includes/libs/Message: Add return type hints, update methods

Applied return type hints to improve type safety in Message classes.
Adjusted constructors to specify parameter types for consistency.
Enhanced `newFromJsonArray` in `ScalarParam`, `DataMessageValue`,
and `MessageParam` to clarify deserialization logic.

Key updates:
- Added `: string` return types for `getLangCode`, `getKey`, `dump`.
- Added `: array` return type for `getParams`, `getData`, `toJsonArray`.
- Revised `ScalarParam` JSON parsing to ensure a consistent return.

These adjustments enhance type safety across the Message classes.

Depends-On: Id7cc576693f5f1830e1d261cb13c626c901fb40f
Depends-On: I260e3f526be10f7b11e5c77a1e300590c6fc5b2b
Depends-On: I3c318273856d15407c71cb9b4ab7c23a0debb314
Change-Id: I7e50be91c01b4b70e1d1380651f001e35bd60523

19 files changed, 88 insertions, 80 deletions

diff --git a/RELEASE-NOTES-1.44 b/RELEASE-NOTES-1.44
index 3f6091e3204f..795f2c5699bb 100644
--- a/RELEASE-NOTES-1.44
+++ b/RELEASE-NOTES-1.44
@@ -116,6 +116,12 @@ because of Phabricator reports.
 * The MediaWiki\Message\Converter class, deprecated in 1.43, has been removed.
   Use MessageValue::newFromSpecifier or Message::newFromSpecifier
   instead (T358779).
+* Return type declarations were added to methods in `includes/libs/Message`,
+  specifically in the `MessageSpecifier`, `ITextFormatter`, and
+  `IMessageFormatterFactory` interfaces. This update enhances type safety but
+  may require updates in any classes implementing these interfaces, including
+  external extensions. Extensions such as Translate, Flow, and ReadingLists
+  require compatibility patches to support these changes.
 * The ParsoidOutputAccess class marked @unstable from creation and
   deprecated with all of its methods in 1.43, has been removed.
 * IDatabase::onAtomicSectionCancel() has been removed without deprecation
diff --git a/includes/language/Message/Message.php b/includes/language/Message/Message.php
index 0e219122e1b2..4fba5d6c7f06 100644
--- a/includes/language/Message/Message.php
+++ b/includes/language/Message/Message.php
@@ -399,7 +399,7 @@ class Message implements Stringable, MessageSpecifier, Serializable {
 	 *
 	 * @return string
 	 */
-	public function getKey() {
+	public function getKey(): string {
 		return $this->key;
 	}
 
@@ -410,7 +410,7 @@ class Message implements Stringable, MessageSpecifier, Serializable {
 	 *
 	 * @return (MessageParam|Message|string|int|float)[]
 	 */
-	public function getParams() {
+	public function getParams(): array {
 		return $this->parameters;
 	}
 
diff --git a/includes/language/Message/MessageFormatterFactory.php b/includes/language/Message/MessageFormatterFactory.php
index 344934ddcab0..25a1d66a15c2 100644
--- a/includes/language/Message/MessageFormatterFactory.php
+++ b/includes/language/Message/MessageFormatterFactory.php
@@ -31,7 +31,7 @@ class MessageFormatterFactory implements IMessageFormatterFactory {
 	/**
 	 * @inheritDoc
 	 */
-	public function getTextFormatter( $langCode ): ITextFormatter {
+	public function getTextFormatter( string $langCode ): ITextFormatter {
 		if ( !isset( $this->textFormatters[$langCode] ) ) {
 			$this->textFormatters[$langCode] = new TextFormatter(
 				$langCode, $this->format );
diff --git a/includes/language/Message/TextFormatter.php b/includes/language/Message/TextFormatter.php
index e463aef0d74e..244c74c4f9ea 100644
--- a/includes/language/Message/TextFormatter.php
+++ b/includes/language/Message/TextFormatter.php
@@ -33,7 +33,7 @@ class TextFormatter implements ITextFormatter {
 		$this->format = $format;
 	}
 
