diff --git a/includes/Title.php b/includes/Title.php
index 5bb8ab80e74..b7359aaff7a 100644
--- a/includes/Title.php
+++ b/includes/Title.php
@@ -156,7 +156,8 @@ class Title implements LinkTarget, IDBAccessObject {
 	/**
 	 * @var int Namespace index when there is no namespace. Don't change the
 	 *   following default, NS_MAIN is hardcoded in several places. See T2696.
-	 *   Zero except in {{transclusion}} tags.
+	 *   Used primarily for {{transclusion}} tags.
+	 * @see newFromText()
 	 */
 	public $mDefaultNamespace = NS_MAIN;
 
@@ -185,6 +186,9 @@ class Title implements LinkTarget, IDBAccessObject {
 
 	/** @var bool|null Would deleting this page be a big deletion? */
 	private $mIsBigDeletion = null;
+
+	/** @var bool|null Is the title known to be valid? */
+	private $mIsValid = null;
 	// @}
 
 	/**
@@ -227,10 +231,9 @@ class Title implements LinkTarget, IDBAccessObject {
 	 */
 	public static function newFromDBkey( $key ) {
 		$t = new self();
-		$t->mDbkeyform = $key;
 
 		try {
-			$t->secureAndSplit();
+			$t->secureAndSplit( $key );
 			return $t;
 		} catch ( MalformedTitleException $ex ) {
 			return null;
@@ -294,11 +297,10 @@ class Title implements LinkTarget, IDBAccessObject {
 	}
 
 	/**
-	 * Create a new Title from text, such as what one would find in a link. De-
-	 * codes any HTML entities in the text.
-	 *
-	 * Title objects returned by this method are guaranteed to be valid, and
-	 * thus return true from the isValid() method.
+	 * Create a new Title from text, such as what one would find in a link.
+	 * Decodes any HTML entities in the text.
+	 * Titles returned by this method are guaranteed to be valid.
+	 * Call canExist() to check if the Title represents an editable page.
 	 *
 	 * @note The Title instance returned by this method is not guaranteed to be a fresh instance.
 	 * It may instead be a cached instance created previously, with references to it remaining
@@ -311,7 +313,8 @@ class Title implements LinkTarget, IDBAccessObject {
 	 *   $text might begin with a namespace prefix, use makeTitle() or
 	 *   makeTitleSafe().
 	 * @throws InvalidArgumentException
-	 * @return Title|null Title or null on an error.
+	 * @return Title|null Title or null if the Title could not be parsed because
+	 *         it is invalid.
 	 */
 	public static function newFromText( $text, $defaultNamespace = NS_MAIN ) {
 		// DWIM: Integers can be passed in here when page titles are used as array keys.
@@ -323,7 +326,7 @@ class Title implements LinkTarget, IDBAccessObject {
 		}
 
 		try {
-			return self::newFromTextThrow( (string)$text, $defaultNamespace );
+			return self::newFromTextThrow( (string)$text, (int)$defaultNamespace );
 		} catch ( MalformedTitleException $ex ) {
 			return null;
 		}
@@ -333,10 +336,8 @@ class Title implements LinkTarget, IDBAccessObject {
 	 * Like Title::newFromText(), but throws MalformedTitleException when the title is invalid,
 	 * rather than returning null.
 	 *
-	 * The exception subclasses encode detailed information about why the title is invalid.
-	 *
-	 * Title objects returned by this method are guaranteed to be valid, and
-	 * thus return true from the isValid() method.
+	 * Titles returned by this method are guaranteed to be valid.
+	 * Call canExist() to check if the Title represents an editable page.
 	 *
 	 * @note The Title instance returned by this method is not guaranteed to be a fresh instance.
 	 * It may instead be a cached instance created previously, with references to it remaining
@@ -347,7 +348,7 @@ class Title implements LinkTarget, IDBAccessObject {
 	 * @since 1.25
 	 * @param string $text Title text to check
 	 * @param int $defaultNamespace
-	 * @throws MalformedTitleException If the title is invalid
+	 * @throws MalformedTitleException If the title is invalid.
 	 * @return Title
 	 */
 	public static function newFromTextThrow( $text, $defaultNamespace = NS_MAIN ) {
@@ -376,10 +377,9 @@ class Title implements LinkTarget, IDBAccessObject {
 		$filteredText = Sanitizer::decodeCharReferencesAndNormalize( $text );
 
