From df8cb3e94c63c266d6388b57f7a0dcea9f087f42 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:16 +0300
Subject: [PATCH] checker: fix veb c compilation error when context not mutable
 (fixes #25361)

---
 vlib/v/checker/fn.v                                 |  7 +++++++
 .../checker/tests/veb_context_param_mutability.out  |  7 +++++++
 .../v/checker/tests/veb_context_param_mutability.vv | 13 +++++++++++++
 3 files changed, 27 insertions(+)
 create mode 100644 vlib/v/checker/tests/veb_context_param_mutability.out
 create mode 100644 vlib/v/checker/tests/veb_context_param_mutability.vv

diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 7d67d0f84..1646f5789 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -788,6 +788,13 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 	// Register implicit context var
 	typ_veb_result := c.table.get_veb_result_type_idx() // c.table.find_type('veb.Result')
 	if node.is_method && node.return_type == typ_veb_result {
+		for param in node.params[1..] {
+			if c.has_veb_context(param.typ) && !param.is_mut {
+				c.error('veb app method `${node.name}` must declare context parameter `${param.name}` as mutable, e.g. `mut ${param.name} ${c.table.type_to_str(param.typ)}`',
+					param.pos)
+				break
+			}
+		}
 		// Find a custom user Context type first
 		mut ctx_idx := c.table.find_type('main.Context')
 		if ctx_idx < 1 {
diff --git a/vlib/v/checker/tests/veb_context_param_mutability.out b/vlib/v/checker/tests/veb_context_param_mutability.out
new file mode 100644
index 000000000..3f6c8cf8a
--- /dev/null
+++ b/vlib/v/checker/tests/veb_context_param_mutability.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/veb_context_param_mutability.vv:11:25: error: veb app method `index` must declare context parameter `ctx` as mutable, e.g. `mut ctx Context`
+    9 | pub struct App {}
+   10 | 
+   11 | pub fn (app &App) index(ctx Context) veb.Result {
+      |                         ~~~
+   12 |     return veb.no_result()
+   13 | }
diff --git a/vlib/v/checker/tests/veb_context_param_mutability.vv b/vlib/v/checker/tests/veb_context_param_mutability.vv
new file mode 100644
index 000000000..4f8955b9b
--- /dev/null
+++ b/vlib/v/checker/tests/veb_context_param_mutability.vv
@@ -0,0 +1,13 @@
+module main
+
+import veb
+
+pub struct Context {
+	veb.Context
+}
+
+pub struct App {}
+
+pub fn (app &App) index(ctx Context) veb.Result {
+	return veb.no_result()
+}
-- 
2.39.5