Check - unreachable conditions (useful for !checkAlwaysTrue* projects)

conf/config.level4.neon

@@ -8,6 +8,7 @@ rules:
 	- PHPStan\Rules\Comparison\ElseIfConstantConditionRule
 	- PHPStan\Rules\Comparison\IfConstantConditionRule
 	- PHPStan\Rules\Comparison\TernaryOperatorConstantConditionRule
+	- PHPStan\Rules\Comparison\UnreachableIfBranchesRule
 
 services:
 	-

src/Rules/Comparison/UnreachableIfBranchesRule.php

@@ -0,0 +1,47 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Comparison;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Rules\RuleError;
+use PHPStan\Rules\RuleErrorBuilder;
+use PHPStan\Type\Constant\ConstantBooleanType;
+
+class UnreachableIfBranchesRule implements \PHPStan\Rules\Rule
+{
+
+	public function getNodeType(): string
+	{
+		return Node\Stmt\If_::class;
+	}
+
+	/**
+	 * @param \PhpParser\Node\Stmt\If_ $node
+	 * @param Scope $scope
+	 * @return RuleError[]
+	 */
+	public function processNode(Node $node, Scope $scope): array
+	{
+		$errors = [];
+		$conditionType = $scope->getType($node->cond)->toBoolean();
+		$nextBranchIsDead = $conditionType instanceof ConstantBooleanType && $conditionType->getValue();
+
+		foreach ($node->elseifs as $elseif) {
+			if ($nextBranchIsDead) {
+				$errors[] = RuleErrorBuilder::message('Elseif branch is unreachable because previous condition is always true.')->line($elseif->getLine())->build();
+				continue;
+			}
+
+			$elseIfConditionType = $scope->getType($elseif->cond)->toBoolean();
+			$nextBranchIsDead = $elseIfConditionType instanceof ConstantBooleanType && $elseIfConditionType->getValue();
+		}
+
+		if ($node->else !== null && $nextBranchIsDead) {
+			$errors[] = RuleErrorBuilder::message('Else branch is unreachable because previous condition is always true.')->line($node->else->getLine())->build();
+		}
+
+		return $errors;
+	}
+
+}

tests/PHPStan/Rules/Comparison/UnreachableIfBranchesRuleTest.php

@@ -0,0 +1,42 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Comparison;
+
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+
+class UnreachableIfBranchesRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new UnreachableIfBranchesRule();
+	}
+
+	public function testRule(): void
+	{
+		$this->analyse([__DIR__ . '/data/unreachable-if-branches.php'], [
+			[
+				'Else branch is unreachable because previous condition is always true.',
+				15,
+			],
+			[
+				'Elseif branch is unreachable because previous condition is always true.',
+				25,
+			],
+			[
+				'Else branch is unreachable because previous condition is always true.',
+				27,
+			],
+			[
+				'Elseif branch is unreachable because previous condition is always true.',
+				39,
+			],
+			[
+				'Else branch is unreachable because previous condition is always true.',
+				41,
+			],
+		]);
+	}
+
+}

tests/PHPStan/Rules/Comparison/data/unreachable-if-branches.php

@@ -0,0 +1,45 @@
+<?php
+
+function () {
+
+	if (true) {
+
+	}
+
+};
+
+function () {
+
+	if (true) {
+
+	} else { // unreachable
+
+	}
+
+};
+
+function (string $str) {
+
+	if (true) {
+
+	} elseif ($str) { // unreachable
+
+	} else { // unreachable
+
+	}
+
+};
+
+function (string $foo, string $bar) {
+
+	if ($str) {
+
+	} elseif (true) {
+
+	} elseif ($bar) { // unreachable
+
+	} else { // unreachable
+
+	}
+
+};