Fix #5946: $this type of static (self, ...) type must remains static

[phpstan-bot]

Summary

When a method with @return $this is called on a static return type, the result type was incorrectly preserved as $this instead of being downgraded to static. This caused PHPStan to miss reporting a type error when returning the result of such a chain from a method that requires $this.

For example:

class Model {
    /** @return static */
    public function getParent() { return new static(); }

    /** @return $this */
    public function getModel(bool $useParent) {
        if ($useParent) {
            return $this->getParent()->getModel(false); // should error but didn't
        }
        return $this;
    }
}

$this->getParent() returns static(Model), then calling getModel() on that should yield static(Model) (not $this(Model)), because $this cannot be guaranteed when the caller is merely static.

Changes

• src/Type/StaticType.php: In transformStaticType(), after changeBaseClass(), check if the resulting type is ThisType and the caller ($this) is not a ThisType — if so, construct a plain StaticType instead
• tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php: Added testBug5946() test method
• tests/PHPStan/Rules/Methods/data/bug-5946.php: New test data file reproducing the issue
• CLAUDE.md: Added documentation about StaticType::transformStaticType and ThisType downgrading

Root cause

StaticType::transformStaticType() uses $type->changeBaseClass($classReflection) to transform return types. Since ThisType extends StaticType, both are caught by the instanceof StaticType check. However, ThisType::changeBaseClass() returns a new ThisType (preserving $this semantics), when it should have been downgraded to a plain StaticType for callers that are static but not $this. The fix explicitly checks for this case and constructs a StaticType instead.

Test

Added a regression test with a Model class that has a getParent() returning static and a getModel() returning $this. The test verifies that both $this->getParent() and $this->getParent()->getModel(false) are correctly reported as returning static(Model) instead of $this(Model).

Fixes phpstan/phpstan#5946