 		$t = new Title();
-		$t->mDbkeyform = strtr( $filteredText, ' ', '_' );
-		$t->mDefaultNamespace = (int)$defaultNamespace;
+		$dbKeyForm = strtr( $filteredText, ' ', '_' );
 
-		$t->secureAndSplit();
+		$t->secureAndSplit( $dbKeyForm, (int)$defaultNamespace );
 		if ( $defaultNamespace == NS_MAIN ) {
 			$titleCache->set( $text, $t );
 		}
@@ -411,10 +411,10 @@ class Title implements LinkTarget, IDBAccessObject {
 			$url = strtr( $url, '+', ' ' );
 		}
 
-		$t->mDbkeyform = strtr( $url, ' ', '_' );
+		$dbKeyForm = strtr( $url, ' ', '_' );
 
 		try {
-			$t->secureAndSplit();
+			$t->secureAndSplit( $dbKeyForm );
 			return $t;
 		} catch ( MalformedTitleException $ex ) {
 			return null;
@@ -600,9 +600,8 @@ class Title implements LinkTarget, IDBAccessObject {
 	 * The parameters will be checked for validity, which is a bit slower
 	 * than makeTitle() but safer for user-provided data.
 	 *
-	 * Title objects returned by makeTitleSafe() are guaranteed to be valid,
-	 * that is, they return true from the isValid() method. If no valid Title
-	 * can be constructed from the input, this method returns null.
+	 * The Title object returned by this method is guaranteed to be valid.
+	 * Call canExist() to check if the Title represents an editable page.
 	 *
 	 * @param int $ns The namespace of the article
 	 * @param string $title Database key form
@@ -619,10 +618,10 @@ class Title implements LinkTarget, IDBAccessObject {
 		}
 
 		$t = new Title();
-		$t->mDbkeyform = self::makeName( $ns, $title, $fragment, $interwiki, true );
+		$dbKeyForm = self::makeName( $ns, $title, $fragment, $interwiki, true );
 
 		try {
-			$t->secureAndSplit();
+			$t->secureAndSplit( $dbKeyForm );
 			return $t;
 		} catch ( MalformedTitleException $ex ) {
 			return null;
@@ -844,47 +843,51 @@ class Title implements LinkTarget, IDBAccessObject {
 	}
 
 	/**
-	 * Returns true if the title is valid, false if it is invalid.
+	 * Returns true if the title is a valid link target, and that it has been
+	 * properly normalized. This method checks that the title is syntactically valid,
+	 * and that the namespace it refers to exists.
 	 *
-	 * Valid titles can be round-tripped via makeTitle() and newFromText().
-	 * Their DB key can be used in the database, though it may not have the correct
-	 * capitalization.
+	 * Titles constructed using newFromText() or makeTitleSafe() are always valid.
 	 *
-	 * Invalid titles may get returned from makeTitle(), and it may be useful to
-	 * allow them to exist, e.g. in order to process log entries about pages in
-	 * namespaces that belong to extensions that are no longer installed.
+	 * @note Code that wants to check whether the title can represent a page that can
+	 * be created and edited should use canExist() instead. Examples of valid titles
+	 * that cannot "exist" are Special pages, interwiki links, and on-page section links
+	 * that only have the fragment part set.
 	 *
-	 * @note This method is relatively expensive. When constructing Title
-	 * objects that need to be valid, use an instantiator method that is guaranteed
-	 * to return valid titles, such as makeTitleSafe() or newFromText().
+	 * @see canExist()
 	 *
 	 * @return bool
 	 */
 	public function isValid() {
-		$services = MediaWikiServices::getInstance();
-		if ( !$services->getNamespaceInfo()->exists( $this->mNamespace ) ) {
-			return false;
+		if ( $this->mIsValid !== null ) {
+			return $this->mIsValid;
 		}
 
 		try {
-			$services->getTitleParser()->parseTitle( $this->mDbkeyform, $this->mNamespace );
+			$text = $this->getFullText();
+			$titleCodec = MediaWikiServices::getInstance()->getTitleParser();
+
+			'@phan-var MediaWikiTitleCodec $titleCodec';
+			$parts = $titleCodec->splitTitleString( $text, $this->mNamespace );
+
+			// Check that nothing changed!
+			// This ensures that $text was already perperly normalized.
+			if ( $parts['fragment'] !== $this->mFragment
+				|| $parts['interwiki'] !== $this->mInterwiki
+				|| $parts['local_interwiki'] !== $this->mLocalInterwiki
+				|| $parts['namespace'] !== $this->mNamespace
+				|| $parts['dbkey'] !== $this->mDbkeyform
+			) {
+				$this->mIsValid = false;
+				return $this->mIsValid;
+			}
 		} catch ( MalformedTitleException $ex ) {
-			return false;
+			$this->mIsValid = false;
+			return $this->mIsValid;
 		}
 
