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Adjust specification of type tests to include promotion to X & S

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Addresses https://github.com/dart-lang/sdk/issues/35314.

Change-Id: I97550187255c5ac896bfea881de85c5e97e9930e
Reviewed-on: https://dart-review.googlesource.com/c/85921
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
This commit is contained in:
Erik Ernst 2018-12-20 13:33:30 +00:00
parent ff44e27c5a
commit 9e48cc7f25
1 changed files with 16 additions and 8 deletions
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24
docs/language/dartLangSpec.tex
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@ -10457,13 +10457,19 @@ Users should test for the null object (\ref{null}) directly rather than via type
\LMHash{}%
The is-expression \code{$e$ \IS{}! $T$} is equivalent to \code{!($e$ \IS{} $T$)}.
% Add flow dependent types
\LMHash{}%
Let $v$ be a local variable (\commentary{which can be a formal parameter}).
%% TODO(eernst): Cf. issue https://github.com/dart-lang/sdk/issues/35314.
An is-expression of the form \code{$v$ \IS{} $T$} shows that $v$ has type $T$
if{}f $T$ is a subtype of the type $S$ of the expression $v$.
An is-expression of the form \code{$v$ \IS{} $T$}
shows that $v$ has type $T$
if $T$ is a subtype of the type of the expression $v$.
%
Otherwise,
if the declared type of $v$ is the type variable $X$,
and $T$ is a subtype of the bound of $X$,
and $X \& T$ is a subtype of the type of the expression $v$,
then $e$ shows that $v$ has type $X \& T$.
%
Otherwise $e$ does not show that $v$ has type $T$ for any $T$.
\rationale{
The motivation for the ``shows that v has type T" relation is to reduce spurious errors thereby enabling a more natural coding style.
@ -10474,10 +10480,12 @@ The rule only applies to locals and parameters, as instance and static variables
It is pointless to deduce a weaker type than what is already known.
Furthermore, this would lead to a situation where multiple types are associated with a variable at a given point, which complicates the specification.
Hence the requirement that \SubtypeNE{T}{S}.
Hence the requirement that the promoted type is a subtype of the current type.
We do not want to refine the type of a variable of type \DYNAMIC{}, as this could lead to more errors rather than fewer.
The opposite requirement, that $T \ne \DYNAMIC{}$ is a safeguard lest $S$ ever be $\bot$.
In any case, it is not an error when a type test does not show
that a given variable does not have a ``better'' type than previously known,
but tools may choose to give a hint in such cases,
if suitable heuristics indicate that a promotion is likely to be intended.
}
\LMHash{}%