Compile-time error errors due to exact types of set literals

[kmillikin]

LanguageFeatures/Set-literals/exact_types_of_literals_A01_t03.dart has:

  LinkedHashSet<int> v11 = {};
  Expect.isTrue(v11 is LinkedHashSet<int>);

The first line is a compile-time error. The empty non-const set/map literal is a set literal, so it has static type Set<T> for some T.

Type inference tries to satisfy Set<T> <: LinkedHashSet<int> by choosing T but there is no such T and so it infers Set<dynamic>.

The rules for exact types of literals require us to reject the assignment of the Set<dynamic> to LinkedHashSet<int> with a compile-time error because LinkedHashSet<int> is not a supertype of LinkedHashSet<dynamic>.

[kmillikin]

For the same reason, LanguageFeatures/Set-literals/semantics_A05_t01.dart has a compile-time error. It has:

LinkedHashSet<int> test4() => {};

which is a compile-time error.

[lrhn]

Correct. The "exact type" for set and map literals is going to be Set<T>/Map<K,V>, and not any of the LinkedHash-subclasses.
I originally said something else in the feature document, but that was based on a misunderstanding of what we currently do for maps. This has (now) been fixed to match the formal language specification (which is now complete and should be used as the source of truth for this feature).

[sgrekhov]

@lrhn, please, update feature specification examples section. It contains

/*
context expression runtime type and const-ness
*/
...
LinkedHashSet< int > v11 = {}; // LinkedHashSet< int >

I can add an issue for Language, if needed

[lrhn]

Good catch. I've fixed it