cgen: allow using of original operation if alias is number and no custom method is defined (#17718)

vlib/time/time_test.v

@@ -286,3 +286,9 @@ fn test_add_seconds_to_time() {
 	future_tm := now_tm.add_seconds(60)
 	assert now_tm.unix < future_tm.unix
 }
+
+fn test_plus_equals_duration() {
+	mut d := time.second
+	d += time.second
+	assert d == 2 * time.second
+}

vlib/v/gen/c/assign.v

@@ -454,6 +454,15 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 					g.expr(val)
 					g.writeln(');')
 					return
+				} else if left_sym.kind == .alias
+					&& g.table.final_sym(g.unwrap_generic(var_type)).is_number()
+					&& !left_sym.has_method(extracted_op) {
+					g.write(' = ')
+					g.expr(left)
+					g.write(' ${extracted_op} ')
+					g.expr(val)
+					g.write(';')
+					return
 				} else {
 					g.write(' = ${styp}_${util.replace_op(extracted_op)}(')
 					method := g.table.find_method(left_sym, extracted_op) or {