Fix #7388: Error when parent class and interface contradicts each other

[phpstan-bot]

Summary

PHPStan failed to report an error when a class extends a parent class and implements an interface, but the parent class's method signature is incompatible with the interface's requirements. For example:

interface FooInterface {
    public function bar(int $i): void;
}
class ParentFoo {
    public function bar(): void {}
}
class Foo extends ParentFoo implements FooInterface {} // Should error, didn't

PHP itself produces a fatal error for this case, but PHPStan silently accepted it.

Changes

• Added new InheritedMethodCompatibilityRule in src/Rules/Methods/InheritedMethodCompatibilityRule.php
  • Processes InClassNode (class-level) at rule level 0
  • For each immediate interface method, checks if the inherited (not overridden) method from the parent class has a compatible signature
  • Checks parameter count, parameter type contravariance, return type covariance, and pass-by-reference compatibility
• Added test class tests/PHPStan/Rules/Methods/InheritedMethodCompatibilityRuleTest.php
• Added test data tests/PHPStan/Rules/Methods/data/bug-7388.php

Root cause

The existing OverridingMethodRule and MethodSignatureRule both process InClassMethodNode, which only fires when a method is explicitly declared in the class. When a class inherits a method from a parent without overriding it, no InClassMethodNode is generated for that method in the child class context. This meant that inherited methods were never validated against interface requirements.

The fix adds a new class-level rule that iterates over interface methods and checks if inherited implementations are compatible.

Test

The regression test (bug-7388.php) covers:

• Basic case: parent method missing a required parameter from the interface (reports error)
• Multiple missing parameters case (reports error)
• Parent method with extra optional params that satisfy the interface (no error)
• Class that overrides the method correctly (no error, handled by existing rules)
• Abstract class that defers implementation (no error)

Fixes phpstan/phpstan#7388