Find unnecessary ternary operators

rules.neon

@@ -41,7 +41,14 @@ rules:
 	- PHPStan\Rules\VariableVariables\VariableStaticPropertyFetchRule
 	- PHPStan\Rules\VariableVariables\VariableVariablesRule
 
+conditionalTags:
+	PHPStan\Rules\Operators\OperandsInTernaryOperatorRule:
+		phpstan.rules.rule: %featureToggles.bleedingEdge%
+
 services:
+	-
+		class: PHPStan\Rules\Operators\OperandsInTernaryOperatorRule
+
 	-
 		class: PHPStan\Rules\BooleansInConditions\BooleanRuleHelper
 

src/Rules/Operators/OperandsInTernaryOperatorRule.php

@@ -0,0 +1,39 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Operators;
+
+use PhpParser\Node;
+use PhpParser\Node\Expr\Ternary;
+use PHPStan\Analyser\Scope;
+use PHPStan\Rules\Rule;
+use PHPStan\Type\ConstantType;
+use PHPStan\Type\VerbosityLevel;
+
+/**
+ * @implements \PHPStan\Rules\Rule<\PhpParser\Node\Expr\Ternary>
+ */
+class OperandsInTernaryOperatorRule implements Rule
+{
+
+	public function getNodeType(): string
+	{
+		return Ternary::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		$ifType = $node->if === null ? $scope->getType($node->cond) : $scope->getType($node->if);
+		$elseType = $scope->getType($node->else);
+		if ($ifType instanceof ConstantType && $elseType instanceof ConstantType) {
+			if ($ifType->equals($elseType)) {
+				return [sprintf(
+					'If and else parts of ternary operator are equal (%s).',
+					$ifType->describe(VerbosityLevel::value())
+				)];
+			}
+		}
+
+		return [];
+	}
+
+}

tests/Rules/Operators/OperandsInTernaryOperatorRuleTest.php

@@ -0,0 +1,44 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Operators;
+
+use PHPStan\Rules\Rule;
+
+/**
+ * @extends \PHPStan\Testing\RuleTestCase<OperandsInTernaryOperatorRule>
+ */
+class OperandsInTernaryOperatorRuleTest extends \PHPStan\Testing\RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new OperandsInTernaryOperatorRule();
+	}
+
+	public function testRule(): void
+	{
+		$this->analyse([__DIR__ . '/data/operators.php'], [
+			[
+				'If and else parts of ternary operator are equal (true).',
+				113,
+			],
+			[
+				'If and else parts of ternary operator are equal (array()).',
+				118,
+			],
+			[
+				'If and else parts of ternary operator are equal (\'string_val\').',
+				119,
+			],
+			[
+				'If and else parts of ternary operator are equal (array(23, 24)).',
+				121,
+			],
+			[
+				'If and else parts of ternary operator are equal (array(1 => 1)).',
+				122,
+			],
+		]);
+	}
+
+}

tests/Rules/Operators/data/operators.php

@@ -109,3 +109,18 @@ function (array $array, int $int, $mixed) {
 
 	explode($mixed, $mixed) + $int;
 };
+
+foo() ? true : true;
+foo() ? true : null;
+foo() ? $object : true;
+foo() ? $object : false;
+foo() ? $object : null;
+foo() ? [] : [];
+foo() ? 'string_val' : 'string_val';
+foo() ? [23, 24] : [23, 25];
+foo() ? [23, 24] : [23, 24];
+foo() ? [1 => 1] : ['1' => 1];
+foo() ? [1 => 1] : ['asd' => 1];
+$object ?: $object;
+foo() ? false : true;
+foo() ? true : false;