Suppress undefined static property error when property_exists() guard is present

[phpstan-bot]

Summary

When property_exists() is called with a class-string first argument (e.g. property_exists(static::class, 'default')), the subsequent static property access static::$default was reported as "Access to an undefined static property" even though the existence check guarded it. This fix teaches PHPStan to recognize that guard.

Changes

• src/Type/Php/PropertyExistsTypeSpecifyingExtension.php: When the first argument to property_exists() is a string type (class-string or constant class name), mark the property_exists() FuncCall expression itself as ConstantBooleanType(true) in scope. Previously, class-string arguments caused the extension to return empty SpecifiedTypes, providing no narrowing. The approach of intersecting HasPropertyType with class-string was not viable because HasPropertyType uses ObjectTypeTrait, making it incompatible with string types in TypeCombinator::intersect() (producing never). This matches the approach already used for non-constant property names.

• src/Rules/Properties/AccessStaticPropertiesCheck.php: Before reporting "Access to an undefined static property", construct a virtual property_exists(ClassName::class, 'propName') FuncCall and check if the scope evaluates it as true. For Name-based class references (static, self, Foo), constructs ClassConstFetch(Name, 'class') as the first argument. For expression-based references ($className::$prop), uses the expression directly.

• phpstan-baseline.neon: Updated expected count for instanceof ConstantStringType in PropertyExistsTypeSpecifyingExtension.php from 2 to 1 (removed the old $objectType instanceof ConstantStringType check).

Analogous cases probed

• Static property assignment (AccessStaticPropertiesInAssignRule): Uses the same AccessStaticPropertiesCheck, so automatically covered. Added regression test.
• Expression-based class access ($className::$prop after property_exists($className, 'prop')): Works because the expression key matches. Added test case.
• Instance properties ($this->prop after property_exists($this, 'prop')): Already handled by existing code in AccessPropertiesCheck (lines 201-209 for dynamic names, and HasPropertyType narrowing for static names).
• HasMethodType vs HasPropertyType with class-string: Investigated why method_exists() correctly narrows class-string types while property_exists() didn't — HasMethodType::isSuperTypeOf checks hasMethod() which class-strings support (for static method calls), but HasPropertyType::isSuperTypeOf checks hasInstanceProperty()/hasStaticProperty() which class-strings don't support. This is why the FuncCall-marking approach was chosen over type intersection.

Root cause

PropertyExistsTypeSpecifyingExtension did not handle class-string first arguments. When the first argument was a ConstantStringType (which includes class-string types), the extension returned empty SpecifiedTypes, so no type narrowing occurred. Meanwhile, AccessStaticPropertiesCheck had no mechanism to detect that a property_exists() guard was present before reporting undefined static properties — unlike AccessPropertiesCheck which already had a virtual property_exists() check for dynamic instance property names.

Test

• tests/PHPStan/Rules/Properties/data/bug-2861.php — Regression test with trait using property_exists(static::class, 'default') and property_exists(self::class, 'default') guards, plus expression-based $className::$prop access. Expects no errors.
• tests/PHPStan/Rules/Properties/data/bug-2861-assign.php — Assignment context regression test with static::$default = 'value' after property_exists() guard. Expects no errors.

Fixes phpstan/phpstan#2861