-	public function getLangCode() {
+	public function getLangCode(): string {
 		return $this->langCode;
 	}
 
diff --git a/includes/language/RawMessage.php b/includes/language/RawMessage.php
index 3c889d11bc29..b2ee93d86f5d 100644
--- a/includes/language/RawMessage.php
+++ b/includes/language/RawMessage.php
@@ -80,7 +80,7 @@ class RawMessage extends Message {
 	 * which is a real message key that can be used with MessageValue and other classes.
 	 * @return string
 	 */
-	public function getKey() {
+	public function getKey(): string {
 		return 'rawmessage';
 	}
 
@@ -89,7 +89,7 @@ class RawMessage extends Message {
 	 * 'rawmessage' message, and can be used with MessageValue and other classes.
 	 * @return string[]
 	 */
-	public function getParams() {
+	public function getParams(): array {
 		// If the provided text is equivalent to 'rawmessage', return the provided params.
 		if ( $this->key === '$1' ) {
 			return $this->parameters;
diff --git a/includes/libs/Message/DataMessageValue.php b/includes/libs/Message/DataMessageValue.php
index 710af0a349c6..79d716af8f67 100644
--- a/includes/libs/Message/DataMessageValue.php
+++ b/includes/libs/Message/DataMessageValue.php
@@ -25,11 +25,8 @@ use Wikimedia\JsonCodec\JsonCodecableTrait;
 class DataMessageValue extends MessageValue {
 	use JsonCodecableTrait;
 
-	/** @var string */
-	private $code;
-
-	/** @var array|null */
-	private $data;
+	private string $code;
+	private ?array $data;
 
 	/**
 	 * @stable to call
@@ -41,7 +38,7 @@ class DataMessageValue extends MessageValue {
 	 * @param array|null $data Structured data representing the concept
 	 *  behind this message.
 	 */
-	public function __construct( $key, $params = [], $code = null, ?array $data = null ) {
+	public function __construct( string $key, array $params = [], ?string $code = null, ?array $data = null ) {
 		parent::__construct( $key, $params );
 
 		$this->code = $code ?? $key;
@@ -50,13 +47,20 @@ class DataMessageValue extends MessageValue {
 
 	/**
 	 * Static constructor for easier chaining of `->params()` methods
+	 *
 	 * @param string $key
 	 * @param (MessageParam|MessageValue|string|int|float)[] $params
 	 * @param string|null $code
 	 * @param array|null $data
+	 *
 	 * @return DataMessageValue
 	 */
-	public static function new( $key, $params = [], $code = null, ?array $data = null ) {
+	public static function new(
+		string $key,
+		array $params = [],
+		?string $code = null,
+		?array $data = null
+	): DataMessageValue {
 		return new DataMessageValue( $key, $params, $code, $data );
 	}
 
@@ -64,7 +68,7 @@ class DataMessageValue extends MessageValue {
 	 * Get the message code
 	 * @return string
 	 */
-	public function getCode() {
+	public function getCode(): string {
 		return $this->code;
 	}
 
@@ -72,11 +76,11 @@ class DataMessageValue extends MessageValue {
 	 * Get the message's structured data
 	 * @return array|null
 	 */
-	public function getData() {
+	public function getData(): ?array {
 		return $this->data;
 	}
 
-	public function dump() {
+	public function dump(): string {
 		$contents = '';
 		if ( $this->getParams() ) {
 			$contents = '<params>';
@@ -100,12 +104,12 @@ class DataMessageValue extends MessageValue {
 		// WARNING: When changing how this class is serialized, follow the instructions
 		// at <https://www.mediawiki.org/wiki/Manual:Parser_cache/Serialization_compatibility>!
 		return parent::toJsonArray() + [
-			'code' => $this->code,
-			'data' => $this->data,
-		];
+				'code' => $this->code,
+				'data' => $this->data,
+			];
 	}
 
