imports: dedup flat decl list by node identity (issue 0056 FIXED)

Impl blocks are anonymous (no declName), so a parameterised-protocol impl in a module reached via a diamond import was appended once per path and registered twice — 'duplicate impl Into for source s64'. mergeFlat and the directory-import merge loop now also dedup by node pointer; a physical AST node is lowered once regardless of how many import paths reach it.

Regression: examples/issue-0056-diamond-param-impl.sx.

src/imports.zig

@@ -315,20 +315,29 @@ pub const ResolvedModule = struct {
     /// `self.scope` — visibility joins per-file scopes at lookup time via
     /// `import_graph`. We only need to append `other.decls` (the full
     /// transitive list) to the global `list` so the lowering pass can
-    /// still resolve transitively-imported callees. Deduped by name.
+    /// still resolve transitively-imported callees.
+    ///
+    /// Deduped two ways: named decls by name (first-wins on cross-module
+    /// collisions), and EVERY decl by node identity. The latter matters for
+    /// anonymous decls — `impl` blocks have no `declName`, so under a diamond
+    /// import the same cached node would otherwise be appended once per path
+    /// and registered twice (e.g. `duplicate impl 'Into'`).
     pub fn mergeFlat(
         self: *ResolvedModule,
         allocator: std.mem.Allocator,
         list: *std.ArrayList(*Node),
         seen_list: *std.StringHashMap(void),
+        seen_nodes: *std.AutoHashMap(*Node, void),
         other: ResolvedModule,
     ) !void {
         _ = self;
         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, {});
             }
+            try seen_nodes.put(decl, {});
             try list.append(allocator, decl);
         }
     }
@@ -420,6 +429,9 @@ pub fn resolveImports(
     // by `mergeFlat` to dedupe across diamond imports now that `mod.scope`
     // is non-transitive and can no longer serve as the dedup key.
     var seen_in_list = std.StringHashMap(void).init(allocator);
+    // Node-identity set for the same purpose, covering anonymous decls
+    // (impl blocks) that carry no name to dedupe on.
+    var seen_nodes = std.AutoHashMap(*Node, void).init(allocator);
 
     for (flat_decls) |decl| {
         if (decl.data == .c_import_decl) {
@@ -559,7 +571,7 @@ pub fn resolveImports(
         if (imp.name) |ns_name| {
             try mod.addNamespace(allocator, &decl_list, &own_decl_list, &seen_in_list, ns_name, imported_mod, decl.span);
         } else {
-            try mod.mergeFlat(allocator, &decl_list, &seen_in_list, imported_mod);
+            try mod.mergeFlat(allocator, &decl_list, &seen_in_list, &seen_nodes, imported_mod);
         }
     }
 
@@ -622,6 +634,7 @@ fn resolveDirectoryImport(
     var decl_list = std.ArrayList(*Node).empty;
     var own_decl_list = std.ArrayList(*Node).empty;
     var seen_in_list = std.StringHashMap(void).init(allocator);
+    var seen_nodes = std.AutoHashMap(*Node, void).init(allocator);
 
     for (file_names.items) |file_name| {
         const file_path = try std.fmt.allocPrint(allocator, "{s}/{s}", .{ dir_path, file_name });
@@ -668,10 +681,12 @@ fn resolveDirectoryImport(
         // register `Event` (a tagged_union) before `handle_event(e: *Event)`
         // triggers the placeholder-struct fallback in `resolveTypeName`.
         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, {});
             }
+            try seen_nodes.put(decl, {});
             try decl_list.append(allocator, decl);
         }
         // Separately track which decls the directory `re-exports` to its