Scala: receiver-type inference for member calls (recover edges suppressed by the weak-member guard) #2384

Summary

Follow-up to #2155 / #2366. With the receiver-aware weak-member guard, a Scala call recv.m(...) whose receiver is a value (values.get(k), xs.foreach(_.register()), underlying.close()) no longer binds an arbitrary project-wide m by short name. That is the right default, but it also drops the edges where the receiver's type is knowable from the caller's own source — a field, parameter or local with an explicit type annotation, or a constructor call in scope.

Measured on twitter/finagle (ca472de) in #2366: of the 9,149 CALLS edges the guard removes, 2,549 (28%) target a class whose name appears in the caller's file (import, type annotation, new). A sample of those is a mix of correct edges (Memcached.serve → Server.serve, CachingPool.checkout → Service.close, TypeTest.testRow → Row.intOrZero) and coincidences (PushStackServerTest → Stack.exists, RefCountedFactoryTest → FailureFlags.isFlagged). The other 72% target a class the caller file never mentions and are almost certainly noise.

Proposal

Receiver-type inference for Scala, applied before the registry resolves a member call:

1. Extraction records, per call, the receiver's root identifier (values in values.get(k), underlying in underlying.close()); today only is_method is set.
2. For the caller's file, build a name → declared type map from the shapes the grammar exposes directly: val x: T = …, var x: T, constructor/method parameters (x: T), val x = new T(...), val x = T(...) / T.apply(...) (companion factory).
3. When the root identifier has an entry T and T resolves to a class in the graph (through the import map or the same file), resolve m as T.m / T$.m — a new strategy such as receiver_type sitting above suffix_match in confidence. When it does not, the call keeps the current suppressed outcome (no weak short-name fallback).
4. Only step 3's positive case adds edges; the guard from fix(scala): suppress weak short-name matches for receiver calls #2366 is unchanged for everything else.

Out of scope for a first cut: inherited members (T.m declared on a parent), generics (Seq[T]), inferred val x = someCall() types, implicit conversions.

Expected effect

Recovers a subset of the 2,549 "owner mentioned" edges — the ones where the type is written down — without reintroducing the 6,600 where it is not. A before/after on finagle with the same edge-identity comparison as #2366 would show recall regained per strategy and the residual.

Related

• Scala: receiver.method(...) calls are bound to an arbitrary same-named symbol by short name (weak-member suppression not applied) #2155 / fix(scala): suppress weak short-name matches for receiver calls #2366 — receiver-aware weak-member suppression for Scala (the guard this builds on)
• Scala: companion object and class share one qualified name, so their members overwrite each other #2154 / fix(scala): keep companion objects distinct from their classes (#2154) #2361 — companion object QNs (T$.m for object members)
• Scala: parse import selectors/aliases/wildcards and resolve imports against the declared package instead of the directory layout #2153 / feat(scala): parse import selectors and resolve package-aware imports (#2153) #2360 — package-aware imports (used to resolve T from the caller's imports)