Fix bug with dynamic unions.

c++/src/capnproto/dynamic-test.c++

@@ -733,6 +733,14 @@ TEST(DynamicApi, ConversionFailures) {
   EXPECT_ANY_THROW(root.set("boolField", 1));
 }
 
+TEST(DynamicApi, LateUnion) {
+  MallocMessageBuilder builder;
+  auto root = builder.initRoot<DynamicStruct>(Schema::from<test::TestLateUnion>());
+
+  root.get("theUnion").as<DynamicUnion>().set("qux", "hello");
+  EXPECT_EQ("hello", root.as<test::TestLateUnion>().getTheUnion().getQux());
+}
+
 }  // namespace
 }  // namespace internal
 }  // namespace capnproto

c++/src/capnproto/schema.c++

@@ -150,7 +150,7 @@ Maybe<StructSchema::Member> StructSchema::Union::findMemberByName(Text::Reader n
 
 StructSchema::Member StructSchema::Union::getMemberByName(Text::Reader name) const {
   Maybe<StructSchema::Member> member = findMemberByName(name);
-  PRECOND(member != nullptr, "struct has no such member", name);
+  PRECOND(member != nullptr, "union has no such member", name);
   return *member;
 }
 

c++/src/capnproto/test.capnp

@@ -339,3 +339,18 @@ struct TestListDefaults {
       textListList = [["foo", "bar"], ["baz"], ["qux", "corge"]],
       structListList = [[(int32Field = 123), (int32Field = 456)], [(int32Field = 789)]]);
 }
+
+struct TestLateUnion {
+  # Test what happens if the unions are the first ordinals in the struct.  At one point this was
+  # broken for the dynamic API.
+
+  foo @0 :Int32;
+  bar @1 :Text;
+  baz @2 :Int16;
+
+  theUnion @3 union {
+    qux @4 :Text;
+    corge @5 :List(Int32);
+    grault @6 :Float32;
+  }
+}

compiler/src/CxxGenerator.hs

@@ -287,8 +287,8 @@ memberTable (DescStruct desc) = let
                $ List.sortBy (compare `on` ordinal) $ structMembers desc
 
     -- Fields of each union.
-    innerMembers = zipWith indexedUnionMembers [1..]
-                 $ List.sortBy (compare `on` unionNumber) $ structUnions desc
+    innerMembers = catMaybes $ zipWith indexedUnionMembers [1..]
+                 $ List.sortBy (compare `on` ordinal) $ structMembers desc
 
     ordinal (DescField f) = fieldNumber f
     ordinal (DescUnion u) = unionNumber u
@@ -298,8 +298,10 @@ memberTable (DescStruct desc) = let
     memberName (DescUnion u) = Just $ unionName u
     memberName _ = Nothing
 
-    indexedUnionMembers i u = zip (memberIndexes i) $ mapMaybe memberName
-                            $ List.sortBy (compare `on` ordinal) $ unionMembers u
+    indexedUnionMembers i (DescUnion u) =
+        Just $ zip (memberIndexes i) $ mapMaybe memberName $
+            List.sortBy (compare `on` ordinal) $ unionMembers u
+    indexedUnionMembers _ _ = Nothing
 
     in concat $ topMembers : innerMembers