-	public static function newFromJsonArray( array $json ) {
+	public static function newFromJsonArray( array $json ): DataMessageValue {
 		// WARNING: When changing how this class is serialized, follow the instructions
 		// at <https://www.mediawiki.org/wiki/Manual:Parser_cache/Serialization_compatibility>!
 		return new self( $json['key'], $json['params'], $json['code'], $json['data'] );
diff --git a/includes/libs/Message/IMessageFormatterFactory.php b/includes/libs/Message/IMessageFormatterFactory.php
index 337ea82436d4..59ed05cef55d 100644
--- a/includes/libs/Message/IMessageFormatterFactory.php
+++ b/includes/libs/Message/IMessageFormatterFactory.php
@@ -14,5 +14,5 @@ interface IMessageFormatterFactory {
 	 * @param string $langCode The language code
 	 * @return ITextFormatter
 	 */
-	public function getTextFormatter( $langCode ): ITextFormatter;
+	public function getTextFormatter( string $langCode ): ITextFormatter;
 }
diff --git a/includes/libs/Message/ITextFormatter.php b/includes/libs/Message/ITextFormatter.php
index fa71a26a95f6..9d6ac9ea597e 100644
--- a/includes/libs/Message/ITextFormatter.php
+++ b/includes/libs/Message/ITextFormatter.php
@@ -20,7 +20,7 @@ interface ITextFormatter {
 	 * Get the internal language code in which format() is
 	 * @return string
 	 */
-	public function getLangCode();
+	public function getLangCode(): string;
 
 	/**
 	 * Convert a MessageSpecifier to text.
diff --git a/includes/libs/Message/ListParam.php b/includes/libs/Message/ListParam.php
index 058bc8525938..27435d7ec641 100644
--- a/includes/libs/Message/ListParam.php
+++ b/includes/libs/Message/ListParam.php
@@ -15,8 +15,7 @@ use Wikimedia\JsonCodec\JsonCodecableTrait;
 class ListParam extends MessageParam {
 	use JsonCodecableTrait;
 
-	/** @var string */
-	private $listType;
+	private string $listType;
 
 	/**
 	 * @stable to call.
@@ -25,7 +24,7 @@ class ListParam extends MessageParam {
 	 * @param (MessageParam|MessageSpecifier|string|int|float)[] $elements Values in the list.
 	 *  Values that are not instances of MessageParam are wrapped using ParamType::TEXT.
 	 */
-	public function __construct( $listType, array $elements ) {
+	public function __construct( string $listType, array $elements ) {
 		if ( !in_array( $listType, ListType::cases() ) ) {
 			throw new InvalidArgumentException( '$listType must be one of the ListType constants' );
 		}
@@ -46,16 +45,16 @@ class ListParam extends MessageParam {
 	 *
 	 * @return string One of the ListType constants
 	 */
-	public function getListType() {
+	public function getListType(): string {
 		return $this->listType;
 	}
 
-	public function dump() {
+	public function dump(): string {
 		$contents = '';
 		foreach ( $this->value as $element ) {
 			$contents .= $element->dump();
 		}
-		return "<{$this->type} listType=\"{$this->listType}\">$contents</{$this->type}>";
+		return "<$this->type listType=\"$this->listType\">$contents</$this->type>";
 	}
 
 	public function toJsonArray(): array {
@@ -67,7 +66,7 @@ class ListParam extends MessageParam {
 		];
 	}
 
-	public static function newFromJsonArray( array $json ) {
+	public static function newFromJsonArray( array $json ): ListParam {
 		// WARNING: When changing how this class is serialized, follow the instructions
 		// at <https://www.mediawiki.org/wiki/Manual:Parser_cache/Serialization_compatibility>!
 		if ( count( $json ) !== 2 || !isset( $json[ParamType::LIST] ) || !isset( $json['type'] ) ) {
diff --git a/includes/libs/Message/MessageParam.php b/includes/libs/Message/MessageParam.php
index 2292b6181db6..fc26a318c4e2 100644
--- a/includes/libs/Message/MessageParam.php
+++ b/includes/libs/Message/MessageParam.php
@@ -11,8 +11,7 @@ use Wikimedia\JsonCodec\JsonCodecable;
  */
 abstract class MessageParam implements JsonCodecable {
 
