SI-1503 don't assume unsound type for ident/literal patterns #3559
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Review by @gkossakowski |
The fix only kicks in under -Xfuture. We also warn under -Xlint. What type should a variable bound to the value matched by a pattern have? To avoid CCEs, it should be a type that's implied by the matching semantics of the pattern. Usually, the type implied by a pattern matching a certain value is the pattern's type, because pattern matching implies instance-of checks. However, Stable Identifier and Literal patterns are matched using `==`, which does not imply a type for the binder that binds the matched value. The change in type checking due to this fix is that programs that used to crash with a CCE (because we blindly cast to the type of the pattern, which a `==` check does not imply) now get a weaker type instead (and no cast). They may still type check, or they may not. To compensate for this fix, change `case x@Foo => x` to `case x: Foo.type => x`, if it's important that `x` have type `Foo.type`. See also: - SI-4577: matching of singleton type patterns uses `eq`, not `==` (so that the types are not a lie). - SI-5024: patmat strips unused bindings, but affects semantics
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ping @gkossakowski |
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LGTM |
gkossakowski
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SI-1503 don't assume unsound type for ident/literal patterns
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eed3si9n
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Apr 9, 2018
Fixes scala/bug#1503 scala#3559 stopped assuming unsound type for ident/literal patterns under -Xfuture in Scala 2.11. Since enough time has elapsed, I am ripping the bandaid off on SI-1503 (scala/bug#1503).
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The fix only kicks in under -Xfuture. We also warn under -Xlint.
What type should a variable bound to the value matched by a pattern have?
To avoid CCEs, it should be a type that's implied by the matching
semantics of the pattern.
Usually, the type implied by a pattern matching a certain value
is the pattern's type, because pattern matching implies instance-of checks.
However, Stable Identifier and Literal patterns are matched using
==,which does not imply a type for the binder that binds the matched value.
The change in type checking due to this fix is that programs that used to crash with a CCE
(because we blindly cast to the type of the pattern, which a
==check does not imply)now get a weaker type instead (and no cast). They may still type check, or they may not.
To compensate for this fix, change
case x@Foo => xtocase x: Foo.type => x,if it's important that
xhave typeFoo.type.See also:
eq,not
==(so that the types are not a lie).