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Change type requirements for {} being a set literal. #129

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Dec 18, 2018
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Change type requirements for {} being a set literal. #129

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a1789bf
Change rules for when a set-or-map-literal is a set literal.
lrhn Dec 10, 2018
01ec8ee
Simplify. Do not attempt to match type variables because they won't w…
lrhn Dec 11, 2018
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17 changes: 12 additions & 5 deletions accepted/future-releases/set-literals/feature-specification.md
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
# Set Literals Design Document
Author: lrn@google.com
Version: 1.1

Solution for [Set Literals Problem](http://github.com/dart-lang/language/issues/36).
Based on feature proposal [Issue 37](http://github.com/dart-lang/language/issues/37)
Expand Down Expand Up @@ -84,12 +85,14 @@ if *s* has a `typeArguments` with two type arguments, *s* is a *map literal*.
(*Three or more type arguments is a compile time error, so the remaining possible case is having no type arguments*).

If *s* is an `emptySetOrMapLiteral` with no `typeArguments` and static context type *C*, then
if `Set<Null>` is assignable to *C* and `Map<Null, Null>` is not assignable to *C*, then *s* is a *set literal*,
otherwise *s* is a *map literal*.
if `LinkedHashSet<Null>` is assignable to *C* and `LinkedHashMap<Null, Null>` is not assignable to *C*,
then *s* is a *set literal*, otherwise *s* is a *map literal*.

(*So if *C* is, for example, `Iterable<int>` or `Set<Object>` or `FutureOr<Iterable<int>>` or a type variable with any of
those as bound, then *s* is a set literal. If *C* is `Object` or `dynamic` or `Null` or `String`, then *s* is a map literal,
*and* potentially a compile-time error due to static typing*).
(*So if *C* is, for example, `Iterable<int>`, `Set<Object>`, `LinkedHashSet<int>` or `FutureOr<Iterable<int>>`,
then *s* is a set literal. If *C* is `Object` or `dynamic` or `Null` or `String`, then *s* is a map literal,
*and* potentially a compile-time error due to static typing*. If *C* is some subclass of `Set<X>` other than `LinkedHashSet`, then the literal is a map literal, but it is also a guaranteed type error even if the literal
is a set. If *C* is a subtype of `LinkedHashSet<X>` *and* a subtype of `LinkedHashMap<Y, Z>`, then *s* is again
a map literal, and a guaranteed run-time type error).

### Map literals

Expand Down Expand Up @@ -357,3 +360,7 @@ var s6 = {...{1: 1}, ...?d}; // Map<dynamic, dynamic>
// var s7 = {...?d}; // Compile-time error, ambiguous
// var s8 = {...{1}, ...{1: 1}}; // Compile-time error, incompatible
```

##Revisions
1.0: Initial version plus type fixes.
1.1: Changed type rules for selecting set literals over map literals.