Allow a parameter's default to be set explicitly to null.

Fixes #363.

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

@@ -2102,8 +2102,28 @@ private:
           auto typeBuilder = slot.initType();
           if (translator.compileType(member.fieldType, typeBuilder, implicitMethodParams)) {
             if (member.hasDefaultValue) {
-              translator.compileBootstrapValue(member.fieldDefaultValue,
-                                               typeBuilder, slot.initDefaultValue());
+              if (member.isParam &&
+                  member.fieldDefaultValue.isRelativeName() &&
+                  member.fieldDefaultValue.getRelativeName().getValue() == "null") {
+                // special case: parameter set null
+                switch (typeBuilder.which()) {
+                  case schema::Type::TEXT:
+                  case schema::Type::DATA:
+                  case schema::Type::LIST:
+                  case schema::Type::STRUCT:
+                  case schema::Type::INTERFACE:
+                  case schema::Type::ANY_POINTER:
+                    break;
+                  default:
+                    errorReporter.addErrorOn(member.fieldDefaultValue.getRelativeName(),
+                        "Only pointer parameters can declare their default as 'null'.");
+                    break;
+                }
+                translator.compileDefaultDefaultValue(typeBuilder, slot.initDefaultValue());
+              } else {
+                translator.compileBootstrapValue(member.fieldDefaultValue,
+                                                 typeBuilder, slot.initDefaultValue());
+              }
               slot.setHadExplicitDefault(true);
             } else {
               translator.compileDefaultDefaultValue(typeBuilder, slot.initDefaultValue());

c++/src/capnp/test.capnp

@@ -840,6 +840,8 @@ interface TestMoreStuff extends(TestCallOrder) {
 
   methodWithDefaults @8 (a :Text, b :UInt32 = 123, c :Text = "foo") -> (d :Text, e :Text = "bar");
 
+  methodWithNullDefault @12 (a :Text, b :TestInterface = null);
+
   getHandle @9 () -> (handle :TestHandle);
   # Get a new handle. Tests have an out-of-band way to check the current number of live handles, so
   # this can be used to test garbage collection.

c++/src/capnp/testdata/errors.capnp.nobuild

@@ -155,3 +155,7 @@ const embedNoSuchFile :Data = embed "no-such-file";
 
 using Baz = import "nosuchfile-unused.capnp".Baz;
 # Check that an import in an unused `using` still reports error.
+
+interface TestInterface {
+  foo @0 (a :UInt32 = null);
+}

c++/src/capnp/testdata/errors.txt

@@ -56,4 +56,5 @@ file:149:18-49: error: Double-application of generic parameters.
 file:150:38-43: error: Sorry, only pointer types can be used as generic parameters.
 file:153:30-44: error: Embeds can only be used when Text, Data, or a struct is expected.
 file:154:37-51: error: Couldn't read file for embed: no-such-file
+file:160:23-27: error: Only pointer parameters can declare their default as 'null'.
 file:156:20-45: error: Import failed: nosuchfile-unused.capnp