Do not flatten template benevolent unions in TypeCombinator::union()

The "transform A | (B | C) to A | B | C" loop spliced the members of any
BenevolentUnionType into the outer union, including
TemplateBenevolentUnionType — destroying the template before the
TemplateType guard (which already protects TemplateUnionType) could
apply. Resolving a PHPDoc union like `K|null` where `K of array-key`
therefore degraded the template to its bound at parse time. The
reconstruction path only restored the template when all resulting union
members came from the benevolent union, which fails as soon as another
member (e.g. `null`) is present.

Exclude TemplateType from the benevolent flattening branch so
TemplateBenevolentUnionType falls through to the same guard as
TemplateUnionType, mirroring how intersect() already handles templates
before benevolent flattening (57e3cbf). The template-rewrapping
machinery in the reconstruction path becomes unreachable and is removed;
plain benevolent unions behave exactly as before.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: gnutix

src/Type/TypeCombinator.php

@@ -199,7 +199,6 @@ public static function union(Type ...$types): Type
 		$alreadyNormalizedCounter = 0;
 
 		$benevolentTypes = [];
-		$benevolentUnionObject = null;
 		$neverCount = 0;
 		// transform A | (B | C) to A | B | C
 		for ($i = 0; $i < $typesCount; $i++) {
@@ -215,10 +214,7 @@ public static function union(Type ...$types): Type
 				$neverCount++;
 				continue;
 			}
-			if ($types[$i] instanceof BenevolentUnionType) {
-				if ($types[$i] instanceof TemplateBenevolentUnionType && $benevolentUnionObject === null) {
-					$benevolentUnionObject = $types[$i];
-				}
+			if ($types[$i] instanceof BenevolentUnionType && !$types[$i] instanceof TemplateType) {
 				$benevolentTypesCount = 0;
 				$typesInner = $types[$i]->getTypes();
 				foreach ($typesInner as $benevolentInnerType) {
@@ -471,10 +467,6 @@ public static function union(Type ...$types): Type
 			}
 
 			if ($tempTypes === []) {
-				if ($benevolentUnionObject instanceof TemplateBenevolentUnionType) {
-					return $benevolentUnionObject->withTypes(array_values($types));
-				}
-
 				return new BenevolentUnionType(array_values($types), true);
 			}
 		}

tests/PHPStan/Analyser/nsrt/bug-7279.php

@@ -0,0 +1,102 @@
+<?php declare(strict_types = 1); // lint >= 8.0
+
+namespace Bug7279;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @template T
+ */
+class Timeline
+{
+
+}
+
+class Percentage
+{
+
+}
+
+/**
+ * @template K of array-key
+ * @template T
+ *
+ * @param array<K, T> $array
+ * @param (callable(T, K): bool) $fn
+ *
+ * @return ($array is non-empty-array ? T|null : null)
+ */
+function find(array $array, callable $fn): mixed
+{
+	foreach ($array as $key => $value) {
+		if ($fn($value, $key)) {
+			return $value;
+		}
+	}
+
+	return null;
+}
+
+/**
+ * @template K of array-key
+ * @template T
+ *
+ * @param array<K, T> $array
+ * @param (callable(T, K): bool) $fn
+ *
+ * @return ($array is non-empty-array ? K|null : null)
+ */
+function findKey(array $array, callable $fn): string|int|null
+{
+	foreach ($array as $key => $value) {
+		if ($fn($value, $key)) {
+			return $key;
+		}
+	}
+
+	return null;
+}
+
+/**
+ * @param callable(mixed): bool $callback
+ * @param array<never, never> $emptyList
+ * @param array{} $emptyMap
+ * @param array<int, string> $unknownList
+ * @param array{id?: int, name?: string} $unknownMap
+ * @param non-empty-array<int, Timeline<Percentage>> $nonEmptyList
+ * @param array{work: Timeline<Percentage>} $nonEmptyMap
+ */
+function assertions(
+	callable $callback,
+	array $emptyList,
+	array $emptyMap,
+	array $unknownList,
+	array $unknownMap,
+	array $nonEmptyList,
+	array $nonEmptyMap,
+): void
+{
+	// Everything works great for find()
+
+	assertType('null', find([], $callback));
+	assertType('null', find($emptyList, $callback));
+	assertType('null', find($emptyMap, $callback));
+
+	assertType('string|null', find($unknownList, $callback));
+	assertType('int|string|null', find($unknownMap, $callback));
+
+	assertType('Bug7279\Timeline<Bug7279\Percentage>|null', find($nonEmptyList, $callback));
+	assertType('Bug7279\Timeline<Bug7279\Percentage>|null', find($nonEmptyMap, $callback));
+
+	// But everything goes to hell for findKey() ?!?
+
+	assertType('null', findKey([], $callback));
+	assertType('null', findKey($emptyList, $callback));
+	assertType('null', findKey($emptyMap, $callback));
+
+	assertType('int|null', findKey($unknownList, $callback));
+	assertType("'id'|'name'|null", findKey($unknownMap, $callback));
+
+	assertType('int|null', findKey($nonEmptyList, $callback));
+	assertType("'work'|null", findKey($nonEmptyMap, $callback));
+}