before this change, format_as() is unable to format a type which
has `auto format_as() -> const another_type&`, and `another_type`
is formattable. because `format_as_result` maps the result type
as it is, and the compiler refuses to compile
`static_cast<T*>(nullptr)`, where T is a reference type. but
it would be handy if we could use `format_as()` to format types
which, for instance, owns / inherit from a formattable type, and
delegate the formatter to these variables instead without creating
a copy of them.
in this change:
* instruct `format_as_result` to map the
result type to the decayed type, so that `type` can be the decayed
type of result type, and this also enables `type` to be formattable,
as long as the decayed type is formattable.
* corresponding test is added to format-test.cc
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
274ba2645b