Fix phpstan/phpstan#6139: False positive for covariant generic parameter

[phpstan-bot]

Summary

When a covariant generic type like Element<T> (where Element declares @template-covariant T) is used as a method parameter type, PHPStan was incorrectly reporting that the covariant template type T occurs in a contravariant position. Since the covariant generic only produces values of type T, using it in a parameter position does not create a true contravariant use of T.

Changes

• Modified src/Rules/Generics/VarianceCheck.php:
  • Added findCovariantTemplatesInCovariantGeneric() method that detects when a covariant template type is used directly as a type argument to a covariant generic type in a parameter position
  • Uses getObjectClassReflections() and getTemplateType() instead of instanceof GenericObjectType to follow PHPStan's type-checking conventions
  • The check() method now skips variance errors for these identified template types
• Updated tests/PHPStan/Rules/Generics/MethodSignatureVarianceRuleTest.php:
  • Removed expected error for @param Out<X> in covariant class test (parameter f)
  • Removed expected error for @param Out<X> in static method covariant class test (parameter c)
  • Added testBug6139() test method
• Added tests/PHPStan/Rules/Generics/data/bug-6139.php regression test

Root cause

The variance check composed the position variance (contravariant for parameters) with the generic type's template variance (covariant) using the standard multiplication rule: contravariant × covariant = contravariant. This treated any covariant template type used as a type argument to a covariant generic in a parameter as a variance violation.

The fix identifies this specific pattern at the top level (in VarianceCheck::check) and skips the error for direct template type arguments that have matching covariant variance with the generic class's template parameter. This avoids the false positive while preserving correct variance checking for nested generic types (e.g., @return In<Out<X>> is still correctly flagged).

Test

Added tests/PHPStan/Rules/Generics/data/bug-6139.php which reproduces the original issue:

• Element<T> interface with @template-covariant T
• FormatLoader<T> interface with @template-covariant T and method addElement(Element<T> $element)
• The test expects no errors (empty expected errors array)

Fixes phpstan/phpstan#6139