-		try {
-			// Title value applies basic syntax checks. Should perhaps be moved elsewhere.
-			new TitleValue(
-				$this->mNamespace,
-				$this->mDbkeyform,
-				$this->mFragment,
-				$this->mInterwiki
-			);
-		} catch ( InvalidArgumentException $ex ) {
-			return false;
-		}
-
-		return true;
+		$this->mIsValid = true;
+		return $this->mIsValid;
 	}
 
 	/**
@@ -1179,12 +1182,49 @@ class Title implements LinkTarget, IDBAccessObject {
 	}
 
 	/**
-	 * Is this in a namespace that allows actual pages?
+	 * Can this title represent a page in the wiki's database?
 	 *
-	 * @return bool
+	 * Titles can exist as pages in the database if they are valid, and they
+	 * are not Special pages, interwiki links, or fragment-only links.
+	 *
+	 * @see isValid()
+	 *
+	 * @return bool true if and only if this title can be used to perform an edit.
 	 */
 	public function canExist() {
-		return $this->mNamespace >= NS_MAIN;
+		// NOTE: Don't use getArticleID(), we don't want to
+		// trigger a database query here. This check is supposed to
+		// act as an optimization, not add extra cost.
+		if ( $this->mArticleID > 0 ) {
+			// It exists, so it can exist.
+			return true;
+		}
+
+		// NOTE: we call the relatively expensive isValid() method further down,
+		// but we can bail out early if we already know the title is invalid.
+		if ( $this->mIsValid === false ) {
+			// It's invalid, so it can't exist.
+			return false;
+		}
+
+		if ( $this->getNamespace() < NS_MAIN ) {
+			// It's a special page, so it can't exist in the database.
+			return false;
+		}
+
+		if ( $this->isExternal() ) {
+			// If it's external, it's not local, so it can't exist.
+			return false;
+		}
+
+		if ( $this->getText() === '' ) {
+			// The title has no text, so it can't exist in the database.
+			// It's probably an on-page section link, like "#something".
+			return false;
+		}
+
+		// Double check that the title is valid.
+		return $this->isValid();
 	}
 
 	/**
@@ -3285,16 +3325,24 @@ class Title implements LinkTarget, IDBAccessObject {
 	/**
 	 * Secure and split - main initialisation function for this object
 	 *
-	 * Assumes that mDbkeyform has been set, and is urldecoded
+	 * Assumes that $text is urldecoded
 	 * and uses underscores, but not otherwise munged.  This function
 	 * removes illegal characters, splits off the interwiki and
 	 * namespace prefixes, sets the other forms, and canonicalizes
 	 * everything.
 	 *
-	 * @throws MalformedTitleException On invalid titles
-	 * @return bool True on success
+	 * If this method returns normally, the Title is valid.
+	 *
+	 * @param string $text
+	 * @param int|null $defaultNamespace
+	 *
+	 * @throws MalformedTitleException On malformed titles
 	 */
-	private function secureAndSplit() {
+	private function secureAndSplit( $text, $defaultNamespace = null ) {
+		if ( $defaultNamespace === null ) {
+			$defaultNamespace = $this->mDefaultNamespace;
+		}
+
 		// @note: splitTitleString() is a temporary hack to allow MediaWikiTitleCodec to share
 		//        the parsing code with Title, while avoiding massive refactoring.
 		// @todo: get rid of secureAndSplit, refactor parsing code.
@@ -3304,25 +3352,27 @@ class Title implements LinkTarget, IDBAccessObject {
 		$titleCodec = MediaWikiServices::getInstance()->getTitleParser();
 		'@phan-var MediaWikiTitleCodec $titleCodec';
 		// MalformedTitleException can be thrown here
-		$parts = $titleCodec->splitTitleString( $this->mDbkeyform, $this->mDefaultNamespace );
+		$parts = $titleCodec->splitTitleString( $text, $defaultNamespace );
 