-	/** @var string */
-	protected $type;
+	protected string $type;
 	/** @var mixed */
 	protected $value;
 
@@ -21,7 +20,7 @@ abstract class MessageParam implements JsonCodecable {
 	 *
 	 * @return string One of the ParamType constants
 	 */
-	public function getType() {
+	public function getType(): string {
 		return $this->type;
 	}
 
@@ -39,5 +38,5 @@ abstract class MessageParam implements JsonCodecable {
 	 *
 	 * @return string
 	 */
-	abstract public function dump();
+	abstract public function dump(): string;
 }
diff --git a/includes/libs/Message/MessageSpecifier.php b/includes/libs/Message/MessageSpecifier.php
index 843e431a8b18..dea4b975df0b 100644
--- a/includes/libs/Message/MessageSpecifier.php
+++ b/includes/libs/Message/MessageSpecifier.php
@@ -33,14 +33,14 @@ interface MessageSpecifier {
 	 *
 	 * @return string
 	 */
-	public function getKey();
+	public function getKey(): string;
 
 	/**
 	 * Returns the message parameters
 	 *
 	 * @return (MessageParam|MessageSpecifier|string|int|float)[]
 	 */
-	public function getParams();
+	public function getParams(): array;
 }
 
 /**
diff --git a/includes/libs/Message/MessageValue.php b/includes/libs/Message/MessageValue.php
index ca25cca2c3c9..79ad579a8a60 100644
--- a/includes/libs/Message/MessageValue.php
+++ b/includes/libs/Message/MessageValue.php
@@ -19,11 +19,10 @@ use Wikimedia\JsonCodec\JsonCodecableTrait;
 class MessageValue implements MessageSpecifier, JsonCodecable {
 	use JsonCodecableTrait;
 
-	/** @var string */
-	private $key;
+	private string $key;
 
 	/** @var MessageParam[] */
-	private $params;
+	private array $params;
 
 	/**
 	 * @stable to call
@@ -32,7 +31,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param (MessageParam|MessageSpecifier|string|int|float)[] $params Values that are not instances
 	 *  of MessageParam are wrapped using ParamType::TEXT.
 	 */
-	public function __construct( $key, $params = [] ) {
+	public function __construct( string $key, array $params = [] ) {
 		$this->key = $key;
 		$this->params = [];
 		$this->params( ...$params );
@@ -44,7 +43,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param (MessageParam|MessageSpecifier|string|int|float)[] $params
 	 * @return MessageValue
 	 */
-	public static function new( $key, $params = [] ) {
+	public static function new( string $key, array $params = [] ): MessageValue {
 		return new MessageValue( $key, $params );
 	}
 
@@ -57,7 +56,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param MessageSpecifier $spec
 	 * @return MessageValue
 	 */
-	public static function newFromSpecifier( MessageSpecifier $spec ) {
+	public static function newFromSpecifier( MessageSpecifier $spec ): MessageValue {
 		if ( $spec instanceof MessageValue ) {
 			return $spec;
 		}
@@ -69,7 +68,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 *
 	 * @return string
 	 */
-	public function getKey() {
+	public function getKey(): string {
 		return $this->key;
 	}
 
@@ -78,7 +77,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 *
 	 * @return MessageParam[]
 	 */
-	public function getParams() {
+	public function getParams(): array {
 		return $this->params;
 	}
 
