Preserve non-empty array guarantee in ConstantArrayType::setOffsetValueType when union key expansion produces all-optional keys

- `ConstantArrayTypeBuilder` now expands union scalar keys (e.g. `'a'|'b'`)
  into individual optional entries even for empty arrays (since 2f66c45).
  Each key is optional because we don't know which branch of the union will
  be taken, but at least one WILL be set, so the result must be non-empty.
- `ArrayType::setOffsetValueType` already intersects with `NonEmptyArrayType`,
  but `ConstantArrayType::setOffsetValueType` did not, causing the array shape
  `array{a?: 1, b?: 1}` to be reported as possibly empty.
- Add `NonEmptyArrayType` intersection in `ConstantArrayType::setOffsetValueType`
  when the builder result's `isIterableAtLeastOnce()` is not `yes`.
- Update pre-existing test that asserted the buggy (possibly-empty) behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Type/Constant/ConstantArrayType.php

@@ -748,7 +748,12 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $uni
 		$builder = ConstantArrayTypeBuilder::createFromConstantArray($this);
 		$builder->setOffsetValueType($offsetType, $valueType);
 
-		return $builder->getArray();
+		$array = $builder->getArray();
+		if (!$array->isIterableAtLeastOnce()->yes()) {
+			$array = TypeCombinator::intersect($array, new NonEmptyArrayType());
+		}
+
+		return $array;
 	}
 
 	public function setExistingOffsetValueType(Type $offsetType, Type $valueType): Type

tests/PHPStan/Analyser/nsrt/bug-14552.php

@@ -0,0 +1,74 @@
+<?php declare(strict_types = 1);
+
+namespace Bug14552;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param non-empty-list<'a'|'b'> $keys
+ */
+function nonEmptyListForeach(array $keys): void
+{
+	$out = [];
+	foreach ($keys as $k) {
+		$out[$k] = 1;
+	}
+	assertType("non-empty-array{a?: 1, b?: 1}", $out);
+}
+
+/**
+ * @param list<'a'|'b'> $keys
+ */
+function possiblyEmptyListForeach(array $keys): void
+{
+	$out = [];
+	foreach ($keys as $k) {
+		$out[$k] = 1;
+	}
+	assertType("array{}|array{a?: 1, b?: 1}", $out);
+}
+
+/**
+ * @param non-empty-list<'x'|'y'|'z'> $keys
+ */
+function nonEmptyListThreeKeys(array $keys): void
+{
+	$out = [];
+	foreach ($keys as $k) {
+		$out[$k] = true;
+	}
+	assertType("non-empty-array{x?: true, y?: true, z?: true}", $out);
+}
+
+/**
+ * Direct assignment (non-foreach) with union key on empty array.
+ * @param 'a'|'b' $key
+ */
+function directAssignment(string $key): void
+{
+	$arr = [];
+	$arr[$key] = 1;
+	assertType("non-empty-array{a?: 1, b?: 1}", $arr);
+}
+
+/**
+ * Direct assignment with integer union key on empty array.
+ * @param 0|1|2 $key
+ */
+function directAssignmentIntKey(int $key): void
+{
+	$arr = [];
+	$arr[$key] = 'val';
+	assertType("non-empty-array{0?: 'val', 1?: 'val', 2?: 'val'}", $arr);
+}
+
+/**
+ * Setting union key on already non-empty array should stay non-empty.
+ * @param 'x'|'y' $key
+ */
+function setOnNonEmptyArray(string $key): void
+{
+	$arr = ['existing' => 0];
+	$arr[$key] = 1;
+	assertType("array{existing: 0, x?: 1, y?: 1}", $arr);
+}

tests/PHPStan/Analyser/nsrt/set-union-offset-preserve-constant-array.php

@@ -13,7 +13,7 @@ public function doFoo(): void
 
 		$k = rand(0, 1) ? 1 : 2;
 		$a[$k] = true;
-		assertType('array{1?: true, 2?: true}', $a);
+		assertType('non-empty-array{1?: true, 2?: true}', $a);
 	}
 
 }