Dynamic Casting: Properly unwrap existential metatype sources #34469
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Existential metatypes are really just existentials that hold metatypes. As
such, they should be handled in the general casting logic in much the same way
as regular existentials: They should generally be ignored by most casting logic,
and unwrapped as necessary at the top level.
In particular, the previous code would fail to correctly handle the following
cast from an existential metatype (`AnyObject.Type`) to an existential
(`AnyObject`):
```
class C {}
let a = C.self as AnyObject.Type
let b = a as! AnyObject
```
With the old code, `b` above would hold a reference to a `__SwiftValue` box
containing the type reference. The correct result would simply store the type
reference directly in `b`. These two are only really distinguishable in that
the correct form permits `a === b` to return `true`.
Fixes rdar://70582753
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mikeash
approved these changes
Oct 28, 2020
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While I'm here, include some comments indicating the missing pieces still needed for Linux support to work. Basically, metatypes on Linux are not currently fully compatible with reference-counted class pointers, which prevents us from fully supporting metatype operations on Linux that we support on macOS.
Whitespace cleanup
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ainu-bot
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that referenced
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Oct 29, 2020
* 'main' of github.com:apple/swift: (22 commits) Dynamic Casting: Properly unwrap existential metatype sources (swiftlang#34469) Add -allow-critical-edges=false to copyforward_ossa.sil Add -allow-critical-edges flag. Sema: Check result builder availability Sema: Pass ExportContext by const reference where possible Sema: Put the AvailabilityContext inside the ExportContext Change interfaces to accept a FileCollectorBase [Parse] Remove unused declaration and diagnostic message [cxx-interop] Use regex instead of operand name to fix C++ constructor tests. [CodeCompletion] Fix a crash in collectPossibleReturnTypesFromContext [test] Update cross_langauge index test for mangling change Temporarily disable this test [Concurrency] SIL: add hop_to_executor instruction sil-opt: add an -enable-experimental-concurrency option. Sema: Don't visit elements of a BraceStmt twice when checking availability Sema: Rename AvailabilityWalker to ExprAvailabilityWalker Sema: Pull diagnoseDeclAvailability() out of AvailabilityWalker Sema: Rename some availability checking entry points SILGen: Stub out support for invoking foreign async methods. Add a mangling for completion block implementation functions. ...
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Existential metatypes are really just existentials that hold metatypes. As
such, they should be handled in the general casting logic in much the same way
as regular existentials: They should generally be ignored by most casting logic,
and unwrapped as necessary at the top level.
In particular, the previous code would fail to correctly handle the following
cast from an existential metatype (
AnyObject.Type) to an existential(
AnyObject):With the old code,
babove would hold a reference to a__SwiftValueboxcontaining the type reference. The correct result would simply store the type
reference directly in
b. These two are only really distinguishable in thatthe correct form permits
a === bto returntrue.Fixes rdar://70582753