Bug 213942

The WPE Release Bot is now using clang and caught[1] the following error:

"
DerivedSources/ForwardingHeaders/wtf/HashTable.h:956:17: error: incomplete type 'WTF::StringHash' named in nested name specifier
            if (HashFunctions::safeToCompareToEmptyOrDeleted) {
                ^~~~~~~~~~~~~~~
DerivedSources/ForwardingHeaders/wtf/HashTable.h:466:56: note: in instantiation of function template specialization 'WTF::HashTable<WTF::String, WTF::String, WTF::IdentityExtractor, WTF::StringHash, WTF::HashTraits<WTF::String>, WTF::HashTraits<WTF::String> >::add<WTF::IdentityHashTranslator<WTF::HashTraits<WTF::String>, WTF::StringHash>, const WTF::String &, const WTF::String &>' requested here
        AddResult add(const ValueType& value) { return add<IdentityTranslatorType>(Extractor::extract(value), value); }
                                                       ^
DerivedSources/ForwardingHeaders/wtf/HashSet.h:239:19: note: in instantiation of member function 'WTF::HashTable<WTF::String, WTF::String, WTF::IdentityExtractor, WTF::StringHash, WTF::HashTraits<WTF::String>, WTF::HashTraits<WTF::String> >::add' requested here
    return m_impl.add(value);
                  ^
../../Source/WebCore/loader/appcache/ApplicationCacheManifestParser.cpp:161:39: note: in instantiation of member function 'WTF::HashSet<WTF::String, WTF::StringHash, WTF::HashTraits<WTF::String> >::add' requested here
                manifest.explicitURLs.add(url.string());
                                      ^
DerivedSources/ForwardingHeaders/wtf/text/StringParsingBuffer.h:119:16: note: in instantiation of function template specialization 'WebCore::parseApplicationCacheManifest(const WTF::URL &, const WTF::String &, const char *, int)::(anonymous class)::operator()<WTF::StringParsingBuffer<unsigned char> >' requested here
        return functor(StringParsingBuffer { string.characters8(), string.length() });
               ^
../../Source/WebCore/loader/appcache/ApplicationCacheManifestParser.cpp:82:12: note: in instantiation of function template specialization 'WTF::readCharactersForParsing<WTF::String &, (lambda at ../../Source/WebCore/loader/appcache/ApplicationCacheManifestParser.cpp:82:53)>' requested here
    return readCharactersForParsing(manifestString, [&](auto buffer) -> Optional<ApplicationCacheManifest> {"

[1] https://build.webkit.org/builders/WPE%20Linux%2064-bit%20Release%20%28Build%29/builds/34853