checker: fixes typechecking for a sum type with an alias type (#13974)

vlib/v/ast/table.v

@@ -1280,25 +1280,54 @@ pub fn (t &Table) sumtype_has_variant(parent Type, variant Type, is_as bool) boo
 	if parent_sym.kind == .sum_type {
 		parent_info := parent_sym.info as SumType
 		var_sym := t.sym(variant)
-		if var_sym.kind == .aggregate {
-			var_info := var_sym.info as Aggregate
-			for var_type in var_info.types {
-				if !t.sumtype_has_variant(parent, var_type, is_as) {
+		match var_sym.kind {
+			.aggregate {
+				return t.sumtype_check_aggregate_variant(parent, variant, is_as)
+			}
+			.alias {
+				return t.sumtype_check_alias_variant(parent, variant, is_as)
+			}
+			else {
+				return t.sumtype_check_variant_in_type(parent_info, variant, is_as)
+			}
+		}
+	}
 	return false
 }
-			}
-			return true
-		} else {
+
+fn (t &Table) sumtype_check_variant_in_type(parent_info SumType, variant Type, is_as bool) bool {
 	for v in parent_info.variants {
 		if v.idx() == variant.idx() && (!is_as || v.nr_muls() == variant.nr_muls()) {
 			return true
 		}
 	}
-		}
-	}
 	return false
 }
 
+fn (t &Table) sumtype_check_aggregate_variant(parent_type Type, aggregate_type &Type, is_as bool) bool {
+	aggregate_sym := t.sym(aggregate_type).info as Aggregate
+	for var_type in aggregate_sym.types {
+		if !t.sumtype_has_variant(parent_type, var_type, is_as) {
+			return false
+		}
+	}
+	return true
+}
+
+fn (t &Table) sumtype_check_alias_variant(parent_type Type, alias_type Type, is_as bool) bool {
+	parent_sym := t.sym(parent_type).info as SumType
+	if !t.sumtype_check_variant_in_type(parent_sym, alias_type, is_as) {
+		alias_info := t.sym(alias_type).info as Alias
+		// The alias is an alias or of the same sumtype parent, or one
+		// of the SumType variant. e.g: alias of another sum type.
+		// https://github.com/vlang/v/issues/14029
+		return parent_type == alias_info.parent_type
+			|| t.sumtype_has_variant(parent_type, alias_info.parent_type, is_as)
+	}
+	// the alias_type is inside one of the variant of the sum type
+	return true
+}
+
 pub fn (t &Table) is_sumtype_or_in_variant(parent Type, typ Type) bool {
 	if typ == 0 {
 		return false

vlib/v/gen/c/testdata/check_combination_of_alias_and_sumtype.c.must_have

@@ -0,0 +1 @@
+// THE END.

vlib/v/gen/c/testdata/check_combination_of_alias_and_sumtype.vv

@@ -0,0 +1,32 @@
+type ParseRes = Result<[]Token, ParseErr>
+
+struct ParseErr {}
+
+type Opt<T> = None<T> | Some<T>
+
+struct None<T> {}
+
+struct Some<T> {
+	value T
+}
+
+type Result<T, U> = Err<U> | Ok<T>
+
+struct Ok<T> {
+	value T
+}
+
+struct Err<U> {
+	value U
+}
+
+fn main() {
+	r := Opt<ParseRes>(None<ParseRes>{})
+	match r {
+		Some<ParseRes> {
+			// make possible cast fo the same type!
+			rx := Result<[]Token, ParseErr>(r.value)
+		}
+		None<ParseRes> {}
+	}
+}

vlib/v/tests/known_errors/testdata/cast_aliases_on_generics_type.vv

@@ -1,8 +1,6 @@
 type ParseRes = Result<[]Token, ParseErr>
 
-struct ParseErr{
-
-}
+struct ParseErr {}
 
 type Opt<T> = None<T> | Some<T>
 
@@ -26,7 +24,7 @@ fn test_report() {
 	r := Opt<ParseRes>(None<ParseRes>{})
 	match r {
 		Some<ParseRes> {
-			rx := Result<[]Token, ParseErr>(r)
+			rx := Result<[]Token, ParseErr>(r.value)
 		}
 		None<ParseRes> {}
 	}