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Add null-safety specification of super-bounded types #1133

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Sep 11, 2020
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Add null-safety specification of super-bounded types #1133

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17de680
Add null-safety specification of super-bounded types
eernstg Aug 5, 2020
cd99123
Reordered a few words
eernstg Aug 5, 2020
0f92fbd
Moved helper predicates to null safety spec, connected them to subtyping
eernstg Aug 13, 2020
f10723f
Change definition of super-bounded to treat "rest" positions same as …
eernstg Aug 13, 2020
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Merge branch 'master' into spec_super_bounds_aug20
eernstg Sep 11, 2020
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25 changes: 22 additions & 3 deletions accepted/future-releases/nnbd/feature-specification.md
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Expand Up @@ -6,9 +6,12 @@ Status: Draft

## CHANGELOG

2020.08.05
- Specify updates to super-bounded type rules for null safety.

2020.07.09
- Specify combined member signature and spread element typing
with null-safety.
with null safety.

2020.06.02
- Fix the diff to the spec for potentially constant instance checks
Expand Down Expand Up @@ -699,11 +702,27 @@ the element, key, and value type of `...e` and `...?e` is `Never`.
*When the static type _S_ of `e` is strictly non-nullable, such as when _S_
is `Never`, `...?e` is a warning, but it may still occur.*

### Instantiate to bounds
### Instantiation to bound

The computation of instantiation to bounds is changed to substitute `Never` for
The computation of instantiation to bound is changed to substitute `Never` for
type variables appearing in contravariant positions instead of `Null`.

### Super-bounded types

Null safety requires two changes to the section 'Super-Bounded Types' in the
language specification.

The definition of a top type is changed: _T_ is a top type if and only if
`Object?` is a subtype of _T_.
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The definition of a super-bounded type is changed such that occurrences of
`Null` are replaced by types involving `Never`, and `Object` is replaced by
`Object?`. This causes one sentence to change, with the following result:

Let _T'_ be the result of replacing every occurrence in _T_ of a top type in
a covariant position by `Never`, and every occurrence in _T_ of a type
_S_ in a contravariant position where _S <: Never_ by `Object?`.

### Least and greatest closure

The definitions of least and greatest closure are changed in null safe libraries
Expand Down