Additional unit tests to confirm behaviour when formulae reference cells within a merge range

tests/PhpSpreadsheetTests/Calculation/MergedCellTest.php

@@ -0,0 +1,51 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Calculation;
+
+use PhpOffice\PhpSpreadsheet\Calculation\Functions;
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PHPStan\Testing\TestCase;
+
+class MergedCellTest extends TestCase
+{
+    /**
+     * @var Spreadsheet
+     */
+    protected $spreadSheet;
+
+    protected function setUp(): void
+    {
+        $this->spreadSheet = new Spreadsheet();
+
+        $dataSheet = $this->spreadSheet->getActiveSheet();
+        $dataSheet->setCellValue('A1', 1.1);
+        $dataSheet->setCellValue('A2', 2.2);
+        $dataSheet->mergeCells('A2:A4');
+        $dataSheet->setCellValue('A5', 3.3);
+    }
+
+    /**
+     * @dataProvider providerWorksheetFormulae
+     */
+    public function testMergedCellBehaviour(string $formula, $expectedResult): void
+    {
+        $worksheet = $this->spreadSheet->getActiveSheet();
+
+        $worksheet->setCellValue('A7', $formula);
+
+        $result = $worksheet->getCell('A7')->getCalculatedValue();
+        self::assertSame($expectedResult, $result);
+    }
+
+    public function providerWorksheetFormulae()
+    {
+        return [
+            ['=SUM(A1:A5)', 6.6],
+            ['=COUNT(A1:A5)', 3],
+            ['=COUNTA(A1:A5)', 3],
+            ['=SUM(A3:A4)', 0],
+            ['=A2+A3+A4', 2.2],
+            ['=A2/A3', Functions::DIV0()],
+        ];
+    }
+}