Add rule checking coalesces

src/Rules/Variables/CoalesceRule.php

@@ -0,0 +1,74 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Variables;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Rules\RuleError;
+use PHPStan\Rules\RuleErrorBuilder;
+use PHPStan\Type\NullType;
+use PHPStan\Type\VerbosityLevel;
+
+/**
+ * @implements \PHPStan\Rules\Rule<\PhpParser\Node\Expr\BinaryOp\Coalesce>
+ */
+class CoalesceRule implements \PHPStan\Rules\Rule
+{
+
+	public function getNodeType(): string
+	{
+		return Node\Expr\BinaryOp\Coalesce::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		$error = $this->canBeCoalesced($node->left, $scope);
+
+		if ($error === null) {
+			return [];
+		}
+
+		return [$error];
+	}
+
+	private function canBeCoalesced(Node $node, Scope $scope): ?RuleError
+	{
+		if ($node instanceof Node\Expr\Variable && is_string($node->name)) {
+
+			$hasVariable = $scope->hasVariableType($node->name);
+
+			if ($hasVariable->no()) {
+				return RuleErrorBuilder::message(
+					sprintf('Coalesce of undefined variable $%s.', $node->name)
+				)->line($node->getLine())->build();
+			}
+
+			$variableType = $scope->getVariableType($node->name);
+
+			if ($variableType->isSuperTypeOf(new NullType())->no()) {
+				return RuleErrorBuilder::message(
+					sprintf('Coalesce of variable $%s, which cannot be null.', $node->name)
+				)->line($node->getLine())->build();
+			}
+
+		} elseif ($node instanceof Node\Expr\ArrayDimFetch && $node->dim !== null) {
+			$type = $scope->getType($node->var);
+			$dimType = $scope->getType($node->dim);
+
+			if ($type->isOffsetAccessible()->no() || $type->hasOffsetValueType($dimType)->no()) {
+				return RuleErrorBuilder::message(
+					sprintf(
+						'Coalesce of invalid offset %s on %s.',
+						$dimType->describe(VerbosityLevel::value()),
+						$type->describe(VerbosityLevel::value())
+					)
+				)->line($node->getLine())->build();
+			}
+
+			return $this->canBeCoalesced($node->var, $scope);
+		}
+
+		return null;
+	}
+
+}

tests/PHPStan/Rules/Variables/CoalesceRuleTest.php

@@ -0,0 +1,39 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Variables;
+
+/**
+ * @extends \PHPStan\Testing\RuleTestCase<CoalesceRule>
+ */
+class CoalesceRuleTest extends \PHPStan\Testing\RuleTestCase
+{
+
+	protected function getRule(): \PHPStan\Rules\Rule
+	{
+		return new CoalesceRule();
+	}
+
+	public function testUnsetRule(): void
+	{
+		require_once __DIR__ . '/data/coalesce.php';
+		$this->analyse([__DIR__ . '/data/coalesce.php'], [
+			[
+				'Coalesce of variable $scalar, which cannot be null.',
+				7,
+			],
+			[
+				'Coalesce of invalid offset \'string\' on array(1, 2, 3).',
+				11,
+			],
+			[
+				'Coalesce of invalid offset \'string\' on array(array(1), array(2), array(3)).',
+				15,
+			],
+			[
+				'Coalesce of undefined variable $doesNotExist.',
+				17,
+			],
+		]);
+	}
+
+}

tests/PHPStan/Rules/Variables/data/coalesce.php

@@ -0,0 +1,19 @@
+<?php
+
+function coalesce () {
+
+	$scalar = 3;
+
+	echo $scalar ?? 4;
+
+	$array = [1, 2, 3];
+
+	echo $array['string'] ?? 0;
+
+	$multiDimArray = [[1], [2], [3]];
+
+	echo $multiDimArray['string'] ?? 0;
+
+	echo $doesNotExist ?? 0;
+
+}