# 0131 — protocol method call with extra arguments compiles and silently drops them

## Symptom

Calling a protocol method with MORE arguments than the protocol
declares is accepted by the compiler; the extra arguments are silently
dropped at the call. Observed: `Allocator.dealloc_bytes` is declared
`(ptr: *void)`, yet `a.dealloc_bytes(p, 12345)` compiles and runs
under both `sx run` and `sx build`. Expected: a compile diagnostic
("expected 1 argument, got 2"), exactly as for plain function calls.

Found via std.http (PLAN-HTTPZ S7a): three `dealloc_bytes(ptr, size)`
calls — written against an imagined two-arg signature — compiled
clean and survived the full example sweep. Corrected in `81fa50c`;
this issue is about the missing diagnostic.

## Reproduction

```sx
#import "modules/std.sx";

main :: () -> i32 {
    gpa := GPA.init();
    a : Allocator = xx gpa;
    p := a.alloc_bytes(64);
    a.dealloc_bytes(p, 12345);   // protocol declares (ptr) — must be rejected
    print("compiled and ran\n");
    return 0;
}
```

Run `sx run repro.sx`: prints "compiled and ran", exit 0. Expected: a
compile error naming the arity mismatch.

## Investigation prompt

The sx compiler accepts protocol method calls with extra trailing
arguments and silently drops them (repro above — `Allocator.
dealloc_bytes(ptr)` called with `(ptr, extra)` compiles and runs).
Plain function calls DO arity-check, so the gap is specific to the
protocol-dispatch call path. Suspected area: the protocol call
planning/lowering in `src/ir/lower/call.zig` (and/or the protocol
method resolution in `src/ir/protocols.zig`) — wherever a protocol
method's parameter list is matched against call-site args, the
arg-count check that plain calls get is likely skipped, and lowering
then truncates args to the method's param count. The fix should emit
the same diagnostic plain calls produce (expected N args, got M) for
both too many AND verify too few is also caught. Verify with the
repro (expect a compile error) plus a negative-test example or a
`call.test.zig` case pinning the diagnostic; then `zig build &&
zig build test && bash tests/run_examples.sh` all green (existing
examples must not regress — if any example relied on dropped extra
args, that example is itself a latent bug to fix in the same
session).

## Resolution

RESOLVED (merged master `d7808f6`, 2026-06-12, Agra-directed in-session
fix). `emitProtocolDispatch` arity-checks exactly (extra AND missing
args) with the standard "expects N arguments" diagnostic at the call
span. Regression pinned by examples/1634-protocol-call-arity; sweep
622/622 + unit tests green — nothing in the tree relied on the
leniency. The three std.http call sites that exposed the gap were
corrected separately in `81fa50c`.
