Report ternary operators that always give one of their branches

conf/bleedingEdge.neon

@@ -23,6 +23,7 @@ parameters:
 		unusedLabel: true
 		newOnNonObject: true
 		unnecessaryNullCoalesce: true
+		unnecessaryTernary: true
 		finiteTypesInHaystack: true
 		switchConditionAlwaysFalse: true
 		checkImportedClassNameCase: true

conf/config.neon

@@ -55,6 +55,7 @@ parameters:
 		unusedLabel: false
 		newOnNonObject: false
 		unnecessaryNullCoalesce: false
+		unnecessaryTernary: false
 		finiteTypesInHaystack: false
 		switchConditionAlwaysFalse: false
 		checkImportedClassNameCase: false

conf/parametersSchema.neon

@@ -52,6 +52,7 @@ parametersSchema:
 		unusedLabel: bool()
 		newOnNonObject: bool()
 		unnecessaryNullCoalesce: bool()
+		unnecessaryTernary: bool()
 		finiteTypesInHaystack: bool()
 		switchConditionAlwaysFalse: bool()
 		checkImportedClassNameCase: bool()

src/Rules/Comparison/UnnecessaryTernaryOperatorRule.php

@@ -0,0 +1,298 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Comparison;
+
+use PhpParser\Node;
+use PhpParser\Node\Expr;
+use PhpParser\NodeFinder;
+use PhpParser\NodeTraverser;
+use PhpParser\NodeVisitor\CloningVisitor;
+use PHPStan\Analyser\CollectedDataEmitter;
+use PHPStan\Analyser\NodeCallbackInvoker;
+use PHPStan\Analyser\Scope;
+use PHPStan\DependencyInjection\AutowiredParameter;
+use PHPStan\DependencyInjection\RegisteredRule;
+use PHPStan\Node\Printer\ExprPrinter;
+use PHPStan\Reflection\ReflectionProvider;
+use PHPStan\Rules\IdentifierRuleError;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+use PHPStan\Type\ObjectWithoutClassType;
+use PHPStan\Type\TypeCombinator;
+use function count;
+use function is_float;
+use function is_string;
+use function sprintf;
+
+/**
+ * Reports `$a ?: X`, `$cond ? $a : X` and `$cond ? X : $a` when the result is
+ * always `$a`: whenever the ternary picks `X`, `$a` already has its value.
+ *
+ * @implements Rule<Node\Expr\Ternary>
+ */
+#[RegisteredRule(level: 4)]
+final class UnnecessaryTernaryOperatorRule implements Rule
+{
+
+	public function __construct(
+		private ConstantConditionRuleHelper $helper,
+		private ConstantConditionInTraitHelper $constantConditionInTraitHelper,
+		private ReflectionProvider $reflectionProvider,
+		private ExprPrinter $exprPrinter,
+		#[AutowiredParameter(ref: '%featureToggles.unnecessaryTernary%')]
+		private bool $unnecessaryTernary,
+		#[AutowiredParameter]
+		private bool $treatPhpDocTypesAsCertain,
+		#[AutowiredParameter(ref: '%tips.treatPhpDocTypesAsCertain%')]
+		private bool $treatPhpDocTypesAsCertainTip,
+	)
+	{
+	}
+
+	public function getNodeType(): string
+	{
+		return Node\Expr\Ternary::class;
+	}
+
+	public function processNode(Node $node, Scope&NodeCallbackInvoker&CollectedDataEmitter $scope): array
+	{
+		if (!$this->unnecessaryTernary) {
+			return [];
+		}
+
+		$error = $this->check($node, $scope);
+		if ($error === null) {
+			$this->constantConditionInTraitHelper->emitNoError(self::class, $scope, $node);
+			return [];
+		}
+
+		if ($scope->isInTrait()) {
+			$this->constantConditionInTraitHelper->emitError(self::class, $scope, $node, true, $error);
+			return [];
+		}
+
+		return [$error];
+	}
+
+	private function check(Node\Expr\Ternary $node, Scope $scope): ?IdentifierRuleError
+	{
+		// TernaryOperatorConstantConditionRule already reports these
+		$conditionType = $this->helper->getBooleanType($scope, $node->cond);
