From bf41d022ce9736a92097f416ab96fca7ab4594ca Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Sun, 23 Aug 2026 14:05:26 +0200
Subject: Reject 0-sized variables

---
 include/elna/boot/ast.h           | 12 ++++++++++++
 include/elna/boot/name_analysis.h |  1 +
 include/elna/boot/symbol.h        | 13 ++++++++++++-
 include/elna/boot/type_check.h    |  9 ++++++++-
 4 files changed, 33 insertions(+), 2 deletions(-)

(limited to 'include')

diff --git a/include/elna/boot/ast.h b/include/elna/boot/ast.h
index 8425b54..94488ff 100644
--- a/include/elna/boot/ast.h
+++ b/include/elna/boot/ast.h
@@ -138,6 +138,7 @@ namespace elna::boot
     class record_type_expression;
     class procedure_type_expression;
     class enumeration_type_expression;
+    class extern_type_expression;
     class named_expression;
     class array_access_expression;
     class field_access_expression;
@@ -182,6 +183,7 @@ namespace elna::boot
         virtual void visit(record_type_expression *) = 0;
         virtual void visit(procedure_type_expression *) = 0;
         virtual void visit(enumeration_type_expression *) = 0;
+        virtual void visit(extern_type_expression *) = 0;
         virtual void visit(named_expression *) = 0;
         virtual void visit(array_access_expression *) = 0;
         virtual void visit(field_access_expression *) = 0;
@@ -210,6 +212,7 @@ namespace elna::boot
         [[noreturn]] void visit(record_type_expression *) override;
         [[noreturn]] void visit(procedure_type_expression *) override;
         [[noreturn]] void visit(enumeration_type_expression *) override;
+        [[noreturn]] void visit(extern_type_expression *) override;
 
         [[noreturn]] void visit(variable_declaration *) override;
         [[noreturn]] void visit(procedure_declaration *) override;
@@ -257,6 +260,7 @@ namespace elna::boot
         void visit(record_type_expression *) override;
         void visit(procedure_type_expression *) override;
         void visit(enumeration_type_expression *) override;
+        void visit(extern_type_expression *) override;
 
         void visit(variable_declaration *) override;
         void visit(procedure_declaration *) override;
@@ -422,6 +426,14 @@ namespace elna::boot
         type_expression& base();
     };
 
+    class extern_type_expression : public type_expression
+    {
+    public:
+        explicit extern_type_expression(const source_position position);
+
+        void accept(parser_visitor *visitor) override;
+    };
+
     using field_declaration = std::pair<std::vector<identifier>, std::shared_ptr<type_expression>>;
 
     class record_type_expression : public type_expression
diff --git a/include/elna/boot/name_analysis.h b/include/elna/boot/name_analysis.h
index 50d9fe6..f2e2eb9 100644
--- a/include/elna/boot/name_analysis.h
+++ b/include/elna/boot/name_analysis.h
@@ -155,6 +155,7 @@ namespace elna::boot
         void visit(slicing_expression *expression) override;
         void visit(procedure_type_expression *expression) override;
         void visit(enumeration_type_expression *expression) override;
+        void visit(extern_type_expression *) override;
 
         void visit(variable_declaration *declaration) override;
         void visit(procedure_declaration *declaration) override;
diff --git a/include/elna/boot/symbol.h b/include/elna/boot/symbol.h
index 1caba83..0c1026d 100644
--- a/include/elna/boot/symbol.h
+++ b/include/elna/boot/symbol.h
@@ -40,6 +40,7 @@ namespace elna::boot
     struct slice_type;
     struct procedure_type;
     struct enumeration_type;
+    struct extern_type;
 
     /**
      * Represents a type stored in the symbol table.
@@ -59,7 +60,8 @@ namespace elna::boot
             std::shared_ptr<array_type>,
             std::shared_ptr<slice_type>,
             std::shared_ptr<procedure_type>,
-            std::shared_ptr<enumeration_type>
+            std::shared_ptr<enumeration_type>,
+            std::shared_ptr<extern_type>
         >;
         Payload payload;
 
@@ -188,6 +190,15 @@ namespace elna::boot
         explicit enumeration_type(const std::vector<std::string>& members);
     };
 
+    /**
+     * An opaque type declared with \c type T = extern.  The representation is
+     * unknown, so the type has no properties and can only be used behind a
+     * pointer.
+     */
+    struct extern_type
+    {
+    };
+
     class type_info;
     class procedure_info;
     class variable_info;
diff --git a/include/elna/boot/type_check.h b/include/elna/boot/type_check.h
index 7a8eece..0b62a4a 100644
--- a/include/elna/boot/type_check.h
+++ b/include/elna/boot/type_check.h
@@ -86,7 +86,14 @@ namespace elna::boot
             condition,
             array_index,
             non_indexable,
-            dereference_of_non_pointer
+            dereference_of_non_pointer,
+            opaque_variable,
+            opaque_parameter,
+            opaque_return,
+            opaque_field,
+            opaque_element,
+            opaque_cast,
+            zero_sized
         };
 
         type_requirement_error(const source_position position,
-- 
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