From a51ce07ac92dcae986b546f2ff8307226945baad Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Sat, 30 May 2026 22:01:42 +0300
Subject: [PATCH] cgen: fix bound interface method values (#27293)

---
 vlib/v/gen/c/cgen.v                           | 15 +++--
 ..._bound_mut_method_value_issue_26986_test.v | 56 +++++++++++++++++++
 2 files changed, 66 insertions(+), 5 deletions(-)
 create mode 100644 vlib/v/tests/interfaces/interface_bound_mut_method_value_issue_26986_test.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index a22e346cb..57a27d013 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -7675,6 +7675,11 @@ fn (mut g Gen) gen_closure_fn(expr_styp string, m ast.Fn, name string) {
 	if rec_sym.info is ast.Interface && rec_sym.info.get_methods().contains(method_name) {
 		methods_struct_name := g.interface_methods_struct_name(receiver.typ)
 		sb.write_string('((${methods_struct_name}*)a0->_methods)->_method_${method_name}(')
+		sb.write_string('a0->_object')
+		for i in 1 .. m.params.len {
+			sb.write_string(', ')
+			sb.write_string('a${i}')
+		}
 	} else {
 		mut full_method_name := '${expr_styp}_${method_name}'
 		if !expr_styp.starts_with('builtin__') {
@@ -7690,12 +7695,12 @@ fn (mut g Gen) gen_closure_fn(expr_styp string, m ast.Fn, name string) {
 		if !receiver.typ.is_ptr() {
 			sb.write_string('*')
 		}
-	}
-	for i in 0 .. m.params.len {
-		if i != 0 {
-			sb.write_string(', ')
+		for i in 0 .. m.params.len {
+			if i != 0 {
+				sb.write_string(', ')
+			}
+			sb.write_string('a${i}')
 		}
-		sb.write_string('a${i}')
 	}
 	sb.writeln(');')
 	sb.writeln('}')
diff --git a/vlib/v/tests/interfaces/interface_bound_mut_method_value_issue_26986_test.v b/vlib/v/tests/interfaces/interface_bound_mut_method_value_issue_26986_test.v
new file mode 100644
index 000000000..ea132c5a8
--- /dev/null
+++ b/vlib/v/tests/interfaces/interface_bound_mut_method_value_issue_26986_test.v
@@ -0,0 +1,56 @@
+struct ProtoReader {
+	value string
+}
+
+fn (pr &ProtoReader) read() string {
+	return pr.value
+}
+
+struct Conn {
+	rd &ProtoReader
+}
+
+fn (c &Conn) with_proto_reader(f fn (rd &ProtoReader) !) ! {
+	f(c.rd)!
+}
+
+interface Cmder {
+mut:
+	read_reply(rd &ProtoReader) !
+}
+
+struct SomeCmd {
+mut:
+	value string
+}
+
+fn (mut sc SomeCmd) read_reply(rd &ProtoReader) ! {
+	sc.value = rd.read()
+}
+
+fn process_via_interface_pointer(mut cmd Cmder) ! {
+	c := &Conn{
+		rd: &ProtoReader{
+			value: 'from pointer'
+		}
+	}
+	mut cmd_ref := unsafe { &cmd }
+	c.with_proto_reader(cmd_ref.read_reply)!
+}
+
+fn process_via_interface_value(mut cmd Cmder) ! {
+	c := &Conn{
+		rd: &ProtoReader{
+			value: 'from value'
+		}
+	}
+	c.with_proto_reader(cmd.read_reply)!
+}
+
+fn test_bound_mut_interface_method_value_mutates_underlying_object() ! {
+	mut cmd := &SomeCmd{}
+	process_via_interface_pointer(mut cmd)!
+	assert cmd.value == 'from pointer'
+	process_via_interface_value(mut cmd)!
+	assert cmd.value == 'from value'
+}
-- 
2.39.5