Linking your change: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1143959

It is using type declaration on the class property, often this makes phan more strict about types.

There is also a check that this is not-null in the same function. Not reachabe for every code path, so this could be a bug.

			if ( !$this->persistenceChangeReason ) {
				$this->persistenceChangeReason = 'token';
			}

Maybe phan is happy with a null-coalesce assignment instead

			$this->persistenceChangeReason ??= 'token';

The revision that generated this error didn't use a type declaration (although then I added one and Phan didn't complain about the suppression being unnecessary, so I suppose it's still confused about whether persistenceChangeReason can be null). The token stuff seems too late to be relevant, by that point Phan decided the property has non-empty-mixed type. (It's actually not mixed, all values are string or null; and this is a private property, so that shouldn' be hard to analyze.)

Upstreamed: https://github.com/phan/phan/issues/4916. Definitely a bug, likely related to the special-cased tracking of properties of $this in local context.

This was released in phan 5.4.6,, we are now on 5.5.2 I think ? Shall we close this ?