refactor(imports): retain dup same-name fn authors + build identity indexes [0102a]

First of four fix-0102 sub-steps. Purely additive: retains data that the
flat/directory merge currently first-wins-drops and builds two identity
indexes for later bare-name disambiguation (fix-0102c). No resolution
change — the existing first-wins bare path still wins; suite unchanged.

- mergeFlat + directory merge: stop dropping a same-name FUNCTION authored
  by a different module/file. Non-function decls keep first-wins dedup; node
  identity dedup is untouched.
- flat_import_graph: a flat-only subset of import_graph, recording an edge
  only for a bare `#import` (imp.name == null), never a namespaced
  `ns :: #import`. Threaded through resolveImports/resolveDirectoryImport
  and into ProgramIndex.
- module_fns (path -> name -> *const FnDecl): per-module authored-function
  index mirroring module_scopes, built in core.zig from the main module +
  cache. Same-name cross-module authors stay distinct under their own paths.
- imports.test.zig: asserts both a.sx/b.sx greet authors are retained in
  module_fns and in the global flat list, and that flat_import_graph
  excludes the namespaced edge while import_graph includes it.

Gate (this worktree): zig build, zig build test (398/398),
bash tests/run_examples.sh (457 passed) all green.

src/core.zig

@@ -26,6 +26,12 @@ pub const Compilation = struct {
     import_sources: std.StringHashMap([:0]const u8),
     module_scopes: std.StringHashMap(std.StringHashMap(void)),
     import_graph: std.StringHashMap(std.StringHashMap(void)),
+    /// Flat-only subset of `import_graph` (bare `#import` edges, no namespaced
+    /// `ns :: #import`). Borrowed by `ProgramIndex.flat_import_graph`.
+    flat_import_graph: std.StringHashMap(std.StringHashMap(void)),
+    /// Per-module authored-function index (`path → name → *const FnDecl`).
+    /// Borrowed by `ProgramIndex.module_fns`.
+    module_fns: imports.ModuleFns,
     ir_emitter: ?ir.LLVMEmitter = null,
     /// Lowered IR module, kept alive past `generateCode` so post-link
     /// callbacks can re-enter the interpreter to invoke sx functions
@@ -52,6 +58,8 @@ pub const Compilation = struct {
             .import_sources = std.StringHashMap([:0]const u8).init(allocator),
             .module_scopes = std.StringHashMap(std.StringHashMap(void)).init(allocator),
             .import_graph = std.StringHashMap(std.StringHashMap(void)).init(allocator),
+            .flat_import_graph = std.StringHashMap(std.StringHashMap(void)).init(allocator),
+            .module_fns = imports.ModuleFns.init(allocator),
             .target_config = target_config,
             .stdlib_paths = stdlib_paths,
         };
@@ -101,6 +109,7 @@ pub const Compilation = struct {
             &self.diagnostics,
             self.stdlib_paths,
             &self.import_graph,
+            &self.flat_import_graph,
             self.comptimeContext(),
         ) catch return error.CompileError;
 
@@ -111,6 +120,11 @@ pub const Compilation = struct {
             self.module_scopes.put(entry.key_ptr.*, entry.value_ptr.scope) catch {};
         }
 
+        // Per-module authored-function index, built from the SAME modules as
+        // `module_scopes` (main + every cache entry). Keyed by path; same-name
+        // cross-module authors stay distinct under their own paths.
+        imports.buildModuleFns(self.allocator, self.file_path, mod, &cache, &self.module_fns) catch {};
+
         // Store main file source in import_sources so error reporting can find it
         self.import_sources.put(self.file_path, self.source) catch {};
 
@@ -276,6 +290,8 @@ pub const Compilation = struct {
         lowering.diagnostics = &self.diagnostics;
         lowering.program_index.module_scopes = &self.module_scopes;
         lowering.program_index.import_graph = &self.import_graph;
+        lowering.program_index.flat_import_graph = &self.flat_import_graph;
+        lowering.program_index.module_fns = &self.module_fns;
         lowering.lowerRoot(root);
         if (self.diagnostics.hasErrors()) return error.CompileError;
 