 		# Fill fields
 		$this->setFragment( '#' . $parts['fragment'] );
 		$this->mInterwiki = $parts['interwiki'];
 		$this->mLocalInterwiki = $parts['local_interwiki'];
 		$this->mNamespace = $parts['namespace'];
+		$this->mDefaultNamespace = $defaultNamespace;
 		$this->mUserCaseDBKey = $parts['user_case_dbkey'];
 
 		$this->mDbkeyform = $parts['dbkey'];
 		$this->mUrlform = wfUrlencode( $this->mDbkeyform );
 		$this->mTextform = strtr( $this->mDbkeyform, '_', ' ' );
 
+		// splitTitleString() guarantees that this title is valid.
+		$this->mIsValid = true;
+
 		# We already know that some pages won't be in the database!
-		if ( $this->isExternal() || $this->isSpecialPage() ) {
+		if ( $this->isExternal() || $this->isSpecialPage() || $this->mTextform === '' ) {
 			$this->mArticleID = 0;
 		}
-
-		return true;
 	}
 
 	/**
diff --git a/includes/changes/ChangesFeed.php b/includes/changes/ChangesFeed.php
index e60cc0937be..6a8d9a9e54a 100644
--- a/includes/changes/ChangesFeed.php
+++ b/includes/changes/ChangesFeed.php
@@ -94,7 +94,7 @@ class ChangesFeed {
 		$nsInfo = MediaWikiServices::getInstance()->getNamespaceInfo();
 		foreach ( $sorted as $obj ) {
 			$title = Title::makeTitle( $obj->rc_namespace, $obj->rc_title );
-			$talkpage = $nsInfo->hasTalkNamespace( $obj->rc_namespace ) && $title->isValid()
+			$talkpage = $nsInfo->hasTalkNamespace( $obj->rc_namespace ) && $title->canExist()
 				? $title->getTalkPage()->getFullURL()
 				: '';
 
diff --git a/includes/logging/BlockLogFormatter.php b/includes/logging/BlockLogFormatter.php
index d49e3c5710d..fc21d880ba2 100644
--- a/includes/logging/BlockLogFormatter.php
+++ b/includes/logging/BlockLogFormatter.php
@@ -129,7 +129,7 @@ class BlockLogFormatter extends LogFormatter {
 	public function getPreloadTitles() {
 		$title = $this->entry->getTarget();
 		// Preload user page for non-autoblocks
-		if ( substr( $title->getText(), 0, 1 ) !== '#' && $title->isValid() ) {
+		if ( substr( $title->getText(), 0, 1 ) !== '#' && $title->canExist() ) {
 			return [ $title->getTalkPage() ];
 		}
 		return [];
diff --git a/includes/title/MediaWikiTitleCodec.php b/includes/title/MediaWikiTitleCodec.php
index 3ebb4430ece..484c51e5c8a 100644
--- a/includes/title/MediaWikiTitleCodec.php
+++ b/includes/title/MediaWikiTitleCodec.php
@@ -167,17 +167,11 @@ class MediaWikiTitleCodec implements TitleFormatter, TitleParser {
 		// Convert things like &eacute; &#257; or &#x3017; into normalized (T16952) text
 		$filteredText = Sanitizer::decodeCharReferencesAndNormalize( $text );
 
-		// NOTE: this is an ugly cludge that allows this class to share the
+		// NOTE: this is an ugly kludge that allows this class to share the
 		// code for parsing with the old Title class. The parser code should
 		// be refactored to avoid this.
 		$parts = $this->splitTitleString( $filteredText, $defaultNamespace );
 
-		// Fragment-only is okay, but only with no namespace
-		if ( $parts['dbkey'] === '' &&
-		( $parts['fragment'] === '' || $parts['namespace'] !== NS_MAIN ) ) {
-			throw new MalformedTitleException( 'title-invalid-empty', $text );
-		}
-
 		return new TitleValue(
 			$parts['namespace'],
 			$parts['dbkey'],
@@ -284,7 +278,8 @@ class MediaWikiTitleCodec implements TitleFormatter, TitleParser {
 	}
 
 	/**
-	 * Normalizes and splits a title string.
+	 * Validates, normalizes and splits a title string.
+	 * This is the "source of truth" for title validity.
 	 *
 	 * This function removes illegal characters, splits off the interwiki and
 	 * namespace prefixes, sets the other forms, and canonicalizes
@@ -494,9 +489,22 @@ class MediaWikiTitleCodec implements TitleFormatter, TitleParser {
 			throw new MalformedTitleException( 'title-invalid-leading-colon', $text );
 		}
 
