Fixes #402

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

@@ -1901,6 +1901,15 @@ TEST(Encoding, UnionInGenerics) {
   builder.initUg();
 }
 
+TEST(Encoding, DefaultListBuilder) {
+  // At one point, this wouldn't compile.
+
+  List<int>::Builder(nullptr);
+  List<TestAllTypes>::Builder(nullptr);
+  List<List<int>>::Builder(nullptr);
+  List<Text>::Builder(nullptr);
+}
+
 }  // namespace
 }  // namespace _ (private)
 }  // namespace capnp

c++/src/capnp/list.h

@@ -140,7 +140,7 @@ struct List<T, Kind::PRIMITIVE> {
     typedef List<T> Builds;
 
     inline Builder(): builder(_::elementSizeForType<T>()) {}
-    inline Builder(decltype(nullptr)) {}
+    inline Builder(decltype(nullptr)): Builder() {}
     inline explicit Builder(_::ListBuilder builder): builder(builder) {}
 
     inline operator Reader() const { return Reader(builder.asReader()); }
@@ -236,7 +236,7 @@ struct List<T, Kind::STRUCT> {
     typedef List<T> Builds;
 
     inline Builder(): builder(ElementSize::INLINE_COMPOSITE) {}
-    inline Builder(decltype(nullptr)) {}
+    inline Builder(decltype(nullptr)): Builder() {}
     inline explicit Builder(_::ListBuilder builder): builder(builder) {}
 
     inline operator Reader() const { return Reader(builder.asReader()); }
@@ -359,7 +359,7 @@ struct List<List<T>, Kind::LIST> {
     typedef List<List<T>> Builds;
 
     inline Builder(): builder(ElementSize::POINTER) {}
-    inline Builder(decltype(nullptr)) {}
+    inline Builder(decltype(nullptr)): Builder() {}
     inline explicit Builder(_::ListBuilder builder): builder(builder) {}
 
     inline operator Reader() const { return Reader(builder.asReader()); }
@@ -467,7 +467,7 @@ struct List<T, Kind::BLOB> {
     typedef List<T> Builds;
 
     inline Builder(): builder(ElementSize::POINTER) {}
-    inline Builder(decltype(nullptr)) {}
+    inline Builder(decltype(nullptr)): Builder() {}
     inline explicit Builder(_::ListBuilder builder): builder(builder) {}
 
     inline operator Reader() const { return Reader(builder.asReader()); }