src/imports.test.zig

@@ -0,0 +1,109 @@
+// Tests for imports.zig — flat-import name-resolution data retention (fix-0102a).
+
+const std = @import("std");
+const ast = @import("ast.zig");
+const parser = @import("parser.zig");
+const imports = @import("imports.zig");
+
+var g_test_threaded: ?std.Io.Threaded = null;
+fn testIo() std.Io {
+    if (g_test_threaded == null) {
+        g_test_threaded = std.Io.Threaded.init(std.heap.page_allocator, .{});
+    }
+    return g_test_threaded.?.io();
+}
+
+// Two flat-imported modules each author `greet`; a third is namespaced. The
+// step retains BOTH `greet` authors under their own paths in `module_fns`, and
+// records the namespaced import in `import_graph` but NOT in `flat_import_graph`.
+test "imports: module_fns retains same-name cross-module fns; flat_import_graph excludes namespaced edge" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    const alloc = arena.allocator();
+    const io = testIo();
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    try tmp.dir.writeFile(io, .{ .sub_path = "a.sx", .data = "greet :: () -> s64 { 1 }\n" });
+    try tmp.dir.writeFile(io, .{ .sub_path = "b.sx", .data = "greet :: () -> s64 { 2 }\n" });
+    try tmp.dir.writeFile(io, .{ .sub_path = "nsmod.sx", .data = "helper :: () -> s64 { 3 }\n" });
+    const main_src =
+        \\#import "a.sx";
+        \\#import "b.sx";
+        \\ns :: #import "nsmod.sx";
+        \\main :: () -> s32 { 0 }
+        \\
+    ;
+    try tmp.dir.writeFile(io, .{ .sub_path = "main.sx", .data = main_src });
+
+    var dirbuf: [4096]u8 = undefined;
+    const dirlen = try tmp.dir.realPath(io, &dirbuf);
+    const absdir = dirbuf[0..dirlen];
+
+    const main_path = try std.fmt.allocPrint(alloc, "{s}/main.sx", .{absdir});
+    const a_path = try std.fmt.allocPrint(alloc, "{s}/a.sx", .{absdir});
+    const b_path = try std.fmt.allocPrint(alloc, "{s}/b.sx", .{absdir});
+    const ns_path = try std.fmt.allocPrint(alloc, "{s}/nsmod.sx", .{absdir});
+
+    const main_bytes = try std.Io.Dir.readFileAlloc(.cwd(), io, main_path, alloc, .limited(1 << 20));
+    const main_source = try alloc.dupeZ(u8, main_bytes);
+    var p = parser.Parser.init(alloc, main_source);
+    const root = p.parse() catch return error.ParseFailed;
+
+    var chain = std.StringHashMap(void).init(alloc);
+    var cache = imports.ModuleCache.init(alloc);
+    var import_graph = std.StringHashMap(std.StringHashMap(void)).init(alloc);
+    var flat_import_graph = std.StringHashMap(std.StringHashMap(void)).init(alloc);
+    const stdlib_paths = [_][]const u8{};
+
+    const mod = try imports.resolveImports(
+        alloc,
+        io,
+        root,
+        absdir,
+        main_path,
+        &chain,
+        &cache,
+        null,
+        null,
+        &stdlib_paths,
+        &import_graph,
+        &flat_import_graph,
+        .{},
+    );
+
+    var module_fns = imports.ModuleFns.init(alloc);
+    try imports.buildModuleFns(alloc, main_path, mod, &cache, &module_fns);
+
+    // mergeFlat no longer first-wins-drops the second `greet`: the global flat
+    // decl list the lowering pass walks now carries BOTH authors.
+    var greet_count: usize = 0;
+    for (mod.decls) |decl| {
+        const name = decl.data.declName() orelse continue;
+        if (std.mem.eql(u8, name, "greet")) greet_count += 1;
+    }
+    try std.testing.expectEqual(@as(usize, 2), greet_count);
+
+    // module_fns retains BOTH authors of `greet`, keyed by their own paths.
+    const a_fns = module_fns.get(a_path) orelse return error.MissingAFns;
+    const b_fns = module_fns.get(b_path) orelse return error.MissingBFns;
+    const a_greet = a_fns.get("greet") orelse return error.MissingAGreet;
+    const b_greet = b_fns.get("greet") orelse return error.MissingBGreet;
+    // Distinct authoring decls — not the same node deduped down to one.
+    try std.testing.expect(a_greet != b_greet);
+
+    // flat_import_graph carries the two bare `#import` edges, NOT the
+    // namespaced `ns :: #import` edge.
+    const flat = flat_import_graph.get(main_path) orelse return error.MissingFlatEdges;
+    try std.testing.expect(flat.contains(a_path));
+    try std.testing.expect(flat.contains(b_path));
+    try std.testing.expect(!flat.contains(ns_path));
+
+    // The full import_graph DOES record the namespaced edge (the contrast that
+    // makes the flat-graph exclusion meaningful).
+    const full = import_graph.get(main_path) orelse return error.MissingFullEdges;
+    try std.testing.expect(full.contains(a_path));
+    try std.testing.expect(full.contains(b_path));
+    try std.testing.expect(full.contains(ns_path));
+}

src/imports.zig

@@ -5,6 +5,32 @@ const errors = @import("errors.zig");
 const c_import = @import("c_import.zig");
 const Node = ast.Node;
 
