diff --git a/phpstan.neon.dist b/phpstan.neon.dist
index de7dee8645..e6d7d0d33c 100644
--- a/phpstan.neon.dist
+++ b/phpstan.neon.dist
@@ -23,6 +23,7 @@ parameters:
     parallel:
         processTimeout: 300.0
     checkMissingIterableValueType: false
+    checkGenericClassInNonGenericObjectType: false
     ignoreErrors:
         # Accept a bit anything for assert methods
         - '~^Parameter \#2 .* of static method PHPUnit\\Framework\\Assert\:\:assert\w+\(\) expects .*, .* given\.$~'
diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index d0e1220348..63b52505b7 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -195,7 +195,7 @@ public static function getLocaleBoolean(string $index): string
      * Excel constant string translations to their PHP equivalents
      * Constant conversion from text name/value to actual (datatyped) value.
      *
-     * @var array<string, mixed>
+     * @var array<string, null|bool>
      */
     private static array $excelConstants = [
         'TRUE' => true,
@@ -208,7 +208,7 @@ public static function keyInExcelConstants(string $key): bool
         return array_key_exists($key, self::$excelConstants);
     }
 
-    public static function getExcelConstants(string $key): mixed
+    public static function getExcelConstants(string $key): bool|null
     {
         return self::$excelConstants[$key];
     }
diff --git a/src/PhpSpreadsheet/Calculation/Database/DatabaseAbstract.php b/src/PhpSpreadsheet/Calculation/Database/DatabaseAbstract.php
index 3f40d475a9..7d9885ee90 100644
--- a/src/PhpSpreadsheet/Calculation/Database/DatabaseAbstract.php
+++ b/src/PhpSpreadsheet/Calculation/Database/DatabaseAbstract.php
@@ -156,7 +156,7 @@ private static function executeQuery(array $database, string $query, array $crit
         return $database;
     }
 
-    private static function processCondition(string $criterion, array $fields, array $dataValues, string $conditions): mixed
+    private static function processCondition(string $criterion, array $fields, array $dataValues, string $conditions): string
     {
         $key = array_search($criterion, $fields, true);
 
diff --git a/src/PhpSpreadsheet/Calculation/DateTimeExcel/Current.php b/src/PhpSpreadsheet/Calculation/DateTimeExcel/Current.php
index 20b48969be..088e3794c7 100644
--- a/src/PhpSpreadsheet/Calculation/DateTimeExcel/Current.php
+++ b/src/PhpSpreadsheet/Calculation/DateTimeExcel/Current.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Calculation\DateTimeExcel;
 
+use DateTime;
 use DateTimeImmutable;
 use PhpOffice\PhpSpreadsheet\Calculation\Information\ExcelError;
 
@@ -21,10 +22,10 @@ class Current
      * Excel Function:
      *        TODAY()
      *
-     * @return mixed Excel date/time serial value, PHP date/time serial value or PHP date/time object,
+     * @return DateTime|float|int|string Excel date/time serial value, PHP date/time serial value or PHP date/time object,
      *                        depending on the value of the ReturnDateType flag
      */
-    public static function today(): mixed
+    public static function today(): DateTime|float|int|string
     {
         $dti = new DateTimeImmutable();
         $dateArray = Helpers::dateParse($dti->format('c'));
@@ -46,10 +47,10 @@ public static function today(): mixed
      * Excel Function:
      *        NOW()
      *
-     * @return mixed Excel date/time serial value, PHP date/time serial value or PHP date/time object,
+     * @return DateTime|float|int|string Excel date/time serial value, PHP date/time serial value or PHP date/time object,
      *                        depending on the value of the ReturnDateType flag
      */
-    public static function now(): mixed
+    public static function now(): DateTime|float|int|string
     {
         $dti = new DateTimeImmutable();
         $dateArray = Helpers::dateParse($dti->format('c'));
diff --git a/src/PhpSpreadsheet/Calculation/DateTimeExcel/Date.php b/src/PhpSpreadsheet/Calculation/DateTimeExcel/Date.php
index c9c6458819..e0e4b25f88 100644
--- a/src/PhpSpreadsheet/Calculation/DateTimeExcel/Date.php
+++ b/src/PhpSpreadsheet/Calculation/DateTimeExcel/Date.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Calculation\DateTimeExcel;
 
