cgen: fix #19407

vlib/v/gen/c/cgen.v

@@ -2265,15 +2265,19 @@ struct SumtypeCastingFn {
 fn (mut g Gen) get_sumtype_casting_fn(got_ ast.Type, exp_ ast.Type) string {
 	got, exp := got_.idx(), exp_.idx()
 	i := got | int(u32(exp) << 16)
-	got_cname, exp_cname := g.table.sym(got).cname, g.table.sym(exp).cname
-	fn_name := '${got_cname}_to_sumtype_${exp_cname}'
+	exp_sym := g.table.sym(exp)
+	mut got_sym := g.table.sym(got)
+	fn_name := '${got_sym.cname}_to_sumtype_${exp_sym.cname}'
 	if got == exp || g.sumtype_definitions[i] {
 		return fn_name
 	}
+	for got_sym.parent_idx != 0 && got_sym.idx !in (exp_sym.info as ast.SumType).variants {
+		got_sym = g.table.sym(got_sym.parent_idx)
+	}
 	g.sumtype_definitions[i] = true
 	g.sumtype_casting_fns << SumtypeCastingFn{
 		fn_name: fn_name
-		got: got
+		got: got_sym.idx
 		exp: exp
 	}
 	return fn_name

vlib/v/tests/alias_to_sum_with_enum_test.v

@@ -0,0 +1,13 @@
+enum Test {
+	abc
+}
+
+type SumTest = Test | u8
+
+type TestAlias = Test
+
+fn test_main() {
+	a := TestAlias.abc
+	b := SumTest(a)
+	assert b is Test
+}