+/// True iff `decl` authors a top-level FUNCTION — either a bare `fn_decl`
+/// (`f :: (…) -> T { … }`) or a `const_decl` whose value is a function
+/// (`f :: (…) { … }` parsed as a const-bound fn). Used by the flat / directory
+/// merge to retain a same-name function authored by a DIFFERENT module instead
+/// of first-wins dropping it (fix-0102a): a flat importer that pulls two modules
+/// each authoring `f` needs BOTH decls reachable downstream. Non-function decls
+/// keep first-wins dedup.
+fn declAuthorsFn(decl: *const Node) bool {
+    return switch (decl.data) {
+        .fn_decl => true,
+        .const_decl => |cd| cd.value.data == .fn_decl,
+        else => false,
+    };
+}
+
+/// The `*const ast.FnDecl` a function-authoring decl carries, or null when the
+/// decl is not a function (mirrors `declAuthorsFn`). Drives the per-module
+/// `module_fns` identity index (fix-0102a).
+fn fnDeclOf(decl: *const Node) ?*const ast.FnDecl {
+    return switch (decl.data) {
+        .fn_decl => &decl.data.fn_decl,
+        .const_decl => |cd| if (cd.value.data == .fn_decl) &cd.value.data.fn_decl else null,
+        else => null,
+    };
+}
+
 /// Comptime evaluation context for the inline-if hoisting pass below.
 /// Mirrors the values `injectComptimeConstants` will later push into the
 /// lowering's `comptime_constants` map (OS / ARCH / POINTER_SIZE), but
@@ -334,8 +360,13 @@ pub const ResolvedModule = struct {
         for (other.decls) |decl| {
             if (seen_nodes.contains(decl)) continue;
             if (decl.data.declName()) |name| {
-                if (seen_list.contains(name)) continue;
-                try seen_list.put(name, {});
+                if (seen_list.contains(name)) {
+                    // First-wins dedup for non-functions; retain a same-name
+                    // FUNCTION authored by a different module (fix-0102a).
+                    if (!declAuthorsFn(decl)) continue;
+                } else {
+                    try seen_list.put(name, {});
+                }
             }
             try seen_nodes.put(decl, {});
             try list.append(allocator, decl);
@@ -392,6 +423,43 @@ pub const ResolvedModule = struct {
 /// Module cache: maps resolved file paths to their ResolvedModules.
 pub const ModuleCache = std.StringHashMap(ResolvedModule);
 