+use DateTime;
 use PhpOffice\PhpSpreadsheet\Calculation\ArrayEnabled;
 use PhpOffice\PhpSpreadsheet\Calculation\Exception;
 use PhpOffice\PhpSpreadsheet\Calculation\Information\ExcelError;
@@ -58,12 +59,12 @@ class Date
      *                                example, DATE(2008,1,-15) returns the serial number representing
      *                                December 16, 2007.
      *
-     * @return mixed Excel date/time serial value, PHP date/time serial value or PHP date/time object,
+     * @return array|DateTime|float|int|string Excel date/time serial value, PHP date/time serial value or PHP date/time object,
      *                        depending on the value of the ReturnDateType flag
      *         If an array of numbers is passed as the argument, then the returned result will also be an array
      *            with the same dimensions
      */
-    public static function fromYMD(array|float|int|string $year, array|float|int|string $month, array|float|int|string $day): mixed
+    public static function fromYMD(array|float|int|string $year, array|float|int|string $month, array|float|int|string $day): float|int|DateTime|string|array
     {
         if (is_array($year) || is_array($month) || is_array($day)) {
             return self::evaluateArrayArguments([self::class, __FUNCTION__], $year, $month, $day);
diff --git a/src/PhpSpreadsheet/Calculation/DateTimeExcel/DateValue.php b/src/PhpSpreadsheet/Calculation/DateTimeExcel/DateValue.php
index 4bf75dd3dc..8c5fa71c94 100644
--- a/src/PhpSpreadsheet/Calculation/DateTimeExcel/DateValue.php
+++ b/src/PhpSpreadsheet/Calculation/DateTimeExcel/DateValue.php
@@ -2,6 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Calculation\DateTimeExcel;
 
+use DateTime;
 use DateTimeImmutable;
 use PhpOffice\PhpSpreadsheet\Calculation\ArrayEnabled;
 use PhpOffice\PhpSpreadsheet\Calculation\Information\ExcelError;
@@ -34,12 +35,12 @@ class DateValue
      *                                    #VALUE! error value if date_text is out of this range.
      *                         Or can be an array of date values
      *
-     * @return mixed Excel date/time serial value, PHP date/time serial value or PHP date/time object,
+     * @return array|DateTime|float|int|string Excel date/time serial value, PHP date/time serial value or PHP date/time object,
      *                        depending on the value of the ReturnDateType flag
      *         If an array of numbers is passed as the argument, then the returned result will also be an array
      *            with the same dimensions
      */
-    public static function fromString(null|array|string|int|bool|float $dateValue): mixed
+    public static function fromString(null|array|string|int|bool|float $dateValue): array|string|float|int|DateTime
     {
         if (is_array($dateValue)) {
             return self::evaluateSingleArgumentArray([self::class, __FUNCTION__], $dateValue);
@@ -131,10 +132,10 @@ private static function setUpArray(string $dateValue, DateTimeImmutable $dti): a
     /**
      * Final results.
      *
-     * @return mixed Excel date/time serial value, PHP date/time serial value or PHP date/time object,
+     * @return DateTime|float|int|string Excel date/time serial value, PHP date/time serial value or PHP date/time object,
      *                        depending on the value of the ReturnDateType flag
      */
-    private static function finalResults(array $PHPDateArray, DateTimeImmutable $dti, int $baseYear): mixed
+    private static function finalResults(array $PHPDateArray, DateTimeImmutable $dti, int $baseYear): string|float|int|DateTime
     {
         $retValue = ExcelError::Value();
         if (Helpers::dateParseSucceeded($PHPDateArray)) {
diff --git a/src/PhpSpreadsheet/Calculation/DateTimeExcel/Helpers.php b/src/PhpSpreadsheet/Calculation/DateTimeExcel/Helpers.php
index 733cecb614..94ffa4c4d3 100644
--- a/src/PhpSpreadsheet/Calculation/DateTimeExcel/Helpers.php
+++ b/src/PhpSpreadsheet/Calculation/DateTimeExcel/Helpers.php
@@ -58,12 +58,13 @@ public static function getDateValue(mixed $dateValue, bool $allowBool = true): f
     /**
      * getTimeValue.
      *
-     * @return mixed Excel date/time serial value, or string if error
+     * @return float|string Excel date/time serial value, or string if error
      */
-    public static function getTimeValue(string $timeValue): mixed
+    public static function getTimeValue(string $timeValue): string|float
     {
         $saveReturnDateType = Functions::getReturnDateType();
         Functions::setReturnDateType(Functions::RETURNDATE_EXCEL);
+        /** @var float|string $timeValue */
         $timeValue = TimeValue::fromString($timeValue);
         Functions::setReturnDateType($saveReturnDateType);
 
