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Adjust specification of type tests to include promotion to X & S
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>
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@ -10457,13 +10457,19 @@ Users should test for the null object (\ref{null}) directly rather than via type
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The is-expression \code{$e$ \IS{}! $T$} is equivalent to \code{!($e$ \IS{} $T$)}.
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% Add flow dependent types
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Let $v$ be a local variable (\commentary{which can be a formal parameter}).
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%% TODO(eernst): Cf. issue https://github.com/dart-lang/sdk/issues/35314.
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An is-expression of the form \code{$v$ \IS{} $T$} shows that $v$ has type $T$
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if{}f $T$ is a subtype of the type $S$ of the expression $v$.
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An is-expression of the form \code{$v$ \IS{} $T$}
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shows that $v$ has type $T$
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if $T$ is a subtype of the type of the expression $v$.
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%
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Otherwise,
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if the declared type of $v$ is the type variable $X$,
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and $T$ is a subtype of the bound of $X$,
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and $X \& T$ is a subtype of the type of the expression $v$,
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then $e$ shows that $v$ has type $X \& T$.
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%
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Otherwise $e$ does not show that $v$ has type $T$ for any $T$.
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\rationale{
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The motivation for the ``shows that v has type T" relation is to reduce spurious errors thereby enabling a more natural coding style.
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@ -10474,10 +10480,12 @@ The rule only applies to locals and parameters, as instance and static variables
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It is pointless to deduce a weaker type than what is already known.
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Furthermore, this would lead to a situation where multiple types are associated with a variable at a given point, which complicates the specification.
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Hence the requirement that \SubtypeNE{T}{S}.
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Hence the requirement that the promoted type is a subtype of the current type.
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We do not want to refine the type of a variable of type \DYNAMIC{}, as this could lead to more errors rather than fewer.
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The opposite requirement, that $T \ne \DYNAMIC{}$ is a safeguard lest $S$ ever be $\bot$.
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In any case, it is not an error when a type test does not show
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that a given variable does not have a ``better'' type than previously known,
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but tools may choose to give a hint in such cases,
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if suitable heuristics indicate that a promotion is likely to be intended.
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}
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\LMHash{}%
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