From 365ea7b16d02b653f406dd2521e6a1150c9aa72e Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 11 Mar 2026 15:54:37 +0300
Subject: [PATCH] checker: immutable instance of a struct can be modified via
 interface (fixes #23509)

---
 vlib/v/checker/checker.v                       | 15 ++++++++++++++-
 ...face_cast_from_immutable_to_mutable_err.out | 13 +++++++++++++
 ...rface_cast_from_immutable_to_mutable_err.vv | 18 ++++++++++++++++++
 3 files changed, 45 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.out
 create mode 100644 vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 050c1c585..4e3ea17b0 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -1183,6 +1183,12 @@ fn (mut c Checker) fail_if_immutable_to_mutable(left_type ast.Type, right_type a
 		return true
 	}
 	match right {
+		ast.CastExpr {
+			if c.cast_expr_aliases_value(right) {
+				mut expr := right.expr
+				c.fail_if_immutable(mut expr)
+			}
+		}
 		ast.Ident {
 			if right.obj is ast.Var {
 				if left_type.is_ptr() && !right.is_mut() && right_type.is_ptr() {
@@ -1243,6 +1249,10 @@ fn (mut c Checker) fail_if_immutable_to_mutable(left_type ast.Type, right_type a
 	return true
 }
 
+fn (mut c Checker) cast_expr_aliases_value(node ast.CastExpr) bool {
+	return c.table.final_sym(c.unwrap_generic(node.typ)).kind == .interface
+}
+
 // returns name and position of variable that needs write lock
 // also sets `is_changed` to true (TODO update the name to reflect this?)
 fn (mut c Checker) fail_if_immutable(mut expr ast.Expr) (string, token.Pos) {
@@ -1251,7 +1261,10 @@ fn (mut c Checker) fail_if_immutable(mut expr ast.Expr) (string, token.Pos) {
 	mut explicit_lock_needed := false
 	match mut expr {
 		ast.CastExpr {
-			// TODO
+			if c.cast_expr_aliases_value(expr) {
+				mut inner_expr := expr.expr
+				return c.fail_if_immutable(mut inner_expr)
+			}
 			return '', expr.pos
 		}
 		ast.ComptimeSelector {
diff --git a/vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.out b/vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.out
new file mode 100644
index 000000000..88e06e42a
--- /dev/null
+++ b/vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.out
@@ -0,0 +1,13 @@
+vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.vv:15:26: error: `bar` is immutable, declare it with `mut` to make it mutable
+   13 |         foo: 'hello'
+   14 |     }
+   15 |     mut foobar := Interface(bar)
+      |                             ~~~
+   16 |     foobar.foo = 'bye'
+   17 |     Interface(bar).foo = 'changed'
+vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.vv:17:12: error: `bar` is immutable, declare it with `mut` to make it mutable
+   15 |     mut foobar := Interface(bar)
+   16 |     foobar.foo = 'bye'
+   17 |     Interface(bar).foo = 'changed'
+      |               ~~~
+   18 | }
diff --git a/vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.vv b/vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.vv
new file mode 100644
index 000000000..21329989d
--- /dev/null
+++ b/vlib/v/checker/tests/interface_cast_from_immutable_to_mutable_err.vv
@@ -0,0 +1,18 @@
+interface Interface {
+mut:
+	foo string
+}
+
+struct Struct {
+mut:
+	foo string
+}
+
+fn main() {
+	bar := Struct{
+		foo: 'hello'
+	}
+	mut foobar := Interface(bar)
+	foobar.foo = 'bye'
+	Interface(bar).foo = 'changed'
+}
-- 
2.39.5