): array
+    {
+        if ($node instanceof Name) {
+            return [strtolower($node->toString())];
+        }
+
+        if (! $node instanceof PHPParserNodeIntersectionType) {
+            return [];
+        }
+
+        $names = [];
+        foreach ($node->types as $type) {
+            if (! $type instanceof Name) {
+                return [];
+            }
+
+            $names[] = strtolower($type->toString());
+        }
+
+        return $names;
+    }
+
     private function resolveUnionTypeNode(PhpParserUnionType $phpParserUnionType): ?PhpParserUnionType
     {
         if (! $this->phpVersionProvider->isAtLeastPhpVersion(PhpVersionFeature::UNION_TYPES)) {

From 2489e65003866a12dc6d9c26756eb7d194fa7727 Mon Sep 17 00:00:00 2001
From: kevin9327 <5299031+kevin9327@users.noreply.github.com>
Date: Mon, 5 Oct 2026 11:31:01 +0900
Subject: [PATCH 2/2] Drop unneeded strtolower in intersection part names

---
 src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php b/src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php
index 89d9d0c752b..c80df7cafdc 100644
--- a/src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php
+++ b/src/PHPStanStaticTypeMapper/TypeMapper/UnionTypeMapper.php
@@ -282,7 +282,7 @@ private function removeRedundantIntersectionTypes(array $phpParserUnionedTypes):
     private function resolveIntersectionPartNames(Node $node): array
     {
         if ($node instanceof Name) {
-            return [strtolower($node->toString())];
+            return [$node->toString()];
         }
 
         if (! $node instanceof PHPParserNodeIntersectionType) {
@@ -295,7 +295,7 @@ private function resolveIntersectionPartNames(Node $node): array
                 return [];
             }
 
-            $names[] = strtolower($type->toString());
+            $names[] = $type->toString();
         }
 
         return $names;