static KeyData GetGraphKeyImpl(ISymbol symbol,
bool allowTypeParameter = false)
{
if (allowTypeParameter && symbol is ITypeParameterSymbol)
{
return(KeyData.FromKey(GetSymbolName(symbol)));
}
var segments = GetGraphKeyImpl(symbol, ImmutableList.Create <Segment>());
return(segments.Aggregate(KeyData.Empty, KeyData.Append));
}