From 504d34a0a89cd3c7bf1b13008f42f76dc825bd9c Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 4 Jun 2025 03:48:50 +0300
Subject: [PATCH] checker: fix mutable const bug (fix #14916)

---
 vlib/v/checker/assign.v                         |  2 +-
 vlib/v/checker/containers.v                     | 10 ++++++++++
 vlib/v/checker/tests/ban_const_ref_mutation.out |  7 +++++++
 vlib/v/checker/tests/ban_const_ref_mutation.vv  |  6 ++----
 4 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/vlib/v/checker/assign.v b/vlib/v/checker/assign.v
index ee04483558..d11f5955d0 100644
--- a/vlib/v/checker/assign.v
+++ b/vlib/v/checker/assign.v
@@ -4,7 +4,7 @@ module checker
 
 import v.ast
 
-// TODO: 600 line function
+// TODO: 980 line function
 fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 	prev_inside_assign := c.inside_assign
 	c.inside_assign = true
diff --git a/vlib/v/checker/containers.v b/vlib/v/checker/containers.v
index 33fc34fd60..3ce44a5cb9 100644
--- a/vlib/v/checker/containers.v
+++ b/vlib/v/checker/containers.v
@@ -795,6 +795,16 @@ fn (mut c Checker) check_append(mut node ast.InfixExpr, left_type ast.Type, righ
 	if !left_value_type.has_flag(.option) && right_type.has_flag(.option) {
 		c.error('unwrapped Option cannot be used in an infix expression', node.pos)
 	}
+
+	right := node.right
+	if right is ast.PrefixExpr && right.op == .amp {
+		mut expr2 := right.right
+		if mut expr2 is ast.Ident {
+			if !node.left.is_blank_ident() && expr2.obj is ast.ConstField {
+				c.error('cannot have mutable reference to const `${expr2.name}`', expr2.pos)
+			}
+		}
+	}
 	if left_value_sym.kind == .interface {
 		if right_final_sym.kind != .array {
 			// []Animal << Cat
diff --git a/vlib/v/checker/tests/ban_const_ref_mutation.out b/vlib/v/checker/tests/ban_const_ref_mutation.out
index e69de29bb2..f162269ac0 100644
--- a/vlib/v/checker/tests/ban_const_ref_mutation.out
+++ b/vlib/v/checker/tests/ban_const_ref_mutation.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/ban_const_ref_mutation.vv:11:14: error: cannot have mutable reference to const `foobar_const`
+    9 |     println('foobar_const.foo: ${foobar_const.foo}') // foobar_const.foo: 123
+   10 |     mut foobars := []&Foobar{}
+   11 |     foobars << &foobar_const
+      |                 ~~~~~~~~~~~~
+   12 |     foobars[0].foo = 456
+   13 |     println('foobar_const.foo: ${foobar_const.foo}') // foobar_const.foo: 456
diff --git a/vlib/v/checker/tests/ban_const_ref_mutation.vv b/vlib/v/checker/tests/ban_const_ref_mutation.vv
index d0209c8cb0..e71d0757cd 100644
--- a/vlib/v/checker/tests/ban_const_ref_mutation.vv
+++ b/vlib/v/checker/tests/ban_const_ref_mutation.vv
@@ -1,4 +1,3 @@
-/*
 struct Foobar {
 mut:
 	foo int
@@ -7,10 +6,9 @@ mut:
 const foobar_const = Foobar{123}
 
 fn main() {
-	println("foobar_const.foo: ${foobar_const.foo}") // foobar_const.foo: 123
+	println('foobar_const.foo: ${foobar_const.foo}') // foobar_const.foo: 123
 	mut foobars := []&Foobar{}
 	foobars << &foobar_const
 	foobars[0].foo = 456
-	println("foobar_const.foo: ${foobar_const.foo}") // foobar_const.foo: 456
+	println('foobar_const.foo: ${foobar_const.foo}') // foobar_const.foo: 456
 }
-*/