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Adjusted circularity prevention rule for type variable subtyping

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We noted in issue https://github.com/dart-lang/sdk/issues/33709 that
the language specification prevents usage of `Null` as the bound
of a type variable, because that makes the bound a subtype of
the type variable itself (which is otherwise a symptom of having
a cyclic declaration like `X extends X`).

This CL adjusts the wording such that it will be possible to use
`Null` (or whatever the denoteable bottom type is called) as a
bound.

Change-Id: I46a5ce6055f3af322a6b93ac38ca2829ce23a26c
Reviewed-on: https://dart-review.googlesource.com/63420
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
This commit is contained in:
Erik Ernst 2018-07-03 10:55:50 +00:00
parent 1467909921
commit 4fd04044fb
1 changed files with 10 additions and 3 deletions
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13
docs/language/dartLangSpec.tex
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@ -2926,8 +2926,15 @@ where each formal type parameter has been replaced by the corresponding actual t
\LMHash{}
A type parameter $T$ may be suffixed with an \EXTENDS{} clause that specifies the {\em upper bound} for $T$.
If no \EXTENDS{} clause is present, the upper bound is \code{Object}.
It is a static type warning if a type parameter is a supertype of its upper bound.
The bounds of type variables are a form of type annotation and have no effect on execution in production mode.
It is a static type warning if a type parameter is a supertype of its upper bound
when that upper bound is itself a type variable.
\commentary{
This prevents circular declarations like
\code{X \EXTENDS{} X}
and
\code{X \EXTENDS{} Y, Y \EXTENDS{} X}.
}
\LMHash{}
Type parameters are declared in the type parameter scope of a class or function.
@ -4053,7 +4060,7 @@ or to a \code{Symbol} instance that is \code{==} to that instance.
\LMHash{}
A symbol literal \code{\#\_$id$}, evaluates to an instance of \code{Symbol}
representing the private identifier \code{_$id} of the containing library.
representing the private identifier \code{\_\id} of the containing library.
All occurences of \code{\#\_$id$} {\em in the same library} evaluate to the same instance,
and no other symbol literals evaluate to that \code{Symbol} instance
or to a \code{Symbol} instance that is \code{==} to that instance.