@@ -238,6 +239,8 @@ public static function validateNumericNull(mixed $number): int|float
 
     /**
      * Many functions accept null/false/true argument treated as 0/0/1.
+     *
+     * @phpstan-assert float $number
      */
     public static function validateNotNegative(mixed $number): float
     {
diff --git a/src/PhpSpreadsheet/Calculation/DateTimeExcel/Month.php b/src/PhpSpreadsheet/Calculation/DateTimeExcel/Month.php
index f653c2fe40..a90c051734 100644
--- a/src/PhpSpreadsheet/Calculation/DateTimeExcel/Month.php
+++ b/src/PhpSpreadsheet/Calculation/DateTimeExcel/Month.php
@@ -73,12 +73,12 @@ public static function adjust(mixed $dateValue, array|string|bool|float|int $adj
      *                                        a negative value yields a past date.
      *                         Or can be an array of adjustment values
      *
-     * @return array|mixed Excel date/time serial value, PHP date/time serial value or PHP date/time object,
+     * @return array|DateTime|float|int|string Excel date/time serial value, PHP date/time serial value or PHP date/time object,
      *                        depending on the value of the ReturnDateType flag
      *         If an array of values is passed as the argument, then the returned result will also be an array
      *            with the same dimensions
      */
-    public static function lastDay(mixed $dateValue, array|float|int|bool|string $adjustmentMonths): mixed
+    public static function lastDay(mixed $dateValue, array|float|int|bool|string $adjustmentMonths): array|string|DateTime|float|int
     {
         if (is_array($dateValue) || is_array($adjustmentMonths)) {
             return self::evaluateArrayArguments([self::class, __FUNCTION__], $dateValue, $adjustmentMonths);
diff --git a/src/PhpSpreadsheet/Calculation/DateTimeExcel/WorkDay.php b/src/PhpSpreadsheet/Calculation/DateTimeExcel/WorkDay.php
index 011f41773c..4e4ed3c853 100644
--- a/src/PhpSpreadsheet/Calculation/DateTimeExcel/WorkDay.php
+++ b/src/PhpSpreadsheet/Calculation/DateTimeExcel/WorkDay.php
@@ -31,12 +31,12 @@ class WorkDay
      *                         Or can be an array of int values
      * @param null|mixed $dateArgs An array of dates (such as holidays) to exclude from the calculation
      *
-     * @return array|mixed Excel date/time serial value, PHP date/time serial value or PHP date/time object,
+     * @return array|DateTime|float|int|string Excel date/time serial value, PHP date/time serial value or PHP date/time object,
      *                        depending on the value of the ReturnDateType flag
      *         If an array of values is passed for the $startDate or $endDays,arguments, then the returned result
      *            will also be an array with matching dimensions
      */
-    public static function date(mixed $startDate, array|int|string $endDays, mixed ...$dateArgs): mixed
+    public static function date(mixed $startDate, array|int|string $endDays, mixed ...$dateArgs): array|float|int|DateTime|string
     {
         if (is_array($startDate) || is_array($endDays)) {
             return self::evaluateArrayArgumentsSubset(
diff --git a/src/PhpSpreadsheet/Calculation/Functions.php b/src/PhpSpreadsheet/Calculation/Functions.php
