1. default as TClass: error
2. $"{default}": use object as target-type for expressions that don't have a natural type
3. using (default) ...: error
4. During return type inference of lambdas, expressions are neither required to be target-typed or to have a natural type.
5. Introduced a new kind of conversion: "default literal" conversion.
6. Object equality is valid when comparing a reference type and the default literal.

Relates to PRs #37596 and #38853

FYI @gafter