Delete default construcrtor on builders, so that people don't incorrectly try to…

Delete default construcrtor on builders, so that people don't incorrectly try to just declare a Builder on the stack and then start setting its fields.  Instead, a Builder can be explicitly assigned to nullptr, which makes it clearer that it isn't safe to use.

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

@@ -1069,7 +1069,9 @@ private:
         "public:\n"
         "  typedef ", unqualifiedParentType, " Builds;\n"
         "\n"
-        "  Builder() = default;\n"
+        "  Builder() = delete;  // Deleted to discourage incorrect usage.\n"
+        "                       // You can explicitly initialize to nullptr instead.\n"
+        "  inline Builder(decltype(nullptr)) {}\n"
         "  inline explicit Builder(::capnp::_::StructBuilder base): _builder(base) {}\n"
         "  inline operator Reader() const { return Reader(_builder.asReader()); }\n"
         "  inline Reader asReader() const { return *this; }\n"

c++/src/capnp/compiler/lexer.capnp.h

@@ -159,7 +159,9 @@ class Token::Builder {
 public:
   typedef Token Builds;
 
-  Builder() = default;
+  Builder() = delete;  // Deleted to discourage incorrect usage.
+                       // You can explicitly initialize to nullptr instead.
+  inline Builder(decltype(nullptr)) {}
   inline explicit Builder(::capnp::_::StructBuilder base): _builder(base) {}
   inline operator Reader() const { return Reader(_builder.asReader()); }
   inline Reader asReader() const { return *this; }
@@ -292,7 +294,9 @@ class Statement::Builder {
 public:
   typedef Statement Builds;
 
-  Builder() = default;
+  Builder() = delete;  // Deleted to discourage incorrect usage.
+                       // You can explicitly initialize to nullptr instead.
+  inline Builder(decltype(nullptr)) {}
   inline explicit Builder(::capnp::_::StructBuilder base): _builder(base) {}
   inline operator Reader() const { return Reader(_builder.asReader()); }
   inline Reader asReader() const { return *this; }
@@ -382,7 +386,9 @@ class LexedTokens::Builder {
 public:
   typedef LexedTokens Builds;
 
-  Builder() = default;
+  Builder() = delete;  // Deleted to discourage incorrect usage.
+                       // You can explicitly initialize to nullptr instead.
+  inline Builder(decltype(nullptr)) {}
   inline explicit Builder(::capnp::_::StructBuilder base): _builder(base) {}
   inline operator Reader() const { return Reader(_builder.asReader()); }
   inline Reader asReader() const { return *this; }
@@ -443,7 +449,9 @@ class LexedStatements::Builder {
 public:
   typedef LexedStatements Builds;
 
-  Builder() = default;
+  Builder() = delete;  // Deleted to discourage incorrect usage.
+                       // You can explicitly initialize to nullptr instead.
+  inline Builder(decltype(nullptr)) {}
   inline explicit Builder(::capnp::_::StructBuilder base): _builder(base) {}
   inline operator Reader() const { return Reader(_builder.asReader()); }
   inline Reader asReader() const { return *this; }

c++/src/capnp/compiler/node-translator.c++

@@ -1031,7 +1031,7 @@ private:
                       StructLayout::StructOrGroup& fieldScope,
                       bool isInUnion)
         : parent(&parent), codeOrder(codeOrder), isInUnion(isInUnion),
-          decl(decl), fieldScope(&fieldScope) {}
+          decl(decl), node(nullptr), fieldScope(&fieldScope) {}
     inline MemberInfo(MemberInfo& parent, uint codeOrder,
                       const Declaration::Reader& decl,
                       schema::Node::Builder node,

c++/src/capnp/dynamic.c++

@@ -1608,9 +1608,7 @@ ReaderFor<typeName> DynamicValue::Reader::AsImpl<typeName>::apply(const Reader&
   return reader.name##Value; \
 } \
 BuilderFor<typeName> DynamicValue::Builder::AsImpl<typeName>::apply(Builder& builder) { \
-  KJ_REQUIRE(builder.type == discrim, "Value type mismatch.") { \
-    return BuilderFor<typeName>(); \
-  } \
+  KJ_REQUIRE(builder.type == discrim, "Value type mismatch."); \
   return builder.name##Value; \
 }
 

c++/src/capnp/dynamic.h

@@ -187,7 +187,8 @@ class DynamicObject::Builder: public kj::DisallowConstCopy {
 public:
   typedef DynamicObject Builds;
 
-  Builder() = default;
+  Builder() = delete;
+  inline Builder(decltype(nullptr)) {}
   Builder(Builder&) = default;
   Builder(Builder&&) = default;
 
@@ -276,7 +277,8 @@ class DynamicStruct::Builder {
 public:
   typedef DynamicStruct Builds;
 
-  Builder() = default;
+  Builder() = delete;
+  inline Builder(decltype(nullptr)) {}
 
   template <typename T, typename = kj::EnableIf<kind<FromBuilder<T>>() == Kind::STRUCT>>
   inline Builder(T&& value): Builder(toDynamic(value)) {}
@@ -428,7 +430,8 @@ class DynamicList::Builder {
 public:
   typedef DynamicList Builds;
 
-  Builder() = default;
+  Builder() = delete;
+  inline Builder(decltype(nullptr)) {}
 
   template <typename T, typename = kj::EnableIf<kind<FromBuilder<T>>() == Kind::LIST>>
   inline Builder(T&& value): Builder(toDynamic(value)) {}

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

@@ -94,6 +94,8 @@ TEST(Encoding, Defaults) {
 
   checkTestMessage(reader.getRoot<TestDefaults>());
   checkTestMessage(readMessageUnchecked<TestDefaults>(nullRoot.words));
+
+  checkTestMessage(TestDefaults::Reader());
 }
 
 TEST(Encoding, DefaultInitialization) {

c++/src/capnp/list.h

@@ -134,7 +134,8 @@ struct List<T, Kind::PRIMITIVE> {
   public:
     typedef List<T> Builds;
 
-    Builder() = default;
+    Builder() = delete;
+    inline Builder(decltype(nullptr)) {}
     inline explicit Builder(_::ListBuilder builder): builder(builder) {}
 
     inline operator Reader() { return Reader(builder.asReader()); }
@@ -241,7 +242,8 @@ struct List<T, Kind::STRUCT> {
   public:
     typedef List<T> Builds;
 
-    Builder() = default;
+    Builder() = delete;
+    inline Builder(decltype(nullptr)) {}
     inline explicit Builder(_::ListBuilder builder): builder(builder) {}
 
     inline operator Reader() { return Reader(builder.asReader()); }
@@ -371,7 +373,8 @@ struct List<List<T>, Kind::LIST> {
   public:
     typedef List<List<T>> Builds;
 
-    Builder() = default;
+    Builder() = delete;
+    inline Builder(decltype(nullptr)) {}
     inline explicit Builder(_::ListBuilder builder): builder(builder) {}
 
     inline operator Reader() { return Reader(builder.asReader()); }
@@ -488,7 +491,8 @@ struct List<T, Kind::BLOB> {
   public:
     typedef List<T> Builds;
 
-    Builder() = default;
+    Builder() = delete;
+    inline Builder(decltype(nullptr)) {}
     inline explicit Builder(_::ListBuilder builder): builder(builder) {}
 
     inline operator Reader() { return Reader(builder.asReader()); }