Fix issue with variants of 1 type and tags (#1485)

* Fix issue with variants of 1 type and tags

* bool cast

Author: stephenberry

include/glaze/json/read.hpp

@@ -2201,7 +2201,7 @@ namespace glz
                      ctx.error = error_code::no_matching_variant_type;
                      return;
                   }
-                  else if constexpr ((type_counts::n_object + type_counts::n_nullable_object) == 1) {
+                  else if constexpr ((type_counts::n_object + type_counts::n_nullable_object) == 1 && tag_v<T>.empty()) {
                      using V = glz::tuple_element_t<0, object_types>;
                      if (!std::holds_alternative<V>(value)) value = V{};
                      read<JSON>::op<opening_handled<Opts>()>(std::get<V>(value), ctx, it, end);

tests/json_test/json_test.cpp

@@ -9941,6 +9941,58 @@ suite custom_nullable_with_specialization = [] {
    };
 };
 
+struct A1
+{
+   int p{};
+};
+
+struct B1
+{
+   float p{};
+};
+
+using X1 = std::variant<A1>;
+using Y1 = std::variant<A1, B1>;
+
+template <>
+struct glz::meta<A1>
+{
+    static constexpr auto value = object(
+        "p", &A1::p);
+};
+
+template <>
+struct glz::meta<B1>
+{
+    static constexpr auto value = object(
+        "p", &B1::p);
+};
+
+template <>
+struct glz::meta<X1>
+{
+    static constexpr std::string_view tag = "tag";
+};
+
+template <>
+struct glz::meta<Y1>
+{
+    static constexpr std::string_view tag = "tag";
+};
+
+suite variant_tag_tests = [] {
+   "variant tag"_test = [] {
+      auto xString = glz::write_json(X1(A1()));
+      expect(xString.has_value());
+
+      auto x = glz::read_json<X1>(*xString);
+      expect(bool(x));
+      if (not x.has_value()) {
+         std::cerr << glz::format_error(x.error(), *xString);
+      }
+   };
+};
+
 int main()
 {
    trace.end("json_test");