index e838ef9cbb..6b74500698 100644
--- a/src/PhpSpreadsheet/Calculation/Functions.php
+++ b/src/PhpSpreadsheet/Calculation/Functions.php
@@ -277,7 +277,7 @@ public static function flattenArrayIndexed($array): array
      *
      * @param mixed $value Array or scalar value
      */
-    public static function flattenSingleValue(mixed $value = ''): mixed
+    public static function flattenSingleValue(mixed $value): mixed
     {
         while (is_array($value)) {
             $value = array_shift($value);
diff --git a/src/PhpSpreadsheet/Calculation/LookupRef/Hyperlink.php b/src/PhpSpreadsheet/Calculation/LookupRef/Hyperlink.php
index 30db8437ed..455442a8eb 100644
--- a/src/PhpSpreadsheet/Calculation/LookupRef/Hyperlink.php
+++ b/src/PhpSpreadsheet/Calculation/LookupRef/Hyperlink.php
@@ -18,9 +18,9 @@ class Hyperlink
      * @param mixed $displayName Expect string. Value to return when testValue is an error condition
      * @param ?Cell $cell The cell to set the hyperlink in
      *
-     * @return mixed The value of $displayName (or $linkURL if $displayName was blank)
+     * @return string The value of $displayName (or $linkURL if $displayName was blank)
      */
-    public static function set(mixed $linkURL = '', mixed $displayName = null, ?Cell $cell = null): mixed
+    public static function set(mixed $linkURL = '', mixed $displayName = null, ?Cell $cell = null): string
     {
         $linkURL = ($linkURL === null) ? '' : Functions::flattenSingleValue($linkURL);
         $displayName = ($displayName === null) ? '' : Functions::flattenSingleValue($displayName);
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/MaxMinBase.php b/src/PhpSpreadsheet/Calculation/Statistical/MaxMinBase.php
index c575721872..a722fbd9c8 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/MaxMinBase.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/MaxMinBase.php
@@ -4,7 +4,7 @@
 
 abstract class MaxMinBase
 {
-    protected static function datatypeAdjustmentAllowStrings(mixed $value): mixed
+    protected static function datatypeAdjustmentAllowStrings(int|float|string|bool $value): int|float
     {
         if (is_bool($value)) {
             return (int) $value;
diff --git a/src/PhpSpreadsheet/Calculation/Statistical/VarianceBase.php b/src/PhpSpreadsheet/Calculation/Statistical/VarianceBase.php
index db6e691726..d5646cf9bc 100644
--- a/src/PhpSpreadsheet/Calculation/Statistical/VarianceBase.php
+++ b/src/PhpSpreadsheet/Calculation/Statistical/VarianceBase.php
@@ -6,7 +6,7 @@
 
 abstract class VarianceBase
 {
-    protected static function datatypeAdjustmentAllowStrings(mixed $value): mixed
+    protected static function datatypeAdjustmentAllowStrings(int|float|string|bool $value): int|float
     {
         if (is_bool($value)) {
             return (int) $value;
diff --git a/src/PhpSpreadsheet/Calculation/TextData/Format.php b/src/PhpSpreadsheet/Calculation/TextData/Format.php
index 38679379fb..67101ca1d0 100644
--- a/src/PhpSpreadsheet/Calculation/TextData/Format.php
+++ b/src/PhpSpreadsheet/Calculation/TextData/Format.php
@@ -126,6 +126,7 @@ public static function TEXTFORMAT(mixed $value, mixed $format): array|string
         $format = Helpers::extractString($format);
 