-		# Fill fields
+		// Fill fields
 		$parts['dbkey'] = $dbkey;
 
+		// Sanity check to ensure that the return value can be used to construct a TitleValue.
+		// All issues should in theory be caught above, this is here to enforce consistency.
+		try {
+			TitleValue::assertValidSpec(
+				$parts['namespace'],
+				$parts['dbkey'],
+				$parts['fragment'],
+				$parts['interwiki']
+			);
+		} catch ( InvalidArgumentException $ex ) {
+			throw new MalformedTitleException( 'title-invalid', $text, [ $ex->getMessage() ] );
+		}
+
 		return $parts;
 	}
 
diff --git a/includes/title/TitleValue.php b/includes/title/TitleValue.php
index 7abe21ba67a..109659fd440 100644
--- a/includes/title/TitleValue.php
+++ b/includes/title/TitleValue.php
@@ -86,6 +86,28 @@ class TitleValue implements LinkTarget {
 	 * @throws InvalidArgumentException
 	 */
 	public function __construct( $namespace, $title, $fragment = '', $interwiki = '' ) {
+		self::assertValidSpec( $namespace, $title, $fragment, $interwiki );
+
+		$this->namespace = $namespace;
+		$this->dbkey = strtr( $title, ' ', '_' );
+		$this->fragment = $fragment;
+		$this->interwiki = $interwiki;
+	}
+
+	/**
+	 * Asserts that the given parameters could be used to construct a TitleValue object.
+	 * Performs basic syntax and consistency checks. Does not perform full validation,
+	 * use TitleParser::makeTitleValueSafe() for that.
+	 *
+	 * @param int $namespace
+	 * @param string $title
+	 * @param string $fragment
+	 * @param string $interwiki
+	 *
+	 * @throws InvalidArgumentException if the combination of parameters is not valid for
+	 *         constructing a TitleValue.
+	 */
+	public static function assertValidSpec( $namespace, $title, $fragment = '', $interwiki = '' ) {
 		if ( !is_int( $namespace ) ) {
 			throw new ParameterTypeException( '$namespace', 'int' );
 		}
@@ -99,20 +121,19 @@ class TitleValue implements LinkTarget {
 			throw new ParameterTypeException( '$interwiki', 'string' );
 		}
 
-		// Sanity check, no full validation or normalization applied here!
 		Assert::parameter( !preg_match( '/^[_ ]|[\r\n\t]|[_ ]$/', $title ), '$title',
 			"invalid name '$title'" );
-		Assert::parameter(
-			$title !== '' ||
-				( $namespace === NS_MAIN && ( $fragment !== '' || $interwiki !== '' ) ),
-			'$title',
-			'should not be empty unless namespace is main and fragment or interwiki is non-empty'
-		);
 
-		$this->namespace = $namespace;
-		$this->dbkey = strtr( $title, ' ', '_' );
-		$this->fragment = $fragment;
-		$this->interwiki = $interwiki;
+		// NOTE: As of MW 1.34, [[#]] is rendered as a valid link, pointing to the empty
+		// page title, effectively leading to the wiki's main page. This means that a completely
+		// empty TitleValue has to be considered valid, for consistency with Title.
+		// Also note that [[#foo]] is a valid on-page section links, and that [[acme:#foo]] is
+		// a valid interwiki link.
+		Assert::parameter(
+			$title !== '' || $namespace === NS_MAIN,
+			'$title',
+			'should not be empty unless namespace is main'
+		);
 	}
 
