sx/issues/0102-flat-import-same-signature-collision.md

# 0102 — flat-import same-name function collision (per-source binding)

**RESOLVED.** Two **flat** imports (bare `#import "a.sx"` / a flat directory
import, NOT a namespaced `ns :: #import`) that each author a top-level free
function with the **same short name** collided in IR lowering. The flat/
directory merge keeps exactly **one** author per name in the merged decl list
(first-wins), and every bare-name consumer site — call dispatch, default-arg
expansion, function-value capture, free-function UFCS, comptime `#run` — read
that one **name-keyed** winner. So when module `b.sx` authored its own `greet`
but `a.sx` was imported first, `b.sx`'s own bare `greet()` silently bound
`a.sx`'s author. Unlike issue 0100 (which crashed on a param-count assert when
the AST/FuncId split across modules), this miscompiled **silently**: the wrong
same-name author ran, with no diagnostic.

The defect had two faces, both rooted in name-keyed identity across a flat
collision:

1. **Lowering** keyed function bodies by short name (`fn_ast_map` /
   `resolveFuncByName` are first-wins), so a shadowed author never got its own
   FuncId or body — there was nothing to bind even if a consumer wanted the
   per-source author.
2. **Resolution** at every bare-name consumer site re-looked-up the winner by
   name, so even once shadow authors had distinct FuncIds, the consumer sites
   kept binding the first-wins winner.

## Fix — four sub-steps (`src/imports.zig`, `src/ir/lower.zig`)

- **0102a — retain dup authors + identity indexes.** The flat/directory merge
  keeps first-wins in the merged scope (unchanged), but now *also* retains
  every dropped same-name author in `program_index.module_fns`
  (`path → name → *FnDecl`) plus a `flat_import_graph` (`file → flat-import
  edges`). Resolution is untouched at this step — the indexes just make the
  shadowed authors addressable.

- **0102b — identity-addressable function lowering.** `fn_decl_fids`
  (`*const ast.FnDecl → FuncId`) lets a body be declared + lowered against a
  **specific** `*FnDecl` (`lowerFunctionBodyInto` / `bareAuthorFuncId`) instead
  of a name. A shadow author gets a fresh same-name FuncId in its own module's
  visibility context; the winner keeps the name-keyed slot. `scanDecls` keys
  `fn_decl_fids` by the stable `module_fns` `*FnDecl`.

- **0102c — THE resolver + call path + param typing.**
  `resolveBareCallee(name, caller_file) -> .func(ResolvedAuthor) | .ambiguous |
  .none` (`src/ir/lower.zig`). It returns `.none` whenever the outcome would
  equal first-wins (single author, or own-author == winner), so every
  single-author / local / parameter / std / qualified / foreign / generic /
  builtin name resolves byte-for-byte as before. Only a genuine flat collision
  reroutes: own-author wins; else the caller's flat-reachable authors — `≥2`
  distinct → `.ambiguous` (loud "qualify the call" diagnostic), exactly one
  differing from the winner → bind it. Routed the **primary call path** and the
  call's **parameter target typing** (so a `*T`-param shadow gets implicit
  address-of, not a value bit-cast to a pointer → segfault).

- **0102d — the four remaining bare-name sites.** Routed the SAME resolver
  through every other site that resolved a bare callee/function-name by
  first-wins, each gated exactly as the call path (plain top-level identifier,
  no scope-mangle / UFCS alias / local shadow; act on `.func` / `.ambiguous`,
  fall through on `.none`):
  1. **Default-argument expansion** (`expandCallDefaults`): omitted trailing
     args fill from the RESOLVED author's defaults, not the winner's.
  2. **Function-value conversion** (`closure(fn)` and the bare-fn-as-value
     `func_ref` / fn-ptr / closure-coercion path): captures the resolved
     author's FuncId.
  3. **Free-function UFCS** (`recv.fn()` → `fn(recv, …)`): dispatches the
     resolved author for the receiver's source.
  4. **Comptime `#run`** of a bare call: `lowerMainAndComptime` now sets
     `current_source_file` per decl, so a `NAME :: #run f()` in an imported
     module resolves `f` from THAT module's flat imports (own-author wins)
     rather than the main file's perspective (where two flat authors made it
     spuriously `.ambiguous` and failed the build).

## Regression tests

`examples/0722`–`0734` (each a focused multi-file flat-collision scene that
fails on pre-fix code and passes after):

- `0722-modules-flat-same-name-own` — own-author wins on the call path.
- `0723-modules-flat-vs-namespaced` — a flat author + a namespaced same-name
  author don't collide.
- `0724-modules-flat-same-name-ambiguous` — `≥2` flat authors, bare call →
  loud diagnostic.
- `0725-modules-flat-dir-same-name` — flat **directory** import collision.
- `0726-modules-flat-same-name-variadic` — per-source variadic packing.
- `0728-modules-flat-same-name-paramtype` — per-source parameter target typing
  (value vs pointer param).
- `0729-modules-flat-same-name-foreign` — same-name `#foreign` authors are NOT
  rerouted (non-plain authors keep first-wins).
- `0730-modules-flat-same-name-default-arg` — per-source default-arg expansion.
- `0731-modules-flat-same-name-closure` — per-source `closure(fn)` + bare
  fn-value capture.
- `0732-modules-flat-same-name-ufcs` — per-source free-function UFCS dispatch.
- `0733-modules-flat-same-name-comptime-run` — per-source comptime `#run`
  callee.
- `0734-modules-flat-same-name-ufcs-ambiguous` — `≥2` flat authors, UFCS call
  → loud diagnostic (pre-fix: silently bound the winner).