check_decode: add Tree's map fields so the map-depth rows test the decoder

CHANGELOG.md

@@ -11,6 +11,14 @@ format changes.
 
 ## [Unreleased]
 
+### Fixed
+
+- **`check_decode`'s `Tree` has `branches` and `tags`** (protowire#449).
+  Without them the PB leg skipped both fields as unknown, so the corpus's
+  map-depth rows could not reach the decoder's map path. The decoders
+  needed no change: they already count a map's `{` and every PB map entry
+  toward `MaxNestingDepth`.
+
 ### Changed
 
 - **The vendored shared schemas are re-synced with [trendvidia/protos](https://github.com/trendvidia/protos) v0.1.2** ([trendvidia/protowire#447](https://github.com/trendvidia/protowire/issues/447)). They gain protos' file options, and `envelope.proto`'s `go_package` now names a published package. Declarations and field numbers are unchanged, and so is this port's public API.

cmd/check_decode/main.cc

@@ -40,6 +40,7 @@
 #include <cstring>
 #include <filesystem>
 #include <fstream>
+#include <map>
 #include <memory>
 #include <span>
 #include <sstream>
@@ -111,10 +112,16 @@ struct Tree {
   std::unique_ptr<Tree> child;
   std::string label;
   std::vector<Tree> children;
+  // Held by pointer so the map's value type is complete here, where the
+  // standard requires it; the wire form is the same as map<string, Tree>.
+  std::map<std::string, std::unique_ptr<Tree>> branches;
+  std::map<std::string, std::string> tags;
   PROTOWIRE_FIELDS(Tree,
                    PROTOWIRE_FIELD(1, child),
                    PROTOWIRE_FIELD(2, label),
-                   PROTOWIRE_FIELD(3, children))
+                   PROTOWIRE_FIELD(3, children),
+                   PROTOWIRE_FIELD(4, branches),
+                   PROTOWIRE_FIELD(5, tags))
 };
 
 struct StringHolder {