cgen: fix fn mut val of interface type (#10568)

vlib/v/gen/c/infix_expr.v

@@ -487,6 +487,9 @@ fn (mut g Gen) infix_expr_left_shift_op(node ast.InfixExpr) {
 			if needs_clone {
 				g.write('string_clone(')
 			}
+			if right.unaliased_sym.kind == .interface_ && node.right.is_auto_deref_var() {
+				g.write('*')
+			}
 			g.expr_with_cast(node.right, node.right_type, array_info.elem_type)
 			if needs_clone {
 				g.write(')')

vlib/v/tests/fn_mut_args_test.v

@@ -43,3 +43,27 @@ fn test_fn_mut_args_of_array_last() {
 	m.ar << 99
 	assert pass_array_mut(mut m.ar) == 99
 }
+
+interface ChildInterface {
+	data int
+}
+
+struct Child {
+	data int
+}
+
+struct Parent {
+mut:
+	children []ChildInterface
+}
+
+fn (mut p Parent) add(mut x ChildInterface) {
+	p.children << x
+}
+
+fn test_fn_mut_args_of_interface() {
+	mut x := Parent{}
+	x.add(mut Child{ data: 123 })
+	println(x.children[0].data)
+	assert x.children[0].data == 123
+}