cgen: fix generic comptimeselector array resolution (#18296)

vlib/v/gen/c/fn.v

@@ -1115,6 +1115,12 @@ fn (mut g Gen) change_comptime_args(func ast.Fn, mut node_ ast.CallExpr, concret
 				}
 			} else if mut call_arg.expr is ast.ComptimeSelector {
 				comptime_args[i] = g.comptime_for_field_type
+				arg_sym := g.table.final_sym(call_arg.typ)
+				param_typ_sym := g.table.sym(param_typ)
+				if arg_sym.kind == .array && param_typ.has_flag(.generic)
+					&& param_typ_sym.kind == .array {
+					comptime_args[i] = g.get_generic_array_element_type(arg_sym.info as ast.Array)
+				}
 				if call_arg.expr.left.is_auto_deref_var() {
 					comptime_args[i] = comptime_args[i].deref()
 				}

vlib/v/tests/generic_comptime_test.v

@@ -0,0 +1,26 @@
+struct EmptyStruct {
+	b []int = [1, 2, 3]
+}
+
+fn encode_array[T](val []T) []int {
+	return val
+}
+
+fn encode_struct[T](val T) []int {
+	$if T is $struct {
+		$for field in T.fields {
+			$if field.is_array {
+				return encode_array(val.$(field.name))
+			}
+		}
+	}
+	return [0]
+}
+
+fn encoder() []int {
+	return encode_struct(EmptyStruct{})
+}
+
+fn test_main() {
+	assert encoder() == [1, 2, 3]
+}