+		if (count($conditionType->getConstantScalarValues()) === 1) {
+			return null;
+		}
+
+		$result = $this->findResult($node, $scope, !$this->treatPhpDocTypesAsCertain);
+		if ($result === null) {
+			return null;
+		}
+
+		$ruleErrorBuilder = RuleErrorBuilder::message(sprintf(
+			$node->if === null
+				? 'Short ternary operator ?: is unnecessary because it always results in %s.'
+				: 'Ternary operator is unnecessary because it always results in %s.',
+			$this->exprPrinter->printExpr($result),
+		))->identifier('ternary.unnecessary');
+
+		if (
+			$this->treatPhpDocTypesAsCertain
+			&& $this->treatPhpDocTypesAsCertainTip
+			&& $this->findResult($node, $scope, true) === null
+		) {
+			$ruleErrorBuilder = $ruleErrorBuilder->treatPhpDocTypesAsCertainTip();
+		}
+
+		return $ruleErrorBuilder->build();
+	}
+
+	/**
+	 * The branch the ternary always results in, or null.
+	 */
+	private function findResult(Node\Expr\Ternary $node, Scope $scope, bool $native): ?Expr
+	{
+		// Without the ternary, the result is evaluated every time, and the
+		// condition and the other branch are not evaluated at all.
+		if (!$this->isPure($node->else, $scope)) {
+			return null;
+		}
+
+		if (!$this->isTruthinessNarrowedExactly($node->cond, $scope, $native)) {
+			return null;
+		}
+
+		if ($node->if === null) {
+			if (!$this->isAlwaysResult($scope->filterByFalseyValue($node->cond), $node->cond, $scope, $node->else, $native)) {
+				return null;
+			}
+
+			return $node->cond;
+		}
+
+		if (!$this->isPure($node->cond, $scope) || !$this->isPure($node->if, $scope)) {
+			return null;
+		}
+
+		// The result then also runs in the scope of the other branch, where a
+		// function call can get an argument it does not accept, for example
+		// `$s === false ? 0 : strlen($s)`.
+		if (!$this->containsCall($node->if) && $this->isAlwaysResult($scope->filterByFalseyValue($node->cond), $node->if, $scope, $node->else, $native)) {
+			return $node->if;
+		}
+		if (!$this->containsCall($node->else) && $this->isAlwaysResult($scope->filterByTruthyValue($node->cond), $node->else, $scope, $node->if, $native)) {
+			return $node->else;
+		}
+
+		return null;
+	}
+
+	/**
+	 * Whether $result always has the single value of $other in $branchScope,
+	 * the scope in which the ternary picks $other.
+	 */
+	private function isAlwaysResult(Scope $branchScope, Expr $result, Scope $scope, Expr $other, bool $native): bool
+	{
+		// a fresh copy, because the walked node keeps the type it had in its own branch
+		$result = (new NodeTraverser(new CloningVisitor()))->traverse([$result])[0];
+		if (!$result instanceof Expr) {
+			return false;
+		}
+
+		if ($native) {
+			$resultType = $branchScope->getNativeType($result);
+			$otherType = $scope->getNativeType($other);
+		} else {
+			$resultType = $branchScope->getType($result);
+			$otherType = $scope->getType($other);
+		}
+
+		// enum cases are not constant values, but each of them is a single value
+		if (!$otherType->isConstantValue()->yes() && count($otherType->getFiniteTypes()) !== 1) {
+			return false;
+		}
+		if (!$resultType->equals($otherType)) {
+			return false;
+		}
+
+		// 0.0 === -0.0, so `$f ?: 0.0` turns -0.0 into 0.0
+		foreach ($otherType->getConstantScalarValues() as $value) {
+			if (is_float($value)) {
+				return false;
+			}
+		}
+
+		return true;
+	}
+
+	/**