+/// Per-module function identity index: function NAME → the `*const FnDecl` that
+/// module AUTHORS. Mirrors a single module's slice of `module_scopes`.
+pub const FnIndex = std.StringHashMap(*const ast.FnDecl);
+
+/// `path → name → *const FnDecl`, mirroring `module_scopes`. One entry per
+/// resolved module keyed by its path (a directory's combined module keyed by
+/// `dir_path`); each entry indexes only what that module AUTHORS. Two modules
+/// each authoring `f` are retained under their own paths — the identity index
+/// fix-0102c's bare-name disambiguation consults to bind a flat call to the
+/// right author.
+pub const ModuleFns = std.StringHashMap(FnIndex);
+
+/// Index a single module's authored functions (`own_decls`) into `out[path]`.
+/// First-wins WITHIN a module mirrors the scan pass; cross-module same-name
+/// authors live under their own `path` keys.
+fn indexModuleFns(allocator: std.mem.Allocator, out: *ModuleFns, path: []const u8, own_decls: []const *Node) !void {
+    const gop = try out.getOrPut(path);
+    if (!gop.found_existing) gop.value_ptr.* = FnIndex.init(allocator);
+    for (own_decls) |decl| {
+        const fd = fnDeclOf(decl) orelse continue;
+        const name = decl.data.declName() orelse continue;
+        if (gop.value_ptr.contains(name)) continue;
+        try gop.value_ptr.put(name, fd);
+    }
+}
+
+/// Build the per-module function index from a resolved program: the main module
+/// (keyed by `main_path`) plus every cached module (keyed by its own path).
+/// Mirrors how `core.zig` fills `module_scopes` from `mod.scope` + the cache.
+pub fn buildModuleFns(allocator: std.mem.Allocator, main_path: []const u8, main_mod: ResolvedModule, cache: *const ModuleCache, out: *ModuleFns) !void {
+    try indexModuleFns(allocator, out, main_path, main_mod.own_decls);
+    var it = cache.iterator();
+    while (it.next()) |entry| {
+        try indexModuleFns(allocator, out, entry.key_ptr.*, entry.value_ptr.own_decls);
+    }
+}
+
 pub fn resolveImports(
     allocator: std.mem.Allocator,
     io: std.Io,
@@ -404,6 +472,7 @@ pub fn resolveImports(
     diagnostics: ?*errors.DiagnosticList,
     stdlib_paths: []const []const u8,
     import_graph: ?*std.StringHashMap(std.StringHashMap(void)),
+    flat_import_graph: ?*std.StringHashMap(std.StringHashMap(void)),
     comptime_ctx: ComptimeContext,
 ) !ResolvedModule {
     // Record this file's edge set so `param_impl_map` lookups can filter
@@ -414,6 +483,15 @@ pub fn resolveImports(
             try g.put(file_path, std.StringHashMap(void).init(allocator));
         }
     }
+    // FLAT-only edge set: identical to `import_graph` but records ONLY bare
+    // `#import "…"` edges (`imp.name == null`), never a namespaced
+    // `ns :: #import "…"`. fix-0102c's bare-name disambiguation walks this to
+    // decide which same-name authors a flat importer can actually reach.
+    if (flat_import_graph) |g| {
+        if (!g.contains(file_path)) {
+            try g.put(file_path, std.StringHashMap(void).init(allocator));
+        }
+    }
     var mod = ResolvedModule{
         .path = file_path,
         .decls = &.{},
@@ -536,6 +614,17 @@ pub fn resolveImports(
                 set.put(resolved_path, {}) catch {};
             }
         }
+        // The same edge, FLAT-only: recorded only for a bare `#import`
+        // (`imp.name == null`), excluding a namespaced `ns :: #import`. Covers
+        // both a flat file import and a flat directory import (`resolved_path`
+        // is the directory in the latter case).
+        if (imp.name == null) {
+            if (flat_import_graph) |g| {
+                if (g.getPtr(file_path)) |set| {
+                    set.put(resolved_path, {}) catch {};
+                }
+            }
+        }
 
         // Circular import check — only along the current chain
         if (chain.contains(resolved_path)) continue;
@@ -563,7 +652,7 @@ pub fn resolveImports(
                 // Push onto chain before recursing, pop after
                 try chain.put(resolved_path, {});
                 const imp_dir = dirName(resolved_path);
-                const result = try resolveImports(allocator, io, imp_root, imp_dir, resolved_path, chain, cache, source_map, diagnostics, stdlib_paths, import_graph, comptime_ctx);
+                const result = try resolveImports(allocator, io, imp_root, imp_dir, resolved_path, chain, cache, source_map, diagnostics, stdlib_paths, import_graph, flat_import_graph, comptime_ctx);
                 _ = chain.remove(resolved_path);
 
                 // Cache
@@ -571,7 +660,7 @@ pub fn resolveImports(
                 break :blk result;
             } else |_| {
                 // File read failed — try as directory import
-                const result = resolveDirectoryImport(allocator, io, resolved_path, chain, cache, source_map, diagnostics, decl.span, stdlib_paths, import_graph, comptime_ctx) catch {
+                const result = resolveDirectoryImport(allocator, io, resolved_path, chain, cache, source_map, diagnostics, decl.span, stdlib_paths, import_graph, flat_import_graph, comptime_ctx) catch {
                     if (diagnostics) |diags| {
                         diags.addFmt(.err, decl.span, "cannot read import '{s}' (not a file or directory)", .{resolved_path});
                     }
@@ -605,6 +694,7 @@ fn resolveDirectoryImport(
     span: ast.Span,
     stdlib_paths: []const []const u8,
     import_graph: ?*std.StringHashMap(std.StringHashMap(void)),
+    flat_import_graph: ?*std.StringHashMap(std.StringHashMap(void)),
     comptime_ctx: ComptimeContext,
 ) anyerror!ResolvedModule {
     // Open the directory with iteration capability
@@ -679,7 +769,7 @@ fn resolveDirectoryImport(
             };
 