 	/**
diff --git a/languages/i18n/en.json b/languages/i18n/en.json
index 63830b50cdf..ca0983ccfa0 100644
--- a/languages/i18n/en.json
+++ b/languages/i18n/en.json
@@ -341,6 +341,7 @@
 	"no-null-revision": "Could not create new null revision for page \"$1\"",
 	"badtitle": "Bad title",
 	"badtitletext": "The requested page title was invalid, empty, or an incorrectly linked inter-language or inter-wiki title.\nIt may contain one or more characters that cannot be used in titles.",
+	"title-invalid": "The requested page title is invalid",
 	"title-invalid-empty": "The requested page title is empty or contains only the name of a namespace.",
 	"title-invalid-utf8": "The requested page title contains an invalid UTF-8 sequence.",
 	"title-invalid-interwiki": "The requested page title contains an interwiki link which cannot be used in titles.",
diff --git a/languages/i18n/qqq.json b/languages/i18n/qqq.json
index 26462e7ea03..06d0c5fba2a 100644
--- a/languages/i18n/qqq.json
+++ b/languages/i18n/qqq.json
@@ -553,6 +553,7 @@
 	"no-null-revision": "Error message shown when no null revision could be created to reflect a protection level change.\n\nAbout \"null revision\":\n* Create a new null-revision for insertion into a page's history. This will not re-save the text, but simply refer to the text from the previous version.\n* Such revisions can for instance identify page rename operations and other such meta-modifications.\n\nParameters:\n* $1 - page title",
 	"badtitle": "The page title when a user requested a page with invalid page name. The content will be {{msg-mw|badtitletext}}.",
 	"badtitletext": "The message shown when a user requested a page with invalid page name. The page title will be {{msg-mw|badtitle}}.\n\nSee also:\n* {{msg-mw|selfmove}}\n* {{msg-mw|immobile-source-namespace}}\n* {{msg-mw|immobile-target-namespace-iw}}\n* {{msg-mw|immobile-target-namespace}}",
+	"title-invalid": "Used as text of error message: invalid title",
 	"title-invalid-empty": "Used as text of error message: empty title",
 	"title-invalid-utf8": "Used as text of error message: invalid UTF8 sequence",
 	"title-invalid-interwiki": "Used as text of error message: invalid interwiki link",
diff --git a/maintenance/uppercaseTitlesForUnicodeTransition.php b/maintenance/uppercaseTitlesForUnicodeTransition.php
index f5bafdeb838..c89cdd6e28f 100644
--- a/maintenance/uppercaseTitlesForUnicodeTransition.php
+++ b/maintenance/uppercaseTitlesForUnicodeTransition.php
@@ -316,7 +316,7 @@ class UppercaseTitlesForUnicodeTransition extends Maintenance {
 			return false;
 		}
 
-		if ( !$newTitle->isValid() ) {
+		if ( !$newTitle->canExist() ) {
 			$this->error(
 				"Cannot move {$oldTitle->getPrefixedText()} → $nt: "
 				. "$munge and munged title '{$newTitle->getPrefixedText()}' is not valid"
diff --git a/tests/phpunit/includes/MovePageTest.php b/tests/phpunit/includes/MovePageTest.php
index c33fef44578..56828518674 100644
--- a/tests/phpunit/includes/MovePageTest.php
+++ b/tests/phpunit/includes/MovePageTest.php
@@ -1,6 +1,7 @@
 <?php
 
 use MediaWiki\Config\ServiceOptions;
+use MediaWiki\Interwiki\InterwikiLookup;
 use MediaWiki\MediaWikiServices;
 use MediaWiki\Page\MovePageFactory;
 use MediaWiki\Permissions\PermissionManager;
@@ -172,6 +173,15 @@ class MovePageTest extends MediaWikiTestCase {
 	public function testIsValidMove(
 		$old, $new, array $expectedErrors, array $extraOptions = []
 	) {
+		$iwLookup = $this->getMock( InterwikiLookup::class );
+		$iwLookup->method( 'isValidInterwiki' )
+			->willReturn( true );
+
+		$this->setService(
+			'InterwikiLookup',
+			$iwLookup
+		);
+
 		if ( is_string( $old ) ) {
 			$old = Title::newFromText( $old );
 		}
@@ -335,6 +345,15 @@ class MovePageTest extends MediaWikiTestCase {
 	 * @param array $extraOptions
 	 */
 	public function testMove( $old, $new, array $expectedErrors, array $extraOptions = [] ) {
+		$iwLookup = $this->getMock( InterwikiLookup::class );
+		$iwLookup->method( 'isValidInterwiki' )
+			->willReturn( true );
+
+		$this->setService(
+			'InterwikiLookup',
+			$iwLookup
+		);
+
 		if ( is_string( $old ) ) {
 			$old = Title::newFromText( $old );
 		}
diff --git a/tests/phpunit/includes/TitleTest.php b/tests/phpunit/includes/TitleTest.php
index bf1d7e509d7..b246419492f 100644
--- a/tests/phpunit/includes/TitleTest.php
+++ b/tests/phpunit/includes/TitleTest.php
@@ -718,7 +718,16 @@ class TitleTest extends MediaWikiTestCase {
 	 * @param bool $isValid
 	 */
 	public function testIsValid( Title $title, $isValid ) {
-		$this->assertEquals( $isValid, $title->isValid(), $title->getPrefixedText() );
+		$iwLookup = $this->getMock( InterwikiLookup::class );
+		$iwLookup->method( 'isValidInterwiki' )
+			->willReturn( true );
+
+		$this->setService(
+			'InterwikiLookup',
+			$iwLookup
+		);
+
+		$this->assertEquals( $isValid, $title->isValid(), $title->getFullText() );
 	}
 
