Carry unsealed slot through ConstantArrayType::flipArray()

`array_flip` swaps keys and values pair-by-pair. For an unsealed source `array{a: 'foo', b: 'bar', ...<int, string>}`, the explicit entries flip as before and the unsealed slot becomes `<string, int>` — `unsealed[1]->toArrayKey()` lands in the new key slot, `unsealed[0]` in the new value slot. Sealed stays sealed. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
ondrejmirtes · May 20, 2026 · Apr 17, 2026 · Apr 22, 2026 · Apr 22, 2026 · Apr 22, 2026
commit 9ee0108f819485441997364afadb60c83bda3bf8
diff --git a/src/Type/Constant/ConstantArrayType.php b/src/Type/Constant/ConstantArrayType.php
@@ -1496,6 +1496,11 @@ public function flipArray(): Type
 			);
 		}
 
+		if ($this->isUnsealed()->yes() && $this->unsealed !== null) {
+			[$unsealedKey, $unsealedValue] = $this->unsealed;
+			$builder->makeUnsealed($unsealedValue->toArrayKey(), $unsealedKey);
+		}
+
 		return $builder->getArray();
 	}
 

diff --git a/tests/PHPStan/Analyser/nsrt/unsealed-derivations.php b/tests/PHPStan/Analyser/nsrt/unsealed-derivations.php
@@ -281,6 +281,30 @@ public function unshiftAssocWithUnpack(array $sealed, array $unknownItems): void
 
 }
 
+class FlipArray
+{
+
+	/**
+	 * @param array{a: 'foo', b: 'bar', ...<int, string>} $arr
+	 */
+	public function flipPreservesUnsealed(array $arr): void
+	{
+		// `array_flip` swaps keys and values pair-by-pair, so the
+		// unsealed `<int, string>` becomes `<string, int>` — the value
+		// type passes through `toArrayKey()` to land in the new key slot.
+		assertType("array{foo: 'a', bar: 'b', ...<string, int>}", array_flip($arr));
+	}
+
+	/**
+	 * @param array{a: 1, b: 2} $sealed
+	 */
+	public function flipSealedStaysSealed(array $sealed): void
+	{
+		assertType("array{1: 'a', 2: 'b'}", array_flip($sealed));
+	}
+
+}
+
 class CountNarrowing
 {