@@ -88,7 +87,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param MessageParam|MessageSpecifier|string|int|float ...$values
 	 * @return $this
 	 */
-	public function params( ...$values ) {
+	public function params( ...$values ): MessageValue {
 		foreach ( $values as $value ) {
 			if ( $value instanceof MessageParam ) {
 				$this->params[] = $value;
@@ -106,7 +105,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param MessageSpecifier|string|int|float ...$values Scalar values
 	 * @return $this
 	 */
-	public function textParamsOfType( $type, ...$values ) {
+	public function textParamsOfType( string $type, ...$values ): MessageValue {
 		foreach ( $values as $value ) {
 			$this->params[] = new ScalarParam( $type, $value );
 		}
@@ -121,7 +120,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 *  is an array of items suitable to pass as $params to ListParam::__construct()
 	 * @return $this
 	 */
-	public function listParamsOfType( $listType, ...$values ) {
+	public function listParamsOfType( string $listType, ...$values ): MessageValue {
 		foreach ( $values as $value ) {
 			$this->params[] = new ListParam( $listType, $value );
 		}
@@ -134,7 +133,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param MessageSpecifier|string|int|float ...$values
 	 * @return $this
 	 */
-	public function textParams( ...$values ) {
+	public function textParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::TEXT, ...$values );
 	}
 
@@ -144,7 +143,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param int|float ...$values
 	 * @return $this
 	 */
-	public function numParams( ...$values ) {
+	public function numParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::NUM, ...$values );
 	}
 
@@ -158,7 +157,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param int|float ...$values
 	 * @return $this
 	 */
-	public function longDurationParams( ...$values ) {
+	public function longDurationParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::DURATION_LONG, ...$values );
 	}
 
@@ -172,7 +171,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param int|float ...$values
 	 * @return $this
 	 */
-	public function shortDurationParams( ...$values ) {
+	public function shortDurationParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::DURATION_SHORT, ...$values );
 	}
 
@@ -183,7 +182,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 *  or "infinity"
 	 * @return $this
 	 */
-	public function expiryParams( ...$values ) {
+	public function expiryParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::EXPIRY, ...$values );
 	}
 
@@ -194,7 +193,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param string ...$values Timestamp as accepted by the Wikimedia\Timestamp library.
 	 * @return $this
 	 */
-	public function dateTimeParams( ...$values ) {
+	public function dateTimeParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::DATETIME, ...$values );
 	}
 
@@ -205,7 +204,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param string ...$values Timestamp as accepted by the Wikimedia\Timestamp library.
 	 * @return $this
 	 */
-	public function dateParams( ...$values ) {
+	public function dateParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::DATE, ...$values );
 	}
 
@@ -216,7 +215,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param string ...$values Timestamp as accepted by the Wikimedia\Timestamp library.
 	 * @return $this
 	 */
-	public function timeParams( ...$values ) {
+	public function timeParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::TIME, ...$values );
 	}
 
@@ -227,7 +226,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param string ...$values User Groups
 	 * @return $this
 	 */
-	public function userGroupParams( ...$values ) {
+	public function userGroupParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::GROUP, ...$values );
 	}
 
@@ -237,7 +236,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param int ...$values
 	 * @return $this
 	 */
-	public function sizeParams( ...$values ) {
+	public function sizeParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::SIZE, ...$values );
 	}
 
@@ -248,7 +247,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param int|float ...$values
 	 * @return $this
 	 */
-	public function bitrateParams( ...$values ) {
+	public function bitrateParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::BITRATE, ...$values );
 	}
 
@@ -262,7 +261,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param string ...$values
 	 * @return $this
 	 */
-	public function rawParams( ...$values ) {
+	public function rawParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::RAW, ...$values );
 	}
 
@@ -276,7 +275,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param string ...$values
 	 * @return $this
 	 */
-	public function plaintextParams( ...$values ) {
+	public function plaintextParams( ...$values ): MessageValue {
 		return $this->textParamsOfType( ParamType::PLAINTEXT, ...$values );
 	}
 
@@ -290,7 +289,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 *  is an array of items suitable to pass as $params to ListParam::__construct()
 	 * @return $this
 	 */
-	public function commaListParams( ...$values ) {
+	public function commaListParams( ...$values ): MessageValue {
 		return $this->listParamsOfType( ListType::COMMA, ...$values );
 	}
 
