capnpc-c++: fix generated code for union inside template

c++/src/capnp/compiler/capnpc-c++.c++

@@ -1134,18 +1134,18 @@ private:
                 templateContext.allDecls(),
                 "inline typename ", scope, titleCase, "::Reader ", scope, "Reader::get", titleCase, "() const {\n",
                 unionDiscrim.check,
-                "  return ", scope, titleCase, "::Reader(_reader);\n"
+                "  return typename ", scope, titleCase, "::Reader(_reader);\n"
                 "}\n",
                 templateContext.allDecls(),
                 "inline typename ", scope, titleCase, "::Builder ", scope, "Builder::get", titleCase, "() {\n",
                 unionDiscrim.check,
-                "  return ", scope, titleCase, "::Builder(_builder);\n"
+                "  return typename ", scope, titleCase, "::Builder(_builder);\n"
                 "}\n",
                 hasDiscriminantValue(proto) ? kj::strTree() : kj::strTree(
                   "#if !CAPNP_LITE\n",
                   templateContext.allDecls(),
                   "inline typename ", scope, titleCase, "::Pipeline ", scope, "Pipeline::get", titleCase, "() {\n",
-                  "  return ", scope, titleCase, "::Pipeline(_typeless.noop());\n"
+                  "  return typename ", scope, titleCase, "::Pipeline(_typeless.noop());\n"
                   "}\n"
                   "#endif  // !CAPNP_LITE\n"),
                 templateContext.allDecls(),
@@ -1166,7 +1166,7 @@ private:
                   }
                   KJ_UNREACHABLE;
                 },
-                "  return ", scope, titleCase, "::Builder(_builder);\n"
+                "  return typename ", scope, titleCase, "::Builder(_builder);\n"
                 "}\n")
           };
       }

c++/src/capnp/encoding-test.c++

@@ -1851,6 +1851,28 @@ TEST(Encoding, GenericDefaults) {
       .getValue().getRev().getFoo().getInt16Field());
 }
 
+TEST(Encoding, UnionInGenerics) {
+  MallocMessageBuilder message;
+  auto builder = message.initRoot<test::TestGenerics<>>();
+  auto reader = builder.asReader();
+
+  //just call the methods to verify that generated code compiles
+  reader.which();
+  builder.which();
+
+  reader.isUv();
+  builder.isUv();
+  reader.getUv();
+  builder.getUv();
+  builder.setUv();
+
+  reader.isUg();
+  builder.isUg();
+  reader.getUg();
+  builder.getUg();
+  builder.initUg();
+}
+
 }  // namespace
 }  // namespace _ (private)
 }  // namespace capnp