Fixes #233

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

@@ -1056,7 +1056,10 @@ private:
         kj::str(
             "  if (which() != ", scope, upperCase, ") return false;\n"),
         kj::str(
-            "  KJ_IREQUIRE(which() == ", scope, upperCase, ",\n"
+            // Extra parens around the condition are needed for when we're compiling a multi-arg
+            // generic type, which will have a comma, which would otherwise mess up the macro.
+            // Ah, C++.
+            "  KJ_IREQUIRE((which() == ", scope, upperCase, "),\n"
             "              \"Must check which() before get()ing a union member.\");\n"),
         kj::str(
             "  _builder.setDataField<", scope, "Which>(\n"

c++/src/capnp/test.capnp

@@ -587,6 +587,15 @@ interface TestImplicitMethodParamsInGeneric(V) {
   call @0 [T, U] (foo :T, bar :U) -> TestGenerics(T, U);
 }
 
+struct TestGenericsUnion(Foo, Bar) {
+  # At one point this failed to compile.
+
+  union {
+    foo @0 :Foo;
+    bar @1 :Bar;
+  }
+}
+
 struct TestUseGenerics $TestGenerics(Text, Data).ann("foo") {
   basic @0 :TestGenerics(TestAllTypes, TestAnyPointer);
   inner @1 :TestGenerics(TestAllTypes, TestAnyPointer).Inner;