@@ -304,7 +303,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 *  is an array of items suitable to pass as $params to ListParam::__construct()
 	 * @return $this
 	 */
-	public function semicolonListParams( ...$values ) {
+	public function semicolonListParams( ...$values ): MessageValue {
 		return $this->listParamsOfType( ListType::SEMICOLON, ...$values );
 	}
 
@@ -318,7 +317,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 *  is an array of items suitable to pass as $params to ListParam::__construct()
 	 * @return $this
 	 */
-	public function pipeListParams( ...$values ) {
+	public function pipeListParams( ...$values ): MessageValue {
 		return $this->listParamsOfType( ListType::PIPE, ...$values );
 	}
 
@@ -332,7 +331,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 * @param (MessageParam|string)[] ...$values
 	 * @return $this
 	 */
-	public function textListParams( ...$values ) {
+	public function textListParams( ...$values ): MessageValue {
 		return $this->listParamsOfType( ListType::AND, ...$values );
 	}
 
@@ -341,7 +340,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 	 *
 	 * @return string
 	 */
-	public function dump() {
+	public function dump(): string {
 		$contents = '';
 		foreach ( $this->params as $param ) {
 			$contents .= $param->dump();
@@ -359,7 +358,7 @@ class MessageValue implements MessageSpecifier, JsonCodecable {
 		];
 	}
 
-	public static function newFromJsonArray( array $json ) {
+	public static function newFromJsonArray( array $json ): MessageValue {
 		// WARNING: When changing how this class is serialized, follow the instructions
 		// at <https://www.mediawiki.org/wiki/Manual:Parser_cache/Serialization_compatibility>!
 		return new self( $json['key'], $json['params'] );
diff --git a/includes/libs/Message/ScalarParam.php b/includes/libs/Message/ScalarParam.php
index 82f6eb4dc56f..e698367ab405 100644
--- a/includes/libs/Message/ScalarParam.php
+++ b/includes/libs/Message/ScalarParam.php
@@ -24,7 +24,7 @@ class ScalarParam extends MessageParam {
 	 * @param string $type One of the ParamType constants.
 	 * @param string|int|float|MessageSpecifier|Stringable $value
 	 */
-	public function __construct( $type, $value ) {
+	public function __construct( string $type, $value ) {
 		if ( !in_array( $type, ParamType::cases() ) ) {
 			throw new InvalidArgumentException( '$type must be one of the ParamType constants' );
 		}
@@ -57,13 +57,13 @@ class ScalarParam extends MessageParam {
 		$this->value = $value;
 	}
 
-	public function dump() {
+	public function dump(): string {
 		if ( $this->value instanceof MessageValue ) {
 			$contents = $this->value->dump();
 		} else {
 			$contents = htmlspecialchars( (string)$this->value );
 		}
-		return "<{$this->type}>" . $contents . "</{$this->type}>";
+		return "<$this->type>" . $contents . "</$this->type>";
 	}
 
 	public function toJsonArray(): array {
@@ -74,15 +74,16 @@ class ScalarParam extends MessageParam {
 		];
 	}
 
-	public static function newFromJsonArray( array $json ) {
+	public static function newFromJsonArray( array $json ): ScalarParam {
 		// WARNING: When changing how this class is serialized, follow the instructions
 		// at <https://www.mediawiki.org/wiki/Manual:Parser_cache/Serialization_compatibility>!
 		if ( count( $json ) !== 1 ) {
 			throw new InvalidArgumentException( 'Invalid format' );
 		}
-		// Use a dummy loop to get the first (and only) key/value pair in the array.
-		foreach ( $json as $type => $value ) {
-			return new self( $type, $value );
-		}
+
+		$type = key( $json );
+		$value = current( $json );
+
+		return new self( $type, $value );
 	}
 }
diff --git a/includes/libs/rdbms/exception/DBExpectedError.php b/includes/libs/rdbms/exception/DBExpectedError.php
index 4397fc293da9..f37c191e4102 100644
--- a/includes/libs/rdbms/exception/DBExpectedError.php
+++ b/includes/libs/rdbms/exception/DBExpectedError.php
@@ -47,11 +47,11 @@ class DBExpectedError extends DBError implements MessageSpecifier {
 		$this->params = $params;
 	}
 