+	 * Narrowing by truthiness treats every object as truthy, but an empty
+	 * SimpleXMLElement or a zero GMP is falsy. Then the scope of the other
+	 * branch does not hold all values that reach it.
+	 */
+	private function isTruthinessNarrowedExactly(Expr $cond, Scope $scope, bool $native): bool
+	{
+		if ($cond instanceof Expr\BooleanNot) {
+			return $this->isTruthinessNarrowedExactly($cond->expr, $scope, $native);
+		}
+
+		if (
+			$cond instanceof Expr\BinaryOp\BooleanAnd
+			|| $cond instanceof Expr\BinaryOp\BooleanOr
+			|| $cond instanceof Expr\BinaryOp\LogicalAnd
+			|| $cond instanceof Expr\BinaryOp\LogicalOr
+		) {
+			return $this->isTruthinessNarrowedExactly($cond->left, $scope, $native) && $this->isTruthinessNarrowedExactly($cond->right, $scope, $native);
+		}
+
+		$type = $native ? $scope->getNativeType($cond) : $scope->getType($cond);
+		if ($type->isObject()->no()) {
+			return true;
+		}
+
+		return TypeCombinator::intersect($type, new ObjectWithoutClassType())->toBoolean()->isTrue()->yes();
+	}
+
+	private function containsCall(Expr $expr): bool
+	{
+		return (new NodeFinder())->findFirstInstanceOf([$expr], Expr\FuncCall::class) !== null;
+	}
+
+	/**
+	 * Whether evaluating the expression can only produce its value, so that
+	 * dropping it or evaluating it once more does not change anything.
+	 */
+	private function isPure(Expr $expr, Scope $scope): bool
+	{
+		if ($expr instanceof Expr\Variable) {
+			return is_string($expr->name);
+		}
+
+		if ($expr instanceof Node\Scalar\Int_ || $expr instanceof Node\Scalar\Float_ || $expr instanceof Node\Scalar\String_) {
+			return true;
+		}
+
+		if ($expr instanceof Expr\ConstFetch) {
+			return true;
+		}
+
+		if ($expr instanceof Expr\ClassConstFetch) {
+			return $expr->class instanceof Node\Name && $expr->name instanceof Node\Identifier;
+		}
+
+		if ($expr instanceof Expr\Array_) {
+			foreach ($expr->items as $item) {
+				if ($item->unpack || $item->byRef) {
+					return false;
+				}
+				if ($item->key !== null && !$this->isPure($item->key, $scope)) {
+					return false;
+				}
+				if (!$this->isPure($item->value, $scope)) {
+					return false;
+				}
+			}
+
+			return true;
+		}
+
+		if (
+			$expr instanceof Expr\BinaryOp\Identical
+			|| $expr instanceof Expr\BinaryOp\NotIdentical
+			|| $expr instanceof Expr\BinaryOp\BooleanAnd
+			|| $expr instanceof Expr\BinaryOp\BooleanOr
+			|| $expr instanceof Expr\BinaryOp\LogicalAnd
+			|| $expr instanceof Expr\BinaryOp\LogicalOr
+		) {
+			return $this->isPure($expr->left, $scope) && $this->isPure($expr->right, $scope);
+		}
+
+		if ($expr instanceof Expr\BooleanNot) {
+			return $this->isPure($expr->expr, $scope);
+		}
+
+		if ($expr instanceof Expr\Instanceof_) {
+			return $expr->class instanceof Node\Name && $this->isPure($expr->expr, $scope);
+		}
+
+		if ($expr instanceof Expr\FuncCall) {
+			if (!$expr->name instanceof Node\Name || $expr->isFirstClassCallable()) {
+				return false;
+			}
+			if (!$this->reflectionProvider->hasFunction($expr->name, $scope)) {
+				return false;
+			}
+			if (!$this->reflectionProvider->getFunction($expr->name, $scope)->hasSideEffects()->no()) {
+				return false;
+			}
+			foreach ($expr->getArgs() as $arg) {
+				if ($arg->unpack || $arg->byRef || !$this->isPure($arg->value, $scope)) {
+					return false;
+				}
+			}
+
+			return true;
+		}
+
+		return false;
+	}
+
+}