diff --git a/src/Type/Accessory/HasOffsetType.php b/src/Type/Accessory/HasOffsetType.php
index da6c176655..6eb0ea07c2 100644
--- a/src/Type/Accessory/HasOffsetType.php
+++ b/src/Type/Accessory/HasOffsetType.php
@@ -52,10 +52,7 @@ public function __construct(private ConstantStringType|ConstantIntegerType $offs
 	{
 	}
 
-	/**
-	 * @return ConstantStringType|ConstantIntegerType
-	 */
-	public function getOffsetType(): Type
+	public function getOffsetType(): ConstantStringType|ConstantIntegerType
 	{
 		return $this->offsetType;
 	}
diff --git a/src/Type/Php/ArrayMergeFunctionDynamicReturnTypeExtension.php b/src/Type/Php/ArrayMergeFunctionDynamicReturnTypeExtension.php
index 1d4f7a1122..1d280be17b 100644
--- a/src/Type/Php/ArrayMergeFunctionDynamicReturnTypeExtension.php
+++ b/src/Type/Php/ArrayMergeFunctionDynamicReturnTypeExtension.php
@@ -8,6 +8,8 @@
 use PHPStan\Reflection\FunctionReflection;
 use PHPStan\TrinaryLogic;
 use PHPStan\Type\Accessory\AccessoryArrayListType;
+use PHPStan\Type\Accessory\HasOffsetType;
+use PHPStan\Type\Accessory\HasOffsetValueType;
 use PHPStan\Type\Accessory\NonEmptyArrayType;
 use PHPStan\Type\ArrayType;
 use PHPStan\Type\Constant\ConstantArrayType;
@@ -16,12 +18,15 @@
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\DynamicFunctionReturnTypeExtension;
 use PHPStan\Type\IntegerType;
+use PHPStan\Type\MixedType;
 use PHPStan\Type\NeverType;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
+use PHPStan\Type\TypeUtils;
 use function array_keys;
 use function count;
 use function in_array;
+use function is_int;
 
 #[AutowiredService]
 final class ArrayMergeFunctionDynamicReturnTypeExtension implements DynamicFunctionReturnTypeExtension
@@ -96,6 +101,43 @@ public function getTypeFromFunctionCall(FunctionReflection $functionReflection,
 			return $newArrayBuilder->getArray();
 		}
 
+		$offsetTypes = [];
+		foreach ($argTypes as $argType) {
+			if ($argType->isConstantArray()->yes()) {
+				foreach ($argType->getConstantArrays() as $constantArray) {
+					foreach ($constantArray->getKeyTypes() as $keyType) {
+						$hasOffsetValue = TrinaryLogic::createFromBoolean($argType->hasOffsetValueType($keyType)->yes());
+						$offsetTypes[$keyType->getValue()] = [
+							$hasOffsetValue,
+							$argType->getOffsetValueType($keyType),
+						];
+					}
+				}
+			} else {
+				foreach ($offsetTypes as $key => [$hasOffsetValue, $offsetValueType]) {
+					$offsetTypes[$key] = [
+						$hasOffsetValue->and(TrinaryLogic::createMaybe()),
+						new MixedType(),
+					];
+				}
+			}
+
+			foreach (TypeUtils::getAccessoryTypes($argType) as $accessoryType) {
+				if (
+					!($accessoryType instanceof HasOffsetType)
+					&& !($accessoryType instanceof HasOffsetValueType)
+				) {
+					continue;
+				}
+
+				$offsetType = $accessoryType->getOffsetType();
+				$offsetTypes[$offsetType->getValue()] = [
+					TrinaryLogic::createYes(),
+					$argType->getOffsetValueType($offsetType),
+				];
+			}
+		}
+
 		$keyTypes = [];
 		$valueTypes = [];
 		$nonEmpty = false;
@@ -132,6 +174,36 @@ public function getTypeFromFunctionCall(FunctionReflection $functionReflection,
 		if ($isList) {
 			$arrayType = TypeCombinator::intersect($arrayType, new AccessoryArrayListType());
 		}
+		if ($offsetTypes !== []) {
+			$knownOffsetValues = [];
+			foreach ($offsetTypes as $key => [$hasOffsetValue, $offsetType]) {
+				if (is_int($key)) {
+					// int keys will be appended and renumbered.
+					// at this point we can't reason about them, because unknown arrays are in the mix.
+					continue;
+				}
+				$keyType = new ConstantStringType($key);
+
+				if ($hasOffsetValue->yes()) {
+					// the last string-keyed offset will overwrite previous values
+					$hasOffsetType = new HasOffsetValueType(
+						$keyType,
+						$offsetType,
+					);
+				} elseif ($hasOffsetValue->maybe()) {
+					$hasOffsetType = new HasOffsetType(
+						$keyType,
+					);
+				} else {
+					continue;
+				}
+
+				$knownOffsetValues[] = $hasOffsetType;
+			}
+			if ($knownOffsetValues !== []) {
+				$arrayType = TypeCombinator::intersect($arrayType, ...$knownOffsetValues);
+			}
+		}
 