 	public static function provideIsValid() {
@@ -727,6 +736,42 @@ class TitleTest extends MediaWikiTestCase {
 			[ Title::makeTitle( NS_MAIN, '<>' ), false ],
 			[ Title::makeTitle( NS_MAIN, '|' ), false ],
 			[ Title::makeTitle( NS_MAIN, '#' ), false ],
+			[ Title::makeTitle( NS_PROJECT, '#' ), false ],
+			[ Title::makeTitle( NS_MAIN, '', 'test' ), true ],
+			[ Title::makeTitle( NS_PROJECT, '#test' ), false ],
+			[ Title::makeTitle( NS_MAIN, '', 'test', 'wikipedia' ), true ],
+			[ Title::makeTitle( NS_MAIN, 'Test', '', 'wikipedia' ), true ],
+			[ Title::makeTitle( NS_MAIN, 'Test' ), true ],
+			[ Title::makeTitle( NS_SPECIAL, 'Test' ), true ],
+			[ Title::makeTitle( NS_MAIN, ' Test' ), false ],
+			[ Title::makeTitle( NS_MAIN, '_Test' ), false ],
+			[ Title::makeTitle( NS_MAIN, 'Test ' ), false ],
+			[ Title::makeTitle( NS_MAIN, 'Test_' ), false ],
+			[ Title::makeTitle( NS_MAIN, "Test\nthis" ), false ],
+			[ Title::makeTitle( NS_MAIN, "Test\tthis" ), false ],
+			[ Title::makeTitle( -33, 'Test' ), false ],
+			[ Title::makeTitle( 77663399, 'Test' ), false ],
+		];
+	}
+
+	/**
+	 * @covers Title::canExist
+	 * @dataProvider provideCanExist
+	 * @param Title $title
+	 * @param bool $canExist
+	 */
+	public function testCanExist( Title $title, $canExist ) {
+		$this->assertEquals( $canExist, $title->canExist(), $title->getFullText() );
+	}
+
+	public static function provideCanExist() {
+		return [
+			[ Title::makeTitle( NS_MAIN, '' ), false ],
+			[ Title::makeTitle( NS_MAIN, '<>' ), false ],
+			[ Title::makeTitle( NS_MAIN, '|' ), false ],
+			[ Title::makeTitle( NS_MAIN, '#' ), false ],
+			[ Title::makeTitle( NS_PROJECT, '#test' ), false ],
+			[ Title::makeTitle( NS_MAIN, '', 'test', 'acme' ), false ],
 			[ Title::makeTitle( NS_MAIN, 'Test' ), true ],
 			[ Title::makeTitle( NS_MAIN, ' Test' ), false ],
 			[ Title::makeTitle( NS_MAIN, '_Test' ), false ],
@@ -736,6 +781,11 @@ class TitleTest extends MediaWikiTestCase {
 			[ Title::makeTitle( NS_MAIN, "Test\tthis" ), false ],
 			[ Title::makeTitle( -33, 'Test' ), false ],
 			[ Title::makeTitle( 77663399, 'Test' ), false ],
+
+			// Valid but can't exist
+			[ Title::makeTitle( NS_MAIN, '', 'test' ), false ],
+			[ Title::makeTitle( NS_SPECIAL, 'Test' ), false ],
+			[ Title::makeTitle( NS_MAIN, 'Test', '', 'acme' ), false ],
 		];
 	}
 
diff --git a/tests/phpunit/includes/title/MediaWikiTitleCodecTest.php b/tests/phpunit/includes/title/MediaWikiTitleCodecTest.php
index 4d88bae9656..7310cda2892 100644
--- a/tests/phpunit/includes/title/MediaWikiTitleCodecTest.php
+++ b/tests/phpunit/includes/title/MediaWikiTitleCodecTest.php
@@ -20,7 +20,6 @@
  */
 
 use MediaWiki\Interwiki\InterwikiLookup;
-use Wikimedia\TestingAccessWrapper;
 
