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Make Null act like the bottom type for all the (default nullable) types.

Browse source 
BUG= http://dartbug.com/28024
R=eernst@google.com

Review URL: https://codereview.chromium.org/2555343004 .
This commit is contained in:
Lasse R.H. Nielsen 2016-12-09 12:59:46 +01:00
parent 925160e2d0
commit 10abaf89fd
1 changed files with 5 additions and 12 deletions
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17
docs/language/dartLangSpec.tex
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@ -2616,10 +2616,7 @@ The null object is the sole instance of the built-in class \code{Null}. Attempti
The \code{Null} class declares no methods except those also declared by \code{Object}.
\LMHash{}
The static type of \NULL{} is $\bot$.
\rationale{The decision to use $\bot$ instead of \code{Null} allows \NULL{} to be be assigned everywhere without complaint by the static checker.
}
The static type of \NULL{} is the \code{Null} type.
\subsection{Numbers}
@ -7369,11 +7366,6 @@ If $I$ is listed in the \EXTENDS{} clause of $J$% or an interface injection decl
\item If $J$ is a mixin application (\ref{mixinApplication}) of the mixin of $I$.
\end{itemize}
\commentary{
}
%\Q{Can wacky stuff happen with interface injection, e.g., a direct superinterface becomes indirect? What about side effects - loading order can affect type relationships.
%}
@ -7381,8 +7373,9 @@ If $I$ is listed in the \EXTENDS{} clause of $J$% or an interface injection decl
A type $T$ is more specific than a type $S$, written $T << S$, if one of the following conditions is met:
\begin{itemize}
\item $T$ is $S$.
\item T is $\bot$.
\item S is \DYNAMIC{}.
\item $T$ is $\bot$.
\item $T$ is \NULL{} and $S$ is not $\bot$.
\item $S$ is \DYNAMIC{}.
\item $S$ is a direct supertype of $T$.
\item $T$ is a type parameter and $S$ is the upper bound of $T$.
\item $T$ is a type parameter and $S$ is \cd{Object}.
@ -7409,7 +7402,7 @@ $S$ is a supertype of $T$, written $S :> T$, iff $T$ is a subtype of $S$.
\commentary{The supertypes of an interface are its direct supertypes and their supertypes. }
\LMHash{}
An interface type $T$ may be assigned to a type $S$, written $T \Longleftrightarrow S$, iff either $T <: S$ or $S <: T$.
An interface type $T$ may be assigned to a type $S$, written $T \Longleftrightarrow S$, iff either $T <: S$, $S <: T$, or either $T$ or $S$ is the \code{Null} type.
\rationale{This rule may surprise readers accustomed to conventional typechecking. The intent of the $\Longleftrightarrow$ relation is not to ensure that an assignment is correct. Instead, it aims to only flag assignments that are almost certain to be erroneous, without precluding assignments that may work.