Add coalesce rule to level one, handle known nullable offsets

conf/config.level1.neon

@@ -12,3 +12,4 @@ rules:
 	- PHPStan\Rules\Constants\ConstantRule
 	- PHPStan\Rules\Functions\UnusedClosureUsesRule
 	- PHPStan\Rules\Variables\VariableCertaintyInIssetRule
+	- PHPStan\Rules\Variables\CoalesceRule

src/Rules/Variables/CoalesceRule.php

@@ -45,7 +45,8 @@ private function canBeCoalesced(Node $node, Scope $scope): ?RuleError
 
 			$variableType = $scope->getVariableType($node->name);
 
-			if ($variableType->isSuperTypeOf(new NullType())->no()) {
+			// For hasVariable->maybe a coalesce is valid
+			if ($hasVariable->yes() && $variableType->isSuperTypeOf(new NullType())->no()) {
 				return RuleErrorBuilder::message(
 					sprintf('Coalesce of variable $%s, which cannot be null.', $node->name)
 				)->line($node->getLine())->build();
@@ -54,8 +55,9 @@ private function canBeCoalesced(Node $node, Scope $scope): ?RuleError
 		} elseif ($node instanceof Node\Expr\ArrayDimFetch && $node->dim !== null) {
 			$type = $scope->getType($node->var);
 			$dimType = $scope->getType($node->dim);
+			$hasOffsetValue = $type->hasOffsetValueType($dimType);
 
-			if ($type->isOffsetAccessible()->no() || $type->hasOffsetValueType($dimType)->no()) {
+			if ($type->isOffsetAccessible()->no() || $hasOffsetValue->no()) {
 				return RuleErrorBuilder::message(
 					sprintf(
 						'Coalesce of invalid offset %s on %s.',
@@ -65,6 +67,20 @@ private function canBeCoalesced(Node $node, Scope $scope): ?RuleError
 				)->line($node->getLine())->build();
 			}
 
+			if ($hasOffsetValue->maybe()) {
+				return null;
+			}
+
+			if ($hasOffsetValue->yes() && $type->isSuperTypeOf(new NullType())->no()) {
+				return RuleErrorBuilder::message(
+					sprintf(
+						'Coalesce of offset %s on %s, which cannot be null.',
+						$dimType->describe(VerbosityLevel::value()),
+						$type->describe(VerbosityLevel::value())
+					)
+				)->line($node->getLine())->build();
+			}
+
 			return $this->canBeCoalesced($node->var, $scope);
 		}
 

tests/PHPStan/Rules/Variables/CoalesceRuleTest.php

@@ -33,6 +33,10 @@ public function testUnsetRule(): void
 				'Coalesce of undefined variable $doesNotExist.',
 				17,
 			],
+			[
+				'Coalesce of offset \'dim\' on array(\'dim\' => 1), which cannot be null.',
+				27,
+			],
 		]);
 	}
 

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

@@ -16,4 +16,21 @@ function coalesce () {
 
 	echo $doesNotExist ?? 0;
 
+	if(rand() > 0.5) {
+		$maybeVariable = 3;
+	}
+
+	echo $maybeVariable ?? 0;
+
+	$fixedDimArray = ['dim' => 1];
+
+	echo $fixedDimArray['dim'] ?? 0;
+}
+
+/**
+ * @param array<string, int> $array
+ */
+function coalesceStringOffset(array $array)
+{
+	echo $array['string'] ?? 0;
 }