 		return $arrayType;
 	}
diff --git a/tests/PHPStan/Analyser/LegacyNodeScopeResolverTest.php b/tests/PHPStan/Analyser/LegacyNodeScopeResolverTest.php
index c9fe4f942a..17c38c25aa 100644
--- a/tests/PHPStan/Analyser/LegacyNodeScopeResolverTest.php
+++ b/tests/PHPStan/Analyser/LegacyNodeScopeResolverTest.php
@@ -4697,11 +4697,11 @@ public static function dataArrayFunctions(): array
 				'array_merge($generalStringKeys, $generalDateTimeValues)',
 			],
 			[
-				'non-empty-array<1|string, int|stdClass>',
+				"non-empty-array<1|string, int|stdClass>&hasOffsetValue('foo', stdClass)",
 				'array_merge($generalStringKeys, $stringOrIntegerKeys)',
 			],
 			[
-				'non-empty-array<1|string, int|stdClass>',
+				"non-empty-array<1|string, int|stdClass>&hasOffset('foo')",
 				'array_merge($stringOrIntegerKeys, $generalStringKeys)',
 			],
 			[
diff --git a/tests/PHPStan/Analyser/nsrt/array-merge-const-non-const.php b/tests/PHPStan/Analyser/nsrt/array-merge-const-non-const.php
new file mode 100644
index 0000000000..586d2b593b
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/array-merge-const-non-const.php
@@ -0,0 +1,123 @@
+<?php
+
+namespace ArrayMergeConstNonConst;
+
+use function PHPStan\Testing\assertType;
+
+function doFoo(array $post): void {
+	assertType(
+		"non-empty-array&hasOffset('a')&hasOffset('b')",
+		array_merge(['a' => 1, 'b' => false, 10 => 99], $post)
+	);
+}
+
+function doBar(array $array): void {
+	assertType(
+		"non-empty-array&hasOffsetValue('a', 1)&hasOffsetValue('b', false)",
+		array_merge($array, ['a' => 1, 'b' => false, 10 => 99])
+	);
+}
+
+function doFooBar(array $array): void {
+	assertType(
+		"non-empty-array&hasOffset('x')&hasOffsetValue('a', 1)&hasOffsetValue('b', false)&hasOffsetValue('c', 'e')",
+		array_merge(['c' => 'd', 'x' => 'y'], $array, ['a' => 1, 'b' => false, 'c' => 'e'])
+	);
+}
+
+function doFooInts(array $array): void {
+	assertType(
+		"non-empty-array&hasOffsetValue('a', 1)&hasOffsetValue('c', 'e')",
+		array_merge([1 => 'd'], $array, ['a' => 1, 3 => false, 'c' => 'e'])
+	);
+}
+
+/**
+ * @param array<string> $array
+ */
+function floatKey(array $array): void {
+	assertType(
+		"non-empty-array<string>&hasOffsetValue('a', '1')&hasOffsetValue('c', 'e')",
+		array_merge([4.23 => 'd'], $array, ['a' => '1', 3 => 'false', 'c' => 'e'])
+	);
+}
+
+function doOptKeys(array $array, array $arr2): void {
+	if (rand(0, 1)) {
+		$array['abc'] = 'def';
+	}
+	assertType("array", array_merge($arr2, $array));
+	assertType("array", array_merge($array, $arr2));
+}
+
+/**
+ * @param array{a?: 1, b: 2} $array
+ */
+function doOptShapeKeys(array $array, array $arr2): void {
+	assertType("non-empty-array&hasOffsetValue('b', 2)", array_merge($arr2, $array));
+	assertType("non-empty-array&hasOffset('b')", array_merge($array, $arr2));
+}
+
+function hasOffsetKeys(array $array, array $arr2): void {
+	if (array_key_exists('b', $array)) {