             try chain.put(file_path, {});
-            const result = try resolveImports(allocator, io, imp_root, dir_path, file_path, chain, cache, source_map, diagnostics, stdlib_paths, import_graph, comptime_ctx);
+            const result = try resolveImports(allocator, io, imp_root, dir_path, file_path, chain, cache, source_map, diagnostics, stdlib_paths, import_graph, flat_import_graph, comptime_ctx);
             _ = chain.remove(file_path);
 
             try cache.put(file_path, result);
@@ -697,8 +787,13 @@ fn resolveDirectoryImport(
         for (file_mod.decls) |decl| {
             if (seen_nodes.contains(decl)) continue;
             if (decl.data.declName()) |name| {
-                if (seen_in_list.contains(name)) continue;
-                try seen_in_list.put(name, {});
+                if (seen_in_list.contains(name)) {
+                    // First-wins dedup for non-functions; retain a same-name
+                    // FUNCTION authored by a different file (fix-0102a).
+                    if (!declAuthorsFn(decl)) continue;
+                } else {
+                    try seen_in_list.put(name, {});
+                }
             }
             try seen_nodes.put(decl, {});
             try decl_list.append(allocator, decl);

src/ir/program_index.zig

@@ -1,5 +1,6 @@
 const std = @import("std");
 const ast = @import("../ast.zig");
+const imports = @import("../imports.zig");
 const types = @import("types.zig");
 const inst = @import("inst.zig");
 const errors = @import("../errors.zig");
@@ -570,9 +571,10 @@ pub const GlobalInfo = struct { id: inst.GlobalId, ty: TypeId };
 /// `self.program_index.<field>`; later phases hand collaborator modules a
 /// `*ProgramIndex` instead of `*Lowering`.
 ///
-/// OWNS the declaration maps below. BORROWS `module_scopes` / `import_graph`
-/// (pointers into maps owned by the compilation driver, `core.zig`) — those
-/// are read-only views and are never freed here.
+/// OWNS the declaration maps below. BORROWS `module_scopes` / `import_graph` /
+/// `flat_import_graph` / `module_fns` (pointers into maps owned by the
+/// compilation driver, `core.zig`) — those are read-only views and are never
+/// freed here.
 ///
 /// Per-map allocators are preserved exactly as they were on `Lowering`:
 /// `import_flags` / `fn_ast_map` / `global_names` use the lowering allocator
@@ -587,6 +589,16 @@ pub const ProgramIndex = struct {
     /// Module path → set of directly imported paths (param_impl visibility
     /// filter). Borrowed view.
     import_graph: ?*std.StringHashMap(std.StringHashMap(void)) = null,
+    /// Module path → set of directly FLAT-imported paths — the subset of
+    /// `import_graph` edges from a bare `#import` (never a namespaced
+    /// `ns :: #import`). fix-0102c's bare-name disambiguation walks this to
+    /// decide which same-name authors a flat importer can reach. Borrowed view.
+    flat_import_graph: ?*std.StringHashMap(std.StringHashMap(void)) = null,
+    /// Module path → (function name → authoring `*const FnDecl`), mirroring
+    /// `module_scopes`. Retains every same-name author under its own path so
+    /// fix-0102c can resolve a flat call to the right module's function.
+    /// Borrowed view.
+    module_fns: ?*imports.ModuleFns = null,
 
     // ── Declaration maps ──
     /// Function name → AST decl.
@@ -627,7 +639,8 @@ pub const ProgramIndex = struct {
     }
 
     pub fn deinit(self: *ProgramIndex) void {
-        // Owned maps only — module_scopes / import_graph are borrowed.
+        // Owned maps only — module_scopes / import_graph / flat_import_graph /
+        // module_fns are borrowed.
         self.import_flags.deinit();
         self.fn_ast_map.deinit();
         self.qualified_fn_source.deinit();

src/root.zig

@@ -13,6 +13,7 @@ pub const trace_runtime_tests = @import("runtime_trace.test.zig");
 pub const sema = @import("sema.zig");
 pub const sema_tests = @import("sema.test.zig");
 pub const imports = @import("imports.zig");
+pub const imports_tests = @import("imports.test.zig");
 pub const core = @import("core.zig");
 pub const c_import = @import("c_import.zig");
 pub const ir = @import("ir/ir.zig");