checker: ensure that the variant SubType exists

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

vlib/v/checker/tests/undefined_type_on_sumtype.vv

@@ -0,0 +1,34 @@
+type ParseRes = Result<[]Token, ParseErr>
+
+// Token type is unknown
+//struct Token {}
+
+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 test_report() {
+	r := Opt<ParseRes>(None<ParseRes>{})
+	match r {
+		Some<ParseRes> {
+			rx := r.value
+		}
+		None<ParseRes> {}
+	}
+}