Fix phpstan/phpstan#10290: Function should return Err<array> but returns Err<array>

[phpstan-bot]

Summary

When a generic class has an invariant template parameter (@template T), PHPStan was rejecting subtypes in the accepts() context. For example, Err<array<mixed>> would not accept Err<non-empty-array<mixed, mixed>> even though non-empty-array is a subtype of array. This caused false positive errors like:

Function f() should return Err<array>|Ok<non-empty-string> but returns Err<array>.

This fix makes the accepts() method (used for error reporting in return types, parameter passing, etc.) treat invariant generic template parameters as covariant — accepting subtypes. The isSuperTypeOf() method (used for type inference) retains strict invariant behavior.

Changes

• Modified src/Type/Generic/GenericObjectType.php: In isSuperTypeOfInternal(), when in acceptsContext and the template is declared invariant, check if the expected type is a supertype of the actual type. If yes, accept it.
• New regression test: tests/PHPStan/Rules/Functions/data/bug-10290.php — Result/Ok/Err pattern with invariant generics
• Updated tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php — added testBug10290
• Updated tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php — testBug4590 now expects no errors
• Updated tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php — removed false positive errors in testGenericVariance, testBug5372, testGenericObjectLowerBound
• Updated tests/PHPStan/Type/Generic/GenericObjectTypeTest.php — added dataAcceptsTypeProjections() to separate accepts vs isSuperTypeOf expectations for type projections
• Updated tests/PHPStan/Type/StaticTypeTest.php — updated GenericStaticType accepts test

Root cause

GenericObjectType::isSuperTypeOfInternal() delegates invariant template checking to TemplateTypeVariance::isValidVariance(), which uses Type::equals() — requiring exact type match. In the accepts() context (parameter $acceptsContext = true), this was too strict. The fix adds a condition: when in accepts context and the template is declared invariant, if the expected type argument is a supertype of the actual type argument ($a->isSuperTypeOf($b)->yes()), accept it instead of requiring equality.

Test

Added tests/PHPStan/Rules/Functions/data/bug-10290.php reproducing the issue with Result/Ok/Err generic classes. The test verifies that:

• Err<non-empty-array<mixed, mixed>> is accepted for return type Err<array<mixed>>
• Ok<non-falsy-string> is accepted for return type Ok<non-empty-string>
• Ok<bool> is still rejected for return type Ok<true> (supertype, not subtype)

Fixes phpstan/phpstan#10290