From 74e75e41fe12f19e5365a3152ffafa1a29376240 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Sat, 19 Sep 2026 23:52:00 +0200
Subject: Reject exporting symbols with the same name from 2 modules

---
 boot/dependency.cc | 20 ++++++++++++++++++--
 boot/symbol.cc     | 20 ++++++++++++++++++--
 2 files changed, 36 insertions(+), 4 deletions(-)

(limited to 'boot')

diff --git a/boot/dependency.cc b/boot/dependency.cc
index 1eb5953..5053a36 100644
--- a/boot/dependency.cc
+++ b/boot/dependency.cc
@@ -72,7 +72,7 @@ namespace elna::boot
     }
 
     analysis_result analyze_semantics(std::unique_ptr<unit>& tree,
-            const std::vector<std::shared_ptr<symbol_table>>& imports,
+            const std::vector<module_import>& imports,
             const std::shared_ptr<symbol_table>& globals, const target_info& target,
             const std::filesystem::path& module_path)
     {
@@ -90,7 +90,23 @@ namespace elna::boot
         }
         for (const auto& import : imports)
         {
-            result.value.add_import(import);
+            const std::vector<import_collision> collisions = result.value.add_import(import.symbols);
+
+            for (const import_collision& collision : collisions)
+            {
+                symbol_declaration_error::redefinition original_definition{
+                    .original = collision.original->position,
+                    .file = collision.original->file
+                };
+                auto error = std::make_unique<symbol_declaration_error>(import.position,
+                        collision.name, original_definition);
+
+                result.errors.push_back(std::move(error));
+            }
+        }
+        if (!result.errors.empty())
+        {
+            return result;
         }
         declaration_visitor declarations(result.value, target, module_path);
         tree->accept(&declarations);
diff --git a/boot/symbol.cc b/boot/symbol.cc
index 58524ad..962808a 100644
--- a/boot/symbol.cc
+++ b/boot/symbol.cc
@@ -17,6 +17,7 @@ along with GCC; see the file COPYING3.  If not see
 
 #include "elna/boot/symbol.h"
 
+#include <algorithm>
 #include <ranges>
 #include <cassert>
 #include <utility>
@@ -511,9 +512,24 @@ namespace elna::boot
         return this->unresolved.insert({ symbol_name, std::move(forward_declaration) }).second;
     }
 
-    void symbol_bag::add_import(const std::shared_ptr<symbol_table>& symbols)
+    std::vector<import_collision> symbol_bag::add_import(const std::shared_ptr<symbol_table>& symbols)
     {
-        this->imports.push_front(symbols);
+        std::vector<import_collision> collisions;
+
+        for (const auto& [name, imported] : *symbols)
+        {
+            std::shared_ptr<info> original = lookup_import(name);
+
+            // The same module imported twice hands out the same symbol, which
+            // doesn't conflict with itself.
+            if (original != nullptr && original != imported)
+            {
+                collisions.push_back({ .name = name, .original = std::move(original) });
+            }
+        }
+        this->imports.push_back(symbols);
+
+        return collisions;
     }
 
     bool symbol_bag::is_global() const
-- 
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