Correctly handle bracketed type in `default_constructed_unit_struct` (#14367)

There were two bugs here. Let's assume `T` is a singleton type
implementing `Default` and that `f()` takes a `T`:

- `f(<T>::default())` cannot be replaced by `f(<T)` as it was (incorrect
spans – this is tricky because the type relative path uses a base span
covering only `T`, not `<T>`) (third commit)
- The argument of `f(<_>::default())` is inferred correctly, but cannot
be replaced by `<_>` or `_`, as this cannot be used to infer an instance
of a singleton type (first commit).

The second commit offers better error messages by pointing at the whole
expression.

Fix #12654

changelog: [`default_constructed_unit_struct`]: do not suggest incorrect
fix when using a type surrounded by brackets

0 files changed, 0 insertions, 0 deletions