         if (!is_numeric($value) && Date::isDateTimeFormatCode($format)) {
+            // @phpstan-ignore-next-line
             $value = DateTimeExcel\DateValue::fromString($value) + DateTimeExcel\TimeValue::fromString($value);
         }
 
diff --git a/src/PhpSpreadsheet/Cell/AddressRange.php b/src/PhpSpreadsheet/Cell/AddressRange.php
index 0217504b58..22e9d11ecc 100644
--- a/src/PhpSpreadsheet/Cell/AddressRange.php
+++ b/src/PhpSpreadsheet/Cell/AddressRange.php
@@ -2,6 +2,9 @@
 
 namespace PhpOffice\PhpSpreadsheet\Cell;
 
+/**
+ * @template T
+ */
 interface AddressRange
 {
     public const MAX_ROW = 1048576;
@@ -10,8 +13,14 @@ interface AddressRange
 
     public const MAX_COLUMN_INT = 16384;
 
+    /**
+     * @return T
+     */
     public function from(): mixed;
 
+    /**
+     * @return T
+     */
     public function to(): mixed;
 
     public function __toString(): string;
diff --git a/src/PhpSpreadsheet/Cell/CellRange.php b/src/PhpSpreadsheet/Cell/CellRange.php
index c3445637de..677009dbe2 100644
--- a/src/PhpSpreadsheet/Cell/CellRange.php
+++ b/src/PhpSpreadsheet/Cell/CellRange.php
@@ -6,6 +6,9 @@
 use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet;
 use Stringable;
 
+/**
+ * @implements AddressRange<CellAddress>
+ */
 class CellRange implements AddressRange, Stringable
 {
     protected CellAddress $from;
diff --git a/src/PhpSpreadsheet/Cell/ColumnRange.php b/src/PhpSpreadsheet/Cell/ColumnRange.php
index e25b9b7e67..7f32a05c28 100644
--- a/src/PhpSpreadsheet/Cell/ColumnRange.php
+++ b/src/PhpSpreadsheet/Cell/ColumnRange.php
@@ -5,6 +5,9 @@
 use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet;
 use Stringable;
 
+/**
+ * @implements AddressRange<string>
+ */
 class ColumnRange implements AddressRange, Stringable
 {
     protected ?Worksheet $worksheet;
diff --git a/src/PhpSpreadsheet/Cell/RowRange.php b/src/PhpSpreadsheet/Cell/RowRange.php
index cb2f6b0e86..4ed232a931 100644
--- a/src/PhpSpreadsheet/Cell/RowRange.php
+++ b/src/PhpSpreadsheet/Cell/RowRange.php
@@ -5,6 +5,9 @@
 use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet;
 use Stringable;
 
+/**
+ * @implements AddressRange<int>
+ */
 class RowRange implements AddressRange, Stringable
 {
     protected ?Worksheet $worksheet;
diff --git a/src/PhpSpreadsheet/Shared/StringHelper.php b/src/PhpSpreadsheet/Shared/StringHelper.php
index 67fba22fee..18f8d0273b 100644
--- a/src/PhpSpreadsheet/Shared/StringHelper.php
+++ b/src/PhpSpreadsheet/Shared/StringHelper.php
@@ -636,9 +636,9 @@ public static function SYLKtoUTF8(string $textValue): string
      * Retrieve any leading numeric part of a string, or return the full string if no leading numeric
      * (handles basic integer or float, but not exponent or non decimal).
      *
-     * @return mixed string or only the leading numeric part of the string
+     * @return float|string string or only the leading numeric part of the string
      */
-    public static function testStringAsNumeric(string $textValue): mixed
+    public static function testStringAsNumeric(string $textValue): float|string
     {
         if (is_numeric($textValue)) {
             return $textValue;
diff --git a/src/PhpSpreadsheet/Style/ConditionalFormatting/ConditionalFormatValueObject.php b/src/PhpSpreadsheet/Style/ConditionalFormatting/ConditionalFormatValueObject.php
index 43f480804b..e6d1035f4e 100644
--- a/src/PhpSpreadsheet/Style/ConditionalFormatting/ConditionalFormatValueObject.php
+++ b/src/PhpSpreadsheet/Style/ConditionalFormatting/ConditionalFormatValueObject.php
@@ -8,9 +8,9 @@ class ConditionalFormatValueObject
 
