feat(types): fluent wither-return covariance through descendant interfaces

[chadmandoo]

What

A PSR-FIG immutable "wither" method (withHeader(): MessageInterface) declares an ancestor interface as its return type but carries @return static. Called through a descendant-interface receiver, the result must keep the receiver's type so a subsequent descendant-only call type-checks — matching PHP's runtime clone $this and PHPStan's @return static.

Previously rejected:

interface Message { public function withHeader(string $v): Message; }
interface Request extends Message {
    public function withMethod(string $m): Request;
    public function method(): string;
}
function chain(Request $r): string {
    // withMethod() was unresolved: withHeader() typed as the ancestor Message
    return $r->withHeader('x')->withMethod('POST')->method();
}

How

Both inference passes resolve a with* method whose declared return is a strict ancestor of the receiver interface back to the receiver:

• checker — infer_method_call_on_interface_type
• EIR lowering — method_call_result_type

Both are required: the checker fix alone leaves the EIR backend re-resolving the chained call to the ancestor and failing native codegen (interface method call to unknown method Message::withMethod).

Also accepts Array(Mixed) where an AssocArray parameter is declared (an untyped list literal passed to an array<int, mixed> param).

Test

test_interface_wither_ancestor_return_stays_receiver — a Message / Request extends Message wither chain, byte-parity vs PHP 8.5 (chain(new Req()) prints POST).

🤖 Generated with Claude Code