+		assertType("non-empty-array&hasOffsetValue('b', mixed)", array_merge($arr2, $array));
+		assertType("non-empty-array&hasOffset('b')", array_merge($array, $arr2));
+	}
+}
+
+function hasOffsetValueKeys(array $hasB, array $mixedArray, array $hasC): void {
+	$hasB['b'] = 123;
+	$hasC['c'] = 'def';
+
+	assertType("non-empty-array&hasOffsetValue('b', 123)", array_merge($mixedArray, $hasB));
+	assertType("non-empty-array&hasOffset('b')", array_merge($hasB, $mixedArray));
+
+	assertType(
+		"non-empty-array&hasOffset('b')&hasOffsetValue('c', 'def')",
+		array_merge($mixedArray, $hasB, $hasC)
+	);
+	assertType(
+		"non-empty-array&hasOffset('b')&hasOffsetValue('c', 'def')",
+		array_merge($hasB, $mixedArray, $hasC)
+	);
+
+	assertType(
+		"non-empty-array&hasOffset('c')&hasOffsetValue('b', 123)",
+		array_merge($hasC, $mixedArray, $hasB)
+	);
+	assertType(
+		"non-empty-array&hasOffset('b')&hasOffset('c')",
+		array_merge($hasC, $hasB, $mixedArray)
+	);
+
+	if (rand(0, 1)) {
+		$hasBorC = ['b' => 1];
+	} else {
+		$hasBorC = ['c' => 2];
+	}
+	assertType('array{b: 1}|array{c: 2}', $hasBorC);
+	assertType("non-empty-array", array_merge($mixedArray, $hasBorC));
+	assertType("non-empty-array", array_merge($hasBorC, $mixedArray));
+
+	if (rand(0, 1)) {
+		$differentCs = ['c' => 10];
+	} else {
+		$differentCs = ['c' => 20];
+	}
+	assertType('array{c: 10}|array{c: 20}', $differentCs);
+	assertType("non-empty-array&hasOffsetValue('c', 10|20)", array_merge($mixedArray, $differentCs));
+	assertType("non-empty-array&hasOffset('c')", array_merge($differentCs, $mixedArray));
+
+	assertType("non-empty-array&hasOffsetValue('c', 10|20)", array_merge($mixedArray, $hasBorC, $differentCs));
+	assertType("non-empty-array", array_merge($differentCs, $mixedArray, $hasBorC)); // could be non-empty-array&hasOffset('c')
+	assertType("non-empty-array&hasOffsetValue('c', 10|20)", array_merge($hasBorC, $mixedArray, $differentCs));
+	assertType("non-empty-array", array_merge($differentCs, $hasBorC, $mixedArray)); // could be non-empty-array&hasOffset('c')
+}
+
+/**
+ * @param array{a?: 1, b?: 2} $allOptional
+ */
+function doAllOptional(array $allOptional, array $arr2): void {
+	assertType("array", array_merge($arr2, $allOptional));
+	assertType("array", array_merge($allOptional, $arr2));
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-2911.php b/tests/PHPStan/Analyser/nsrt/bug-2911.php
index 3cfbd308f1..1b189148b3 100644
--- a/tests/PHPStan/Analyser/nsrt/bug-2911.php
+++ b/tests/PHPStan/Analyser/nsrt/bug-2911.php
@@ -49,23 +49,23 @@ public function __construct(MutatorConfig $config)
 	private function getResultSettings(array $settings): array
 	{
 		$settings = array_merge(self::DEFAULT_SETTINGS, $settings);
-		assertType('non-empty-array<string, mixed>', $settings);
+		assertType("non-empty-array<string, mixed>&hasOffset('limit')&hasOffset('remove')", $settings);
 