 /**
  * @covers MediaWikiTitleCodec
@@ -366,6 +365,7 @@ class MediaWikiTitleCodecTest extends MediaWikiTestCase {
 			[ 'X#n&#x303;', NS_MAIN, 'en', new TitleValue( NS_MAIN, 'X', 'ñ' ) ],
 			// target section parsing
 			'empty fragment' => [ 'X#', NS_MAIN, 'en', new TitleValue( NS_MAIN, 'X' ) ],
+			'only fragment' => [ '#', NS_MAIN, 'en', new TitleValue( NS_MAIN, '' ) ],
 			'double hash' => [ 'X##', NS_MAIN, 'en', new TitleValue( NS_MAIN, 'X', '#' ) ],
 			'fragment with hash' => [ 'X#z#z', NS_MAIN, 'en', new TitleValue( NS_MAIN, 'X', 'z#z' ) ],
 			'fragment with space' => [ 'X#z z', NS_MAIN, 'en', new TitleValue( NS_MAIN, 'X', 'z z' ) ],
@@ -396,7 +396,7 @@ class MediaWikiTitleCodecTest extends MediaWikiTestCase {
 		// TODO: test unicode errors
 
 		return [
-			[ '#' ],
+			[ 'User:#' ],
 			[ '::' ],
 			[ '::xx' ],
 			[ '::##' ],
@@ -490,14 +490,13 @@ class MediaWikiTitleCodecTest extends MediaWikiTestCase {
 		$this->setService( 'TitleFormatter', $codec );
 
 		$actual = Title::makeTitleSafe( $ns, $text, $fragment, $interwiki );
-		// We need to reset some members so equality testing works
-		if ( $actual ) {
-			$wrapper = TestingAccessWrapper::newFromObject( $actual );
-			$wrapper->mArticleID = $actual->getNamespace() === NS_SPECIAL ? 0 : -1;
-			$wrapper->mLocalInterwiki = false;
-			$wrapper->mUserCaseDBKey = null;
+
+		if ( $expected ) {
+			$this->assertNotNull( $actual );
+			$this->assertTrue( Title::castFromLinkTarget( $expected )->equals( $actual ) );
+		} else {
+			$this->assertNull( $actual );
 		}
-		$this->assertEquals( Title::castFromLinkTarget( $expected ), $actual );
 	}
 
 	public static function provideMakeTitleValueSafe() {
diff --git a/tests/phpunit/unit/includes/title/TitleValueTest.php b/tests/phpunit/unit/includes/title/TitleValueTest.php
index 39e264bc63b..8b91ee5cb8d 100644
--- a/tests/phpunit/unit/includes/title/TitleValueTest.php
+++ b/tests/phpunit/unit/includes/title/TitleValueTest.php
@@ -35,6 +35,7 @@ class TitleValueTest extends \MediaWikiUnitTestCase {
 			[ NS_USER, 'TestThis', '', 'baz', false, true ],
 			[ NS_MAIN, 'foo bar', '', '', false, false ],
 			[ NS_MAIN, 'foo_bar', '', '', false, false ],
+			[ NS_MAIN, '', '', '', false, false ],
 		];
 	}
 
@@ -56,6 +57,14 @@ class TitleValueTest extends \MediaWikiUnitTestCase {
 		$this->assertEquals( $hasInterwiki, $title->isExternal() );
 	}
 
+	/**
+	 * @dataProvider goodConstructorProvider
+	 */
+	public function testAssertValidSpec( $ns, $text, $fragment, $interwiki ) {
+		TitleValue::assertValidSpec( $ns, $text, $fragment, $interwiki );
+		$this->assertTrue( true ); // we are just checking that no exception is thrown
+	}
+
 	public function badConstructorProvider() {
 		return [
 			[ 'foo', 'title', 'fragment', '' ],
@@ -88,6 +97,14 @@ class TitleValueTest extends \MediaWikiUnitTestCase {
 		new TitleValue( $ns, $text, $fragment, $interwiki );
 	}
 
+	/**
+	 * @dataProvider badConstructorProvider
+	 */
+	public function testAssertValidSpecErrors( $ns, $text, $fragment, $interwiki ) {
+		$this->setExpectedException( InvalidArgumentException::class );
+		TitleValue::assertValidSpec( $ns, $text, $fragment, $interwiki );
+	}
+
 	public function fragmentTitleProvider() {
 		return [
 			[ new TitleValue( NS_MAIN, 'Test' ), 'foo' ],