schemagen: fix nullable root types (#13)

The C++ code generator now handles schemas where the top-level type is
nullable ("type": ["object", "null"], ["string", "null"], or
["array", "null"]).

Changes to weaseljson_schemagen.py:
- Rename the inner object struct when the root is a nullable object, so
  the `using Root = std::optional<...>` alias no longer conflicts with
  `struct Root`.
- Treat nullable root objects and arrays as container roots, emplacing
  the inner value before pushing the root frame and pointing the frame at
  the contained value.
- For nullable root scalars/enums, engage() now returns a pointer to the
  value inside the optional rather than to the optional wrapper itself.
- cbNull() now safely accepts a top-level null when the root is nullable
  and rejects it otherwise.

Regression tests added:
- nullable_object.schema.json + test_nullable_root.cpp
- nullable_string.schema.json
- nullable_array.schema.json

Closes #13

contrib/schemagen/weaseljson_schemagen.py

@@ -493,6 +493,18 @@ class Emitter:
         self.root_ty, self.root_nullable = Builder._unpack(
             self.b.build_type(self.b.root_schema, "Root")
         )
+
+        # A nullable root object would otherwise produce
+        #   using Root = std::optional<Root>;
+        # which conflicts with the struct named Root.  Rename the inner struct.
+        if isinstance(self.root_ty, TObj) and self.root_nullable:
+            old_name = self.root_ty.name
+            new_name = self.b.unique_name("RootInner")
+            obj = self.b.objects.pop(old_name)
+            obj.name = new_name
+            self.b.objects[new_name] = obj
+            self.root_ty = TObj(new_name)
+
         break_cycles(self.b.objects)
 
         # register all array kinds (walk every field + root)
@@ -582,9 +594,7 @@ namespace {ns} {{"""
     def _kind_enum(self):
         kinds = list(self.b.objects.keys())
         kinds += [n for n, _ in self.arr_types]
-        root_is_container = (
-            isinstance(self.root_ty, (TObj, TArr)) and not self.root_nullable
-        )
+        root_is_container = isinstance(self.root_ty, (TObj, TArr))
         if not root_is_container:
             kinds.append("RootScalar")
         kinds.append("Skip")
@@ -593,9 +603,9 @@ namespace {ns} {{"""
 
     def _root_info(self):
         """Return (root_cat, root_container_kind_or_None)."""
-        if isinstance(self.root_ty, TObj) and not self.root_nullable:
+        if isinstance(self.root_ty, TObj):
             return ("Obj", f"Kind::{self.root_ty.name}")
-        if isinstance(self.root_ty, TArr) and not self.root_nullable:
+        if isinstance(self.root_ty, TArr):
             return ("Arr", f"Kind::{self.arr_kind(self.root_ty)}")
         return (self.cat(self.root_ty), None)
 
@@ -665,7 +675,12 @@ namespace {ns} {{"""
             lines.append("    }")
         root_cat, root_kind = self._root_info()
         if root_kind is None:
-            lines.append("    case Kind::RootScalar: return &result_;")
+            if self.root_nullable:
+                lines.append(
+                    "    case Kind::RootScalar: { if (!result_) result_.emplace(); return &*result_; }"
+                )
+            else:
+                lines.append("    case Kind::RootScalar: return &result_;")
         lines.append("    default: return nullptr;")
         lines.append("    }")
         lines.append("  }")
@@ -807,7 +822,12 @@ namespace {ns} {{"""
         lines = ["    if (error_) return;"]
         if root_kind is not None and root_cat == event_cat:
             lines.append("    if (stack_.empty()) {")
-            lines.append(f"      stack_.push_back(Frame{{{root_kind}, &result_}});")
+            if self.root_nullable:
+                lines.append("      result_.emplace();")
+                dest = "&*result_"
+            else:
+                dest = "&result_"
+            lines.append(f"      stack_.push_back(Frame{{{root_kind}, {dest}}});")
             if event_cat == "Obj":
                 lines.append("      {")
                 lines.append(f"        int n = fieldCount({root_kind});")
@@ -839,6 +859,9 @@ namespace {ns} {{"""
         root_cat, root_kind = self._root_info()
         begin_obj = self._begin_container("Obj", root_kind)
         begin_arr = self._begin_container("Arr", root_kind)
+        null_at_root = (
+            "done_ = true; return;" if self.root_nullable else "reject(); return;"
+        )
 
         return f"""
 class RootBuilder {{
@@ -1065,6 +1088,9 @@ private:
 
   void cbNull() {{
     if (error_) return;
+    if (stack_.empty()) {{
+      {null_at_root}
+    }}
     Frame &f = stack_.back();
     if (f.kind == Kind::Skip) return;
     SlotInfo si = slotInfoG(f);