Fix: appliesTo() incorrectly excludes mutually exclusive class states

[fain182]

Summary

Fixes #560 - IsNotAbstract (and other similar expressions) incorrectly excluded final classes when used in that() clauses, preventing subsequent rules from being evaluated.

Problem Description

The Bug

When using IsNotAbstract as a filter in that(), final classes were incorrectly excluded:

Rule::allClasses()
    ->that(new IsNotAbstract())  // BUG: Excluded final classes!
    ->should(new HaveNameMatching('*Test'))
    ->because('all tests must follow naming convention');

Result: Final test classes with wrong names were NOT flagged as violations.

Root Cause

The appliesTo() method was confusing two different concepts:

1. ✅ Type incompatibility: Interface/Trait/Enum don't have the concept of abstract/final → correctly excluded
2. ❌ Mutually exclusive states: Final ↔ Abstract are opposites → incorrectly excluded

Buggy implementation (from PR #478):

class IsNotAbstract {
    public function appliesTo(ClassDescription $theClass): bool {
        return !($theClass->isInterface() || $theClass->isTrait() || 
                 $theClass->isEnum() || $theClass->isFinal());  // ← BUG!
    }
}

The logic error:

• Final classes ARE non-abstract by definition
• Excluding them from IsNotAbstract is semantically wrong
• Same bug existed in IsFinal, IsNotFinal, IsAbstract

Historical Context

• PR Adds appliesTo to verify if a given rule should be applied #454: Introduced appliesTo() to avoid false positives on incompatible types
• PR Violation can now contain a reference to the file path #478: Implemented appliesTo() but incorrectly extended it to mutually exclusive states
• PR Fix Is(Not)Abstract expression so that it does not ignore final classes #560: Bug discovered 9 months later by real-world use case

Solution

Changed Semantics

New appliesTo() semantics:

"Does it make sense to verify this property for this type of class?"

Rules:

• ✅ Return true for regular classes, final classes, abstract classes
• ❌ Return false ONLY for Interface/Trait/Enum (where abstract/final concepts don't apply)
• ✅ Do NOT exclude mutually exclusive states (abstract ↔ final)

Code Changes

// Before (WRONG)
class IsNotAbstract {
    public function appliesTo(ClassDescription $theClass): bool {
        return !($theClass->isInterface() || $theClass->isTrait() || 
                 $theClass->isEnum() || $theClass->isFinal());
    }
}

// After (CORRECT)
class IsNotAbstract {
    public function appliesTo(ClassDescription $theClass): bool {
        return !($theClass->isInterface() || $theClass->isTrait() || 
                 $theClass->isEnum());
        // Final classes removed - they ARE non-abstract!
    }
}

Same fix applied to:

• IsAbstract (removed || $theClass->isFinal())
• IsNotAbstract (removed || $theClass->isFinal())
• IsFinal (removed || $theClass->isAbstract())
• IsNotFinal (removed || $theClass->isAbstract())

Why This Is Correct

PHP Semantics

In PHP, abstract and final are mutually exclusive:

• final class Foo {} → isAbstract() always returns false
• abstract class Bar {} → isFinal() always returns false

Rule Evaluation Flow

In that() clauses (filtering):

if (!$spec->appliesTo($classDescription)) {
    return false;  // Skip entire rule chain
}

In should() clauses (validation):

$expression->evaluate($classDescription, $violations, $because);
// appliesTo() is never called here

Expected Behavior Table

Class Type	IsAbstract::appliesTo()	IsNotAbstract::appliesTo()	Why
Regular class	true	true	Can check abstract property
Final class	true ✅	true ✅	Can check abstract property (always false)
Abstract class	true ✅	true ✅	Can check abstract property (always true)
Interface	false	false	Abstract concept doesn't apply
Trait	false	false	Abstract concept doesn't apply
Enum	false	false	Abstract concept doesn't apply

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.80%. Comparing base (c600ddd) to head (806256c).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #561   +/-   ##
=========================================
  Coverage     97.80%   97.80%           
+ Complexity      632      628    -4     
=========================================
  Files            81       81           
  Lines          1822     1822           
=========================================
  Hits           1782     1782           
  Misses           40       40           

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