-	public function getKey() {
+	public function getKey(): string {
 		return 'databaseerror-text';
 	}
 
-	public function getParams() {
+	public function getParams(): array {
 		return $this->params;
 	}
 }
diff --git a/includes/libs/rdbms/exception/DBQueryTimeoutError.php b/includes/libs/rdbms/exception/DBQueryTimeoutError.php
index 8317ad0d2486..546381deaceb 100644
--- a/includes/libs/rdbms/exception/DBQueryTimeoutError.php
+++ b/includes/libs/rdbms/exception/DBQueryTimeoutError.php
@@ -45,7 +45,7 @@ class DBQueryTimeoutError extends DBQueryError {
 		parent::__construct( $db, $error, $errno, $sql, $fname, $message );
 	}
 
-	public function getKey() {
+	public function getKey(): string {
 		return 'transaction-max-statement-time-exceeded';
 	}
 
diff --git a/includes/libs/rdbms/exception/DBTransactionSizeError.php b/includes/libs/rdbms/exception/DBTransactionSizeError.php
index bea1300c5b08..64b454e42139 100644
--- a/includes/libs/rdbms/exception/DBTransactionSizeError.php
+++ b/includes/libs/rdbms/exception/DBTransactionSizeError.php
@@ -24,7 +24,7 @@ namespace Wikimedia\Rdbms;
  * @ingroup Database
  */
 class DBTransactionSizeError extends DBTransactionError {
-	public function getKey() {
+	public function getKey(): string {
 		return 'transaction-duration-limit-exceeded';
 	}
 }
diff --git a/tests/phpunit/integration/includes/Rest/Handler/DiscoveryHandlerTest.php b/tests/phpunit/integration/includes/Rest/Handler/DiscoveryHandlerTest.php
index e99e3da3abc7..429e86646c6a 100644
--- a/tests/phpunit/integration/includes/Rest/Handler/DiscoveryHandlerTest.php
+++ b/tests/phpunit/integration/includes/Rest/Handler/DiscoveryHandlerTest.php
@@ -46,7 +46,7 @@ class DiscoveryHandlerTest extends MediaWikiIntegrationTestCase {
 		);
 
 		$formatter = new class implements ITextFormatter {
-			public function getLangCode() {
+			public function getLangCode(): string {
 				return 'qqx';
 			}
 
diff --git a/tests/phpunit/integration/includes/Rest/Handler/ModuleSpecHandlerTest.php b/tests/phpunit/integration/includes/Rest/Handler/ModuleSpecHandlerTest.php
index e58b6d3fbff5..3a2695894f33 100644
--- a/tests/phpunit/integration/includes/Rest/Handler/ModuleSpecHandlerTest.php
+++ b/tests/phpunit/integration/includes/Rest/Handler/ModuleSpecHandlerTest.php
@@ -48,7 +48,7 @@ class ModuleSpecHandlerTest extends MediaWikiIntegrationTestCase {
 		);
 
 		$formatter = new class implements ITextFormatter {
-			public function getLangCode() {
+			public function getLangCode(): string {
 				return 'qqx';
 			}
 
diff --git a/tests/phpunit/mocks/DummyServicesTrait.php b/tests/phpunit/mocks/DummyServicesTrait.php
index 99272229537a..160c0386b8ae 100644
--- a/tests/phpunit/mocks/DummyServicesTrait.php
+++ b/tests/phpunit/mocks/DummyServicesTrait.php
@@ -471,7 +471,7 @@ trait DummyServicesTrait {
 				$this->dumpMessages = $dumpMessages;
 			}
 
-			public function getLangCode() {
+			public function getLangCode(): string {
 				return 'qqx';
 			}