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

[chadmandoo]

Closing as superseded. #473 now implements sound covariant self-return validation for interface methods and removed the with*-prefixed return-type heuristic this PR added — reworking the same inference/objects/methods.rs and type_compat/unions.rs paths (e183b1b, 4f66615). Assuming the receiver interface for any with* method is unsound in the general case, so the sound self-return path in #473 is the right home for this. Reproducer/credit can carry over there. Thanks!

[nahime0]

Superseded by #578.

#518 identified the real PSR-style fluent-chain failure and provided the original reproducer. The replacement PR carries that reproducer and contributor credit forward, but removes the method-name heuristic and addresses the underlying type-system issue instead:

• native : static is preserved as a late-bound method return;
• checker and EIR bind it consistently at the call-site receiver;
• class hierarchies, interfaces, static factories, nullable/union returns, override conformance, and Reflection metadata are covered;
• withdraw()-style false positives remain rejected;
• the unrelated array-compatibility change is not included.

Keeping #518 closed in favor of #578 so review and CI continue on the complete replacement. Thanks @chadmandoo for surfacing the original issue and reproducer.