From 6cf401dfcd4a8e4f3b058974f0a1aba0a038b77f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicol=C3=B2=20Ribaudo?= <nribaudo@igalia.com>
Date: Wed, 18 Mar 2026 19:32:52 +0100
Subject: [PATCH] Lint against re-declaring parameters

---
 src/lint/rules/variable-use-def.ts |  3 +-
 test/lint-variable-use-def.ts      | 48 ++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/src/lint/rules/variable-use-def.ts b/src/lint/rules/variable-use-def.ts
index 3a9e9361..b364a4fe 100644
--- a/src/lint/rules/variable-use-def.ts
+++ b/src/lint/rules/variable-use-def.ts
@@ -141,9 +141,10 @@ export function checkVariableUsage(
       preceding.querySelector('emu-alg') == null &&
       preceding.textContent != null
     ) {
+      const isParameter = preceding.nodeName === 'H1';
       // `__` is for <del>_x_</del><ins>_y_</ins>, which has textContent `_x__y_`
       for (const name of preceding.textContent.matchAll(/(?<=\b|_)_([a-zA-Z0-9]+)_(?=\b|_)/g)) {
-        scope.declare(name[1], null, undefined, true);
+        scope.declare(name[1], null, isParameter ? 'parameter' : undefined, !isParameter);
       }
     }
     preceding = previousOrParent(preceding, parentClause);
diff --git a/test/lint-variable-use-def.ts b/test/lint-variable-use-def.ts
index f13a5eac..bb4ced15 100644
--- a/test/lint-variable-use-def.ts
+++ b/test/lint-variable-use-def.ts
@@ -699,4 +699,52 @@ describe('variables cannot be redeclared', () => {
       `,
     );
   });
+
+  it('parameters cannot be redeclared', async () => {
+    await assertLint(
+      positioned`
+        <emu-clause id="example" type="abstract operation">
+          <h1>
+            Example (
+              _x_: ~foo~,
+            ): ~foo~ or ~bar~
+          </h1>
+          <dl class="header"></dl>
+          <emu-alg>
+            1. Let ${M}_x_ be ~bar~.
+            1. Return _x_.
+          </emu-alg>
+        </emu-clause>
+      `,
+      {
+        ruleId: 're-declaration',
+        nodeType: 'emu-alg',
+        message: '"x" is already declared',
+      },
+    );
+  });
+
+  it('optional parameters cannot be redeclared', async () => {
+    await assertLint(
+      positioned`
+        <emu-clause id="example" type="abstract operation">
+          <h1>
+            Example (
+              optional _x_: ~foo~,
+            ): ~foo~ or ~bar~
+          </h1>
+          <dl class="header"></dl>
+          <emu-alg>
+            1. If _x_ is not present, let ${M}_x_ be ~bar~.
+            1. Return _x_.
+          </emu-alg>
+        </emu-clause>
+      `,
+      {
+        ruleId: 're-declaration',
+        nodeType: 'emu-alg',
+        message: '"x" is already declared',
+      },
+    );
+  });
 });