From 349e84c693c1444606e877f4d8cdc1a39aa91634 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Fri, 17 Jul 2026 17:57:40 +0200
Subject: Reject const in const nesting

---
 boot/name_analysis.cc                        | 15 +++++++++++++++
 include/elna/boot/name_analysis.h            | 12 ++++++++++++
 testsuite/compilable/const_alias.elna        | 10 ++++++++++
 testsuite/fail_compilation/double_const.elna |  7 +++++++
 4 files changed, 44 insertions(+)
 create mode 100644 testsuite/compilable/const_alias.elna
 create mode 100644 testsuite/fail_compilation/double_const.elna

diff --git a/boot/name_analysis.cc b/boot/name_analysis.cc
index 48251cd..5255878 100644
--- a/boot/name_analysis.cc
+++ b/boot/name_analysis.cc
@@ -70,6 +70,16 @@ namespace elna::boot
         return "const must be written before the array size, not after";
     }
 
+    double_const_error::double_const_error(const source_position position)
+        : error(position)
+    {
+    }
+
+    std::string double_const_error::what() const
+    {
+        return "Duplicate 'const' qualifier is not allowed";
+    }
+
     // Members of a constant aggregate are constant themselves.
     static type qualify_member_type(const type& element, const type& aggregate)
     {
@@ -179,6 +189,11 @@ namespace elna::boot
     void name_analysis_visitor::visit(constant_type_expression *expression)
     {
         walking_visitor::visit(expression);
+
+        if (this->current_type.get<constant_type>() != nullptr)
+        {
+            add_error<double_const_error>(expression->position());
+        }
         this->current_type = type(std::make_shared<constant_type>(this->current_type));
     }
 
diff --git a/include/elna/boot/name_analysis.h b/include/elna/boot/name_analysis.h
index feef3bf..40bdf16 100644
--- a/include/elna/boot/name_analysis.h
+++ b/include/elna/boot/name_analysis.h
@@ -80,6 +80,18 @@ namespace elna::boot
         std::string what() const override;
     };
 
+    /**
+     * Direct \c const \c const T is not valid — write it once.
+     * Merging through an alias is fine (C++ rule).
+     */
+    class double_const_error : public error
+    {
+    public:
+        explicit double_const_error(const source_position position);
+
+        std::string what() const override;
+    };
+
     /**
      * Origin of a field in a composite type.
      */
diff --git a/testsuite/compilable/const_alias.elna b/testsuite/compilable/const_alias.elna
new file mode 100644
index 0000000..9cb9ffb
--- /dev/null
+++ b/testsuite/compilable/const_alias.elna
@@ -0,0 +1,10 @@
+type
+  CI = const Int
+  CCI = const CI
+
+var
+  x: CCI
+  y: const CI
+
+begin
+end.
diff --git a/testsuite/fail_compilation/double_const.elna b/testsuite/fail_compilation/double_const.elna
new file mode 100644
index 0000000..204a7f7
--- /dev/null
+++ b/testsuite/fail_compilation/double_const.elna
@@ -0,0 +1,7 @@
+proc f()
+var
+  x: const const Int (* @Error Duplicate 'const' qualifier is not allowed *)
+begin
+return
+
+end.
-- 
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