Fix unbox_value_or problems with explicit type and implicit hstring #656
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In a call to `unbox_value_or<T>(obj, def)`, if the
second parameter happens to be a valid constructor
parameter for `param::hstring`, then the hstring
version of `unbox_value_or` will be used, even though
the user clearly wants `T`.
```cpp
// expected to unbox obj to an IPropertyValue,
// but actually unboxes to an hstring!
auto pv = unbox_value_or<IPropertyValue>(obj, {});
```
Add an explicit `enable_if` to remove the hstring
version if the template parameter is not `hstring`.
There's the converse problem, where you want
`unbox_value_or` to infer `hstring`, but it can't:
```cpp
// Doesn't compile due to inability to infer T.
auto str = unbox_value_or<hstring>(obj, hstring{});
```
Add a default type `hstring` for `T` in the hstring
version of `unbox_value_or.
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In a call to
unbox_value_or<T>(obj, def), if the second parameter happens to be a valid constructor parameter forparam::hstring, then the hstring version ofunbox_value_orwill be used, even though the user clearly wantsT.Add an explicit
enable_ifto remove the hstring version if the template parameter is nothstring.There's the converse problem, where you want
unbox_value_orto inferhstring, but it can't:Add a default type
hstringforTin the hstring version ofunbox_value_or.