     private null|float|int|string $value;
 
-    private mixed $cellFormula;
+    private ?string $cellFormula;
 
-    public function __construct(string $type, null|float|int|string $value = null, mixed $cellFormula = null)
+    public function __construct(string $type, null|float|int|string $value = null, ?string $cellFormula = null)
     {
         $this->type = $type;
         $this->value = $value;
@@ -41,12 +41,12 @@ public function setValue(null|float|int|string $value): self
         return $this;
     }
 
-    public function getCellFormula(): mixed
+    public function getCellFormula(): ?string
     {
         return $this->cellFormula;
     }
 
-    public function setCellFormula(mixed $cellFormula): self
+    public function setCellFormula(?string $cellFormula): self
     {
         $this->cellFormula = $cellFormula;
 
diff --git a/src/PhpSpreadsheet/Style/ConditionalFormatting/ConditionalFormattingRuleExtension.php b/src/PhpSpreadsheet/Style/ConditionalFormatting/ConditionalFormattingRuleExtension.php
index c4ef7eb620..c6c648ba24 100644
--- a/src/PhpSpreadsheet/Style/ConditionalFormatting/ConditionalFormattingRuleExtension.php
+++ b/src/PhpSpreadsheet/Style/ConditionalFormatting/ConditionalFormattingRuleExtension.php
@@ -162,12 +162,12 @@ private static function parseExtDataBarElementChildrenFromXml(ConditionalDataBar
         }
     }
 
-    public function getId(): mixed
+    public function getId(): string
     {
         return $this->id;
     }
 
-    public function setId(mixed $id): self
+    public function setId(string $id): self
     {
         $this->id = $id;
 
diff --git a/src/PhpSpreadsheet/Writer/Xls/Parser.php b/src/PhpSpreadsheet/Writer/Xls/Parser.php
index 3b078b4344..c51b490c92 100644
--- a/src/PhpSpreadsheet/Writer/Xls/Parser.php
+++ b/src/PhpSpreadsheet/Writer/Xls/Parser.php
@@ -1246,9 +1246,9 @@ private function parenthesizedExpression(): array
      * It parses a term. It assumes the following rule:
      * Term -> Fact [("*" | "/") Fact].
      *
-     * @return mixed The parsed ptg'd tree on success
+     * @return array The parsed ptg'd tree on success
      */
-    private function term(): mixed
+    private function term(): array
     {
         $result = $this->fact();
         while (
@@ -1277,9 +1277,9 @@ private function term(): mixed
      *       | Number
      *       | Function.
      *
-     * @return mixed The parsed ptg'd tree on success
+     * @return array The parsed ptg'd tree on success
      */
-    private function fact(): mixed
+    private function fact(): array
     {
         $currentToken = $this->currentToken;
         if ($currentToken === '(') {
@@ -1376,9 +1376,9 @@ private function fact(): mixed
      * It parses a function call. It assumes the following rule:
      * Func -> ( Expr [,Expr]* ).
      *
-     * @return mixed The parsed ptg'd tree on success
+     * @return array The parsed ptg'd tree on success
      */
-    private function func(): mixed
+    private function func(): array
     {
         $num_args = 0; // number of arguments received
         $function = strtoupper($this->currentToken);