Taking this parameter by const reference forces us to copy it (because
we know we're going to store it). Taking it by r-value reference would
suggest that we _might_ take ownership over it and would also force the
user to make a copy if they wish to retain the original value.
Taking this parameter by value however clearly gives us ownership of its
content without forcing a copy if it's implicit conversion from
`const char*` or explicitly handed over to us by the user via std::move.
current code mistakenly allows the following TOML file.
```toml
a.b = 42 # table "a" is defined here, implicitly
a = {c = 3.14} # table "a" is overwritten here
```
But we need to allow the following (structually similar) TOML file.
```toml
a.b = 42 # table "a" is defined here, implicitly
a.c = 3.14 # table "a" is merged with {c = 3.14}
```
To distinguish those, we check whether the current table is defined as
an inline table or via dotted key. If the table we are inserting is
defined via dotted key, we accept it and merge the table. If the table
being inserted is defined as an inline table, then we report an error.
As described in issue #173, this warning is raised on various platforms
and in various build types. For example, g++ 11 in release mode will
cause this warning to be raised. This change fixes this warning.
Without a region, error messages in exceptions are unable to print
a filename.
By retaining the location in a zero-length region and detecting this
when formatting the exception text it is possible to print the filename
and explicitly state that the file is completely empty.
Fixes#160
constructor sometimes has `template<T> ctor(const T&)` and it causes
ambiguity. To avoid it, from<T> and T.from_toml precedes any
constructor. But, to check the ambiguity between from<T> and from_toml,
they do not precede each other. If anyone define both from<T> and
from_toml, it causes compilation error.
