Fix compile error caused by customized mapping of enums during JSONSerialization

[2horse9sun]

Changes

In JSONSerialization.h, if I map an enum into json by implementing ScalarBitSetTraits::bitset() method, the result of the logical operation of two enums (which is int) will be assigned to an enum, which causes the compile error.

The changes are:

• Use static_cast<T> to convert the result in all bitset related methods.

Rationale

I'm trying to extract the Swift and Objective-C AST into separate JSON files. During the extraction, I take advantage of swift's JSONSerialization tools to perform the mapping from AST nodes to json objects. However, some compile errors occur during the process. The following demo is a simplified version of what I'm doing:

// Map Decl::ObjCDeclQualifier into json
template <>
struct ScalarBitSetTraits<Decl::ObjCDeclQualifier> {
    static void bitset(Output &out, Decl::ObjCDeclQualifier &kind) {
        out.bitSetCase(kind, "none",   Decl::ObjCDeclQualifier::OBJC_TQ_None);
        out.bitSetCase(kind, "in",   Decl::ObjCDeclQualifier::OBJC_TQ_In);
        out.bitSetCase(kind, "inout",   Decl::ObjCDeclQualifier::OBJC_TQ_Inout);
        out.bitSetCase(kind, "bycopy",   Decl::ObjCDeclQualifier::OBJC_TQ_Bycopy);
        out.bitSetCase(kind, "byref",   Decl::ObjCDeclQualifier::OBJC_TQ_Byref);
        out.bitSetCase(kind, "oneway",   Decl::ObjCDeclQualifier::OBJC_TQ_Oneway);
        out.bitSetCase(kind, "cs_nullability",   Decl::ObjCDeclQualifier::OBJC_TQ_CSNullability);
    }
};

int main(int argc, const char *argv[]) {
    std::error_code stream_error;
    llvm::raw_fd_ostream outputStream(StringRef("-"), stream_error);
    Output serializer(outputStream, {}, false);
    serializer.beginObject();
    Decl::ObjCDeclQualifier value = Decl::ObjCDeclQualifier::OBJC_TQ_None;
    serializer.mapRequired("key", value);
    serializer.endObject();
    return 0;
}

The above demo is built as a swift tool (the detail is omitted). When I run the demo, the compiler complains that:

error: assigning to 'clang::Decl::ObjCDeclQualifier' from incompatible type 'int'
      Val = Val | ConstVal;

It turns out that Decl::ObjCDeclQualifier is enum but the result of Val | ConstVal is int. That seems to be the root cause of the error. Then I modify the source code of JSONSerialization.h by statically cast the result:

  template <typename T>
  void bitSetCase(T &Val, const char* Str, const T ConstVal) {
    if (bitSetMatch(Str, (Val & ConstVal) == ConstVal)) {
      Val = static_cast<T>(Val | ConstVal);
      // Val = Val | ConstVal;
    }
  }

After that, it works and the json serialization results are correct.

[2horse9sun]

@swift-ci Please test

[DougGregor]

@swift-ci please smoke test