Preserve constant array shape when setting union of constant scalar keys

- Modified ConstantArrayTypeBuilder::setOffsetValueType() to add unmatched
  scalar keys as optional entries instead of degrading to a general array,
  when none of the union members match existing keys and the array is non-empty
- New regression test in tests/PHPStan/Analyser/nsrt/bug-12665.php
- New test for non-loop union offset setting in nsrt/set-constant-union-offset-on-constant-array.php
- Updated array-fill-keys test expectations to reflect improved type precision

Closes phpstan/phpstan#12665

Author: github-actions[bot]

src/Type/Constant/ConstantArrayTypeBuilder.php

@@ -258,6 +258,7 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 			}
 			if (count($scalarTypes) > 0 && count($scalarTypes) < self::ARRAY_COUNT_LIMIT) {
 				$match = true;
+				$hasMatch = false;
 				$valueTypes = $this->valueTypes;
 				foreach ($scalarTypes as $scalarType) {
 					$offsetMatch = false;
@@ -273,6 +274,7 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 					}
 
 					if ($offsetMatch) {
+						$hasMatch = true;
 						continue;
 					}
 
@@ -283,6 +285,26 @@ public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $opt
 					$this->valueTypes = $valueTypes;
 					return;
 				}
+
+				if (!$hasMatch && count($this->keyTypes) > 0) {
+					foreach ($scalarTypes as $scalarType) {
+						$this->keyTypes[] = $scalarType;
+						$this->valueTypes[] = $valueType;
+						$this->optionalKeys[] = count($this->keyTypes) - 1;
+					}
+
+					$this->isList = TrinaryLogic::createNo();
+
+					if (
+						!$this->disableArrayDegradation
+						&& count($this->keyTypes) > self::ARRAY_COUNT_LIMIT
+					) {
+						$this->degradeToGeneralArray = true;
+						$this->oversized = true;
+					}
+
+					return;
+				}
 			}
 
 			$this->isList = TrinaryLogic::createNo();

tests/PHPStan/Analyser/nsrt/array-fill-keys.php

@@ -54,14 +54,14 @@ function withObjectKey() : array
 function withUnionKeys(): void
 {
 	$arr1 = ['foo', rand(0, 1) ? 'bar1' : 'bar2', 'baz'];
-	assertType("non-empty-array<'bar1'|'bar2'|'baz'|'foo', 'b'>", array_fill_keys($arr1, 'b'));
+	assertType("array{foo: 'b', bar1?: 'b', bar2?: 'b', baz: 'b'}", array_fill_keys($arr1, 'b'));
 
 	$arr2 = ['foo'];
 	if (rand(0, 1)) {
 		$arr2[] = 'bar';
 	}
 	$arr2[] = 'baz';
-	assertType("non-empty-array<'bar'|'baz'|'foo', 'b'>", array_fill_keys($arr2, 'b'));
+	assertType("array{foo: 'b', bar?: 'b', baz?: 'b'}", array_fill_keys($arr2, 'b'));
 }
 
 function withOptionalKeys(): void

tests/PHPStan/Analyser/nsrt/bug-12665.php

@@ -0,0 +1,19 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12665;
+
+use function PHPStan\Testing\assertType;
+
+class Broken
+{
+	/** @return array{a: string, b: int, c: int} */
+	public function break(string $s, int $i): array
+	{
+		$array = ['a' => $s];
+		foreach (['b', 'c'] as $letter) {
+			$array[$letter] = $i;
+		}
+		assertType('array{a: string, b?: int, c?: int}', $array);
+		return $array;
+	}
+}

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

@@ -0,0 +1,20 @@
+<?php
+
+namespace SetConstantUnionOffsetOnConstantArray;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+
+	/**
+	 * @param array{foo: int}  $a
+	 */
+	public function doFoo(array $a): void
+	{
+		$k = rand(0, 1) ? 'a' : 'b';
+		$a[$k] = 256;
+		assertType('array{foo: int, a?: 256, b?: 256}', $a);
+	}
+
+}