add a new rule to detect duplicate switch cases

conf/bleedingEdge.neon

@@ -20,3 +20,4 @@ parameters:
 		reportMethodPurityOverride: true
 		checkDynamicConstantNameValues: true
 		unusedLabel: true
+		duplicateCasesInSwitch: true

conf/config.level0.neon

@@ -14,6 +14,8 @@ conditionalTags:
 		phpstan.restrictedFunctionUsageExtension: %featureToggles.internalTag%
 	PHPStan\Rules\Classes\NewStaticInAbstractClassStaticMethodRule:
 		phpstan.rules.rule: %featureToggles.newStaticInAbstractClassStaticMethod%
+	PHPStan\Rules\Comparison\DuplicateCasesInSwitchRule:
+		phpstan.rules.rule: %featureToggles.duplicateCasesInSwitch%
 
 services:
 	-
@@ -27,3 +29,6 @@ services:
 
 	-
 		class: PHPStan\Rules\InternalTag\RestrictedInternalFunctionUsageExtension
+
+	-
+		class: PHPStan\Rules\Comparison\DuplicateCasesInSwitchRule

conf/config.neon

@@ -47,6 +47,7 @@ parameters:
 		reportMethodPurityOverride: false
 		checkDynamicConstantNameValues: false
 		unusedLabel: false
+		duplicateCasesInSwitch: false
 	fileExtensions:
 		- php
 	checkAdvancedIsset: false

conf/parametersSchema.neon

@@ -49,6 +49,7 @@ parametersSchema:
 		reportMethodPurityOverride: bool()
 		checkDynamicConstantNameValues: bool()
 		unusedLabel: bool()
+		duplicateCasesInSwitch: bool()
 	])
 	fileExtensions: listOf(string())
 	checkAdvancedIsset: bool()

src/Rules/Comparison/DuplicateCasesInSwitchRule.php

@@ -0,0 +1,83 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Comparison;
+
+use PhpParser\Node;
+use PhpParser\Node\Stmt\Switch_;
+use PHPStan\Analyser\Scope;
+use PHPStan\Node\Printer\ExprPrinter;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+use function count;
+use function sprintf;
+
+/**
+ * @implements Rule<Switch_>
+ */
+final class DuplicateCasesInSwitchRule implements Rule
+{
+
+	public function __construct(private ExprPrinter $exprPrinter)
+	{
+	}
+
+	public function getNodeType(): string
+	{
+		return Switch_::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		$errors = [];
+		$seenCases = [];
+
+		foreach ($node->cases as $case) {
+			if ($case->cond === null) {
+				continue;
+			}
+
+			$condType = $scope->getType($case->cond);
+			$scalarValues = $condType->getConstantScalarValues();
+
+			if (count($scalarValues) === 1) {
+				$key = ['scalar', $scalarValues[0]];
+			} else {
+				$enumCases = $condType->getEnumCases();
+
+				if (count($enumCases) !== 1) {
+					continue;
+				}
+
+				$key = ['enum', $enumCases[0]->getClassName(), $enumCases[0]->getEnumCaseName()];
+			}
+
+			$firstSeen = null;
+
+			foreach ($seenCases as $seenCase) {
+				if ($seenCase['key'] === $key) {
+					$firstSeen = $seenCase;
+					break;
+				}
+			}
+
+			if ($firstSeen === null) {
+				$seenCases[] = [
+					'key' => $key,
+					'printed' => $this->exprPrinter->printExpr($case->cond),
+					'line' => $case->cond->getStartLine(),
+				];
+				continue;
+			}
+
+			$errors[] = RuleErrorBuilder::message(sprintf(
+				'Case %s in switch is a duplicate of case %s on line %d.',
+				$this->exprPrinter->printExpr($case->cond),
+				$firstSeen['printed'],
+				$firstSeen['line'],
+			))->identifier('switch.duplicateCase')->line($case->getStartLine())->build();
+		}
+
+		return $errors;
+	}
+
+}

tests/PHPStan/Rules/Comparison/DuplicateCasesInSwitchRuleTest.php

@@ -0,0 +1,85 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Comparison;
+
+use PHPStan\Node\Printer\ExprPrinter;
+use PHPStan\Node\Printer\Printer;
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+use function define;
+use function defined;
+
+/**
+ * @extends RuleTestCase<DuplicateCasesInSwitchRule>
+ */
+class DuplicateCasesInSwitchRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new DuplicateCasesInSwitchRule(
+			new ExprPrinter(new Printer()),
+		);
+	}
+
+	public function testDuplicateCases(): void
+	{
+		if (!defined('DUPLICATE_SWITCH_CASE_CONST')) {
+			define('DUPLICATE_SWITCH_CASE_CONST', 'unknown');
+		}
+
+		$this->analyse([__DIR__ . '/data/duplicate-cases-in-switch.php'], [
+			[
+				'Case \'lb\' in switch is a duplicate of case \'lb\' on line 24.',
+				30,
+			],
+			[
+				'Case \'oz\' in switch is a duplicate of case \'oz\' on line 27.',
+				33,
+			],
+			[
+				'Case 1 in switch is a duplicate of case 1 on line 42.',
+				46,
+			],
+			[
+				'Case \'x\' in switch is a duplicate of case \'x\' on line 54.',
+				58,
+			],
+			[
+				'Case \'x\' in switch is a duplicate of case \'x\' on line 54.',
+				60,
+			],
+			[
+				'Case self::EQ in switch is a duplicate of case \'=\' on line 68.',
+				72,
+			],
+			[
+				'Case DUPLICATE_SWITCH_CASE_CONST in switch is a duplicate of case \'unknown\' on line 80.',
+				82,
+			],
+			[
+				'Case \'a\' in switch is a duplicate of case \'a\' on line 90.',
+				94,
+			],
+			[
+				'Case true in switch is a duplicate of case true on line 103.',
+				107,
+			],
+			[
+				'Case null in switch is a duplicate of case null on line 105.',
+				109,
+			],
+		]);
+	}
+
+	public function testDuplicateCasesEnum(): void
+	{
+		$this->analyse([__DIR__ . '/data/duplicate-cases-in-switch-enum.php'], [
+			[
+				'Case \DuplicateCasesInSwitchEnum\Status::Active in switch is a duplicate of case \DuplicateCasesInSwitchEnum\Status::Active on line 20.',
+				24,
+			],
+		]);
+	}
+
+}

tests/PHPStan/Rules/Comparison/data/duplicate-cases-in-switch-enum.php

@@ -0,0 +1,55 @@
+<?php // lint >= 8.1
+
+namespace DuplicateCasesInSwitchEnum;
+
+enum Status: string
+{
+
+	case Active = 'active';
+	case Inactive = 'inactive';
+	case Pending = 'pending';
+
+}
+
+class Foo
+{
+
+	public function doFoo(Status $status): void
+	{
+		switch ($status) {
+			case Status::Active:
+				break;
+			case Status::Inactive:
+				break;
+			case Status::Active:
+				break;
+		}
+	}
+
+	public function noDuplicates(Status $status): void
+	{
+		switch ($status) {
+			case Status::Active:
+				break;
+			case Status::Inactive:
+				break;
+			case Status::Pending:
+				break;
+		}
+	}
+
+	public function backedEnumCaseIsNotADuplicateOfItsBackingValue(Status|string $value): void
+	{
+		switch ($value) {
+			case 'active':
+				break;
+			case Status::Active:
+				break;
+			case Status::Inactive:
+				break;
+			case 'inactive':
+				break;
+		}
+	}
+
+}