[APX403]

Closes phpstan/phpstan#15128

A child class that binds the parent's template type and overrides a method without a PHPDoc of its own gets an error about the parent's PHPDoc. The issue's snippet, shortened:

/**
 * @template T of int|string
 */
class A {
	/**
	 * @return (T is int ? string : int)
	 */
	public function foo(): int|string { /* ... */ }
}

/**
 * @extends A<string>
 */
class B extends A {
	#[Override]
	public function foo(): int { /* ... */ }
}

PHPStan reports on B::foo():

Conditional return type uses subject type string which is not part of PHPDoc @template tags.

B::foo() inherits the tags through ResolvedPhpDocBlock::resolveTemplateTypeInTag(), which replaces T with string but keeps the conditional type, so ConditionalReturnTypeRuleHelper checks (string is int ? string : int) as if it had been written on B::foo(). An inherited @param tag, an interface method and an abstract trait method get the same error, and a child declared with @template U of int and @extends A<U> gets Condition "U of int is int" in conditional return type is always true.

As suggested in the issue, the inherited conditional type is now resolved once its condition no longer depends on a template type, or the bound of that template type decides it. A static subject stays as it is, since it depends on the class the method is called on, and so does a conditional type that would resolve to another one, including a conditional on a parameter, because MethodSignatureRule compares it with the parent's conditional type branch by branch.

A conditional on a parameter, such as ($val is int ? string : int) with an inherited @param T $val, is left as it is. Whether it is decided depends on the parameter's final type, which a = null default or the native type can still change, and call sites already resolve it against the argument.