Fix resolving type of isset($arr['key'])

It was buggy when $arr['key'] might be null.

Fixes phpstan/phpstan#1924

src/Analyser/MutatingScope.php

@@ -552,23 +552,26 @@ private function resolveType(Expr $node): Type
 			$result = new ConstantBooleanType(true);
 			foreach ($node->vars as $var) {
 				if ($var instanceof Expr\ArrayDimFetch && $var->dim !== null) {
-					$hasOffset = $this->getType($var->var)->hasOffsetValueType(
-						$this->getType($var->dim)
-					)->toBooleanType();
-					if ($hasOffset instanceof ConstantBooleanType) {
-						if (!$hasOffset->getValue()) {
-							return $hasOffset;
+					$variableType = $this->getType($var->var);
+					$dimType = $this->getType($var->dim);
+					$hasOffset = $variableType->hasOffsetValueType($dimType);
+					$offsetValueType = $variableType->getOffsetValueType($dimType);
+					$offsetValueIsNotNull = (new NullType())->isSuperTypeOf($offsetValueType)->negate();
+					$isset = $hasOffset->and($offsetValueIsNotNull)->toBooleanType();
+					if ($isset instanceof ConstantBooleanType) {
+						if (!$isset->getValue()) {
+							return $isset;
 						}
 
 						continue;
 					}
 
-					$result = $hasOffset;
+					$result = $isset;
 					continue;
 				}
 
 				if ($var instanceof Expr\Variable && is_string($var->name)) {
-					$variableType = $this->resolveType($var);
+					$variableType = $this->getType($var);
 					$isNullSuperType = (new NullType())->isSuperTypeOf($variableType);
 					$has = $this->hasVariableType($var->name);
 					if ($has->no() || $isNullSuperType->yes()) {

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -10194,6 +10194,11 @@ public function dataBug3961(): array
 		return $this->gatherAssertTypes(__DIR__ . '/data/bug-3961.php');
 	}
 
+	public function dataBug1924(): array
+	{
+		return $this->gatherAssertTypes(__DIR__ . '/data/bug-1924.php');
+	}
+
 	/**
 	 * @dataProvider dataBug2574
 	 * @dataProvider dataBug2577
@@ -10275,6 +10280,7 @@ public function dataBug3961(): array
 	 * @dataProvider dataNotEmptyArray
 	 * @dataProvider dataClassConstantOnExpression
 	 * @dataProvider dataBug3961
+	 * @dataProvider dataBug1924
 	 * @param string $assertType
 	 * @param string $file
 	 * @param mixed ...$args

tests/PHPStan/Analyser/data/bug-1924.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Bug1924;
+
+use function PHPStan\Analyser\assertType;
+
+class Bug1924
+{
+
+	function getArrayOrNull(): ?array
+	{
+		return rand(0, 1) ? [1, 2, 3] : null;
+	}
+
+	function foo(): void
+	{
+		$arr = [
+			'a' => $this->getArrayOrNull(),
+			'b' => $this->getArrayOrNull(),
+		];
+		assertType('array(\'a\' => array|null, \'b\' => array|null)', $arr);
+
+		$cond = isset($arr['a']) && isset($arr['b']);
+		assertType('bool', $cond);
+	}
+
+}