Fix wrong property type after assigning iterable type

Author: ondrejmirtes

src/Type/IterableType.php

@@ -236,7 +236,23 @@ public function toArrayKey(): Type
 
 	public function toCoercedArgumentType(bool $strictTypes): Type
 	{
-		return TypeCombinator::union($this, new ArrayType(new MixedType(true), new MixedType(true)), new ObjectType(Traversable::class));
+		return TypeCombinator::union(
+			$this,
+			new ArrayType(
+				TypeCombinator::intersect(
+					$this->keyType->toArrayKey(),
+					new UnionType([
+						new IntegerType(),
+						new StringType(),
+					]),
+				),
+				$this->itemType,
+			),
+			new GenericObjectType(Traversable::class, [
+				$this->keyType,
+				$this->itemType,
+			]),
+		);
 	}
 
 	public function isOffsetAccessLegal(): TrinaryLogic

tests/PHPStan/Analyser/nsrt/bug-12891.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Bug12891;
+
+use function PHPStan\Testing\assertType;
+
+class Handler
+{
+	/** @var iterable<array-key, string> */
+	private iterable $builders;
+
+	/**
+	 * @param iterable<array-key, string> $builders
+	 */
+	public function __construct(iterable $builders) {
+		$this->builders = $builders;
+		assertType('iterable<(int|string), string>', $builders);
+		assertType('iterable<(int|string), string>', $this->builders);
+	}
+}