 		if (!is_string($settings['remove'])) {
 			throw $this->configException($settings, 'remove');
 		}
 
-		assertType("non-empty-array<string, mixed>&hasOffsetValue('remove', string)", $settings);
+		assertType("non-empty-array<string, mixed>&hasOffset('limit')&hasOffsetValue('remove', string)", $settings);
 
 		$settings['remove'] = strtolower($settings['remove']);
 
-		assertType("non-empty-array<string, mixed>&hasOffsetValue('remove', lowercase-string)", $settings);
+		assertType("non-empty-array<string, mixed>&hasOffset('limit')&hasOffsetValue('remove', lowercase-string)", $settings);
 
 		if (!in_array($settings['remove'], ['first', 'last', 'all'], true)) {
 			throw $this->configException($settings, 'remove');
 		}
 
-		assertType("non-empty-array<string, mixed>&hasOffsetValue('remove', 'all'|'first'|'last')", $settings);
+		assertType("non-empty-array<string, mixed>&hasOffset('limit')&hasOffsetValue('remove', 'all'|'first'|'last')", $settings);
 
 		if (!is_numeric($settings['limit']) || $settings['limit'] < 1) {
 			throw $this->configException($settings, 'limit');
@@ -110,13 +110,13 @@ private function getResultSettings(array $settings): array
 	{
 		$settings = array_merge(self::DEFAULT_SETTINGS, $settings);
 
-		assertType('non-empty-array<string, mixed>', $settings);
+		assertType("non-empty-array<string, mixed>&hasOffset('limit')&hasOffset('remove')", $settings);
 
 		if (!is_string($settings['remove'])) {
 			throw new Exception();
 		}
 
-		assertType("non-empty-array<string, mixed>&hasOffsetValue('remove', string)", $settings);
+		assertType("non-empty-array<string, mixed>&hasOffset('limit')&hasOffsetValue('remove', string)", $settings);
 
 		if (!is_int($settings['limit'])) {
 			throw new Exception();
diff --git a/tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php b/tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php
index dd51c3df3a..0d4161ee54 100644
--- a/tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php
+++ b/tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php
@@ -1276,4 +1276,9 @@ public function testBug9494(): void
 		$this->analyse([__DIR__ . '/data/bug-9494.php'], []);
 	}
 
+	public function testBug8438(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-8438.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Methods/data/bug-8438.php b/tests/PHPStan/Rules/Methods/data/bug-8438.php
new file mode 100644
index 0000000000..2459ff616e
--- /dev/null
+++ b/tests/PHPStan/Rules/Methods/data/bug-8438.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace Bug8438;
+
+class HelloWorld
+{
+	/**
+	 * @param array<string, string> $array
+	 *
+	 * @return array{expr: mixed, ...}
+	 */
+	protected function foo(array $array): array
+	{
+		$rnd = mt_rand();
+		if ($rnd === 0) {
+			return ['expr' => 'test'];
+		} elseif ($rnd === 1) {
+			// no error with checkBenevolentUnionTypes: false (default even with l9 + strict rules)
+			return ['expr' => 'test', 1 => 'ok'];
+		} else {
+			// phpstan must understand 'expr' key is always present in the result,
+			// then there will be no error here neither
+			return array_merge($array, ['expr' => 'test', 1 => 'ok']);
+		}
+	}
+}