simple support for mixed pseudo-type

src/FunctionSignatureAssertionError.php

@@ -91,4 +91,12 @@ final class FunctionSignatureAssertionError extends \AssertionError {
 			"Expected return type of type '$expectedReturnType', but given function declares '$actualReturnType'."
 		);
 	}
+
+	public static function parameterDoesNotAcceptAllRequiredValues(\ReflectionFunction $reflection, \ReflectionType $reflectionType): self
+	{
+		return new self(
+			$reflection,
+			"Parameter of given function does not accept all required values. Parameter restricts input values to {$reflectionType}, assertion requires mixed. (=must accept everything)"
+		);
+	}
 };

src/SignatureAssertionUtil.php

@@ -55,6 +55,17 @@ final class SignatureAssertionUtil
 	): void
 	{
 		$reflectionType = $parameterReflection->getType();
+		if ($expectedParameterType === 'mixed') {
+			// must accept everything
+			if ($reflectionType === NULL) {
+				return;
+			}
+
+			throw FunctionSignatureAssertionError::parameterDoesNotAcceptAllRequiredValues(
+				$functionReflection,
+				$reflectionType
+			);
+		}
 		assert($reflectionType instanceof \ReflectionType);
 
 		// checks:
@@ -80,7 +91,7 @@ final class SignatureAssertionUtil
 	// Return-type:
 	private static function checkReturnType(?string $expectedReturnType, \ReflectionFunction $reflection): void
 	{
-		if ($expectedReturnType === NULL) {
+		if ($expectedReturnType === NULL || $expectedReturnType === 'mixed') {
 			return;
 		}
 

tests/fn.assertSignature.mixed-pseudotype.phpt

+<?php declare(strict_types=1);
+namespace MyTestNamespace;
+require __DIR__ . '/bootstrap.php';
+require __DIR__ . '/testClasses.php';
+use function Grifart\AssertFunction\{assertSignature, nullable, params};
+use Grifart\AssertFunction\FunctionSignatureAssertionError;
+use Tester\Assert;
+$place = 'tests/' . basename(__FILE__) . ':' . (__LINE__ +2) . ' ';
+
+$fn = function(string $p1, $p2) {};
+
+assertSignature($fn, params('string', 'mixed'), 'mixed');
+
+Assert::true(true); // if assertion do not fail, we are OK