List initialization problem

```cpp
boost::fusion::vector{ 123 }; // ok
boost::fusion::vector{ { 123 } }; // fail
boost::optional{ 123 }; // ok
boost::optional{ { 123 } }; // ok
boost::fusion::vector>{ 123 }; // ok
boost::fusion::vector>{ { 123 } }; // fail
boost::fusion::vector>{ { { 123 } } }; // fail

boost::fusion::vector{ "hello" }; // ok
boost::fusion::vector{ { "hello" } }; // fail
boost::optional{ "hello" }; // ok
boost::optional{ { "hello" } }; // ok
boost::fusion::vector>{ "hello" }; // fail
boost::fusion::vector>{ { "hello" } }; // fail
boost::fusion::vector>{ { { "hello" } } }; // fail
```

Start by reproducing the listed C++ initialization examples and comparing the failing nested-brace cases with the successful ones. The issue names no source files or tests, so completion would require identifying the relevant Boost.Fusion vector initialization entry point and confirming that the failing forms behave consistently with the intended forms.

Written by the indexing model from the issue text.