Fix phpstan/phpstan#11705: recompute ArrayDimFetch type in MutatingScope when key is narrowed

Revert the ArrayKeyExistsFunctionTypeSpecifyingExtension approach (shouldStoreArrayDimFetchType)
which caused regressions in bug-11276 and bug-13526 by not storing the value type needed by the
NonexistentOffset rule. Instead, fix in MutatingScope::resolveType(): when an ArrayDimFetch has
a stored expression type and its dim has been narrowed to a constant scalar, recompute the offset
value type and use it if strictly more specific than the stored type.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Analyser/MutatingScope.php

@@ -1043,7 +1043,24 @@ private function resolveType(string $exprString, Expr $node): Type
 			&& !$node instanceof Expr\ArrowFunction
 			&& $this->hasExpressionType($node)->yes()
 		) {
-			return $this->expressionTypes[$exprString]->getType();
+			$type = $this->expressionTypes[$exprString]->getType();
+
+			if ($node instanceof Expr\ArrayDimFetch && $node->dim !== null) {
+				$dimType = $this->getType($node->dim);
+				if ($dimType->isConstantScalarValue()->yes()) {
+					$arrayType = $this->getType($node->var);
+					$offsetValueType = $arrayType->getOffsetValueType($dimType);
+					if (
+						!$offsetValueType instanceof ErrorType
+						&& $type->isSuperTypeOf($offsetValueType)->yes()
+						&& !$offsetValueType->isSuperTypeOf($type)->yes()
+					) {
+						$type = $offsetValueType;
+					}
+				}
+			}
+
+			return $type;
 		}
 
 		/** @var ExprHandler<Expr> $exprHandler */

src/Type/Php/ArrayKeyExistsFunctionTypeSpecifyingExtension.php

@@ -23,7 +23,6 @@
 use PHPStan\Type\FunctionTypeSpecifyingExtension;
 use PHPStan\Type\IntersectionType;
 use PHPStan\Type\MixedType;
-use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
 use function count;
 use function in_array;
@@ -55,21 +54,6 @@ public function isFunctionSupported(
 			&& !$context->null();
 	}
 
-	private function shouldStoreArrayDimFetchType(Type $arrayType): bool
-	{
-		if (!$arrayType->isConstantArray()->yes()) {
-			return true;
-		}
-
-		foreach ($arrayType->getConstantArrays() as $constantArray) {
-			if (count($constantArray->getOptionalKeys()) > 0) {
-				return true;
-			}
-		}
-
-		return false;
-	}
-
 	public function specifyTypes(
 		FunctionReflection $functionReflection,
 		FuncCall $node,
@@ -125,10 +109,6 @@ public function specifyTypes(
 					$key,
 				);
 
-				if (!$this->shouldStoreArrayDimFetchType($arrayType)) {
-					return $specifiedTypes->setRootExpr(new Identical($arrayDimFetch, new ConstFetch(new Name('__PHPSTAN_FAUX_CONSTANT'))));
-				}
-
 				return $specifiedTypes->unionWith($this->typeSpecifier->create(
 					$arrayDimFetch,
 					$arrayType->getIterableValueType(),