Detect parameter type widening violation before PHP 7.2

src/Php/PhpVersion.php

@@ -81,4 +81,9 @@ public function supportsPromotedProperties(): bool
 		return $this->versionId >= 80000;
 	}
 
+	public function supportsParameterTypeWidening(): bool
+	{
+		return $this->versionId >= 70200;
+	}
+
 }

src/Rules/Methods/OverridingMethodRule.php

@@ -290,6 +290,24 @@ public function processNode(Node $node, Scope $scope): array
 			}
 
 			$prototypeParameterType = $prototypeParameter->getNativeType();
+			if (!$this->phpVersion->supportsParameterTypeWidening()) {
+				if (!$methodParameterType->equals($prototypeParameterType)) {
+					$messages[] = RuleErrorBuilder::message(sprintf(
+						'Parameter #%d $%s (%s) of method %s::%s() does not match parameter #%d $%s (%s) of method %s::%s().',
+						$i + 1,
+						$methodParameter->getName(),
+						$methodParameterType->describe(VerbosityLevel::typeOnly()),
+						$method->getDeclaringClass()->getDisplayName(),
+						$method->getName(),
+						$i + 1,
+						$prototypeParameter->getName(),
+						$prototypeParameterType->describe(VerbosityLevel::typeOnly()),
+						$prototype->getDeclaringClass()->getDisplayName(),
+						$prototype->getName()
+					))->nonIgnorable()->build();
+				}
+				continue;
+			}
 
 			if ($this->isTypeCompatible($methodParameterType, $prototypeParameterType, $this->phpVersion->supportsParameterContravariance())) {
 				continue;

tests/PHPStan/Rules/Methods/OverridingMethodRuleTest.php

@@ -460,4 +460,37 @@ public function testLessOverridenParametersWithVariadic(int $phpVersionId, array
 		$this->analyse([__DIR__ . '/data/less-parameters-variadics.php'], $errors);
 	}
 
+	public function dataParameterTypeWidening(): array
+	{
+		return [
+			[
+				70100,
+				[
+					[
+						'Parameter #1 $foo (mixed) of method ParameterTypeWidening\Bar::doFoo() does not match parameter #1 $foo (string) of method ParameterTypeWidening\Foo::doFoo().',
+						18,
+					],
+				],
+			],
+			[
+				70200,
+				[],
+			],
+		];
+	}
+
+	/**
+	 * @dataProvider dataParameterTypeWidening
+	 * @param int $phpVersionId
+	 * @param mixed[] $errors
+	 */
+	public function testParameterTypeWidening(int $phpVersionId, array $errors): void
+	{
+		if (!self::$useStaticReflectionProvider) {
+			$this->markTestSkipped('Test requires static reflection.');
+		}
+		$this->phpVersionId = $phpVersionId;
+		$this->analyse([__DIR__ . '/data/parameter-type-widening.php'], $errors);
+	}
+
 }

tests/PHPStan/Rules/Methods/data/parameter-type-widening.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace ParameterTypeWidening;
+
+class Foo
+{
+
+	public function doFoo(string $foo): void
+	{
+
+	}
+
+}
+
+class Bar extends Foo
+{
+
+	public function doFoo($foo): void
+	{
+
+	}
+
+}