Fix phpstan/phpstan#11430: Two unbounded generics in conditional return are assumed to be always the same

[phpstan-bot]

Summary

When a method had two unbounded template types (e.g. T and S) used in a conditional return type like (T is S ? None : Some<T>), PHPStan incorrectly reported "Method never returns Some so it can be removed from the return type." This was a false positive — the method can clearly return either branch depending on the arguments.

Changes

• Modified TemplateTypeTrait::tryRemove() in src/Type/Generic/TemplateTypeTrait.php to return null when asked to remove a TemplateType, preventing meaningless template bound narrowing
• Added regression test in tests/PHPStan/Rules/TooWideTypehints/data/bug-11430.php
• Added test method testBug11430 in tests/PHPStan/Rules/TooWideTypehints/TooWideMethodReturnTypehintRuleTest.php

Root cause

When resolving the conditional return type (T is S ? None : Some<T>), ConditionalType::getNormalizedElse() replaces the subject T in the else branch with TypeCombinator::remove(T, S). This called TemplateTypeTrait::tryRemove(S), which in turn called TypeCombinator::remove(mixed, S). Since plain MixedType::tryRemove considers mixed a supertype of any template type, it succeeded and returned mixed~S (mixed minus S). This created a new template type T of mixed~S.

The problem: when narrowType later compared the declared return type None|Some<T of mixed~S> against the actual return type None|Some<T>, the invariant generic variance check used equals() to compare T of mixed~S with T. Since these aren't equal (different bounds), isValidVariance returned No, causing Some<T of mixed~S> to be reported as never returned.

The fix prevents subtracting a TemplateType from another TemplateType's bound, since template types represent unknown types and their subtraction doesn't produce useful narrowing information.

Test

Added a regression test reproducing the exact scenario from the issue: a final class with a static method using two unbounded generics in a conditional return type. The test expects no errors (the false positive is eliminated).

Fixes phpstan/phpstan#11430