[SR-7799] Matching optional enums against non-optional values

[martinr448]

Previous ID	SR-7799
Radar	rdar://problem/41494702
Original Reporter	@martinr448
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@theblixguy
Priority	Medium

md5: 866262f404c0af00426fbd898e2b0060

Issue Description:

We can match an optional Int against a non-optional value:

let x: Int? = 1

switch x {
case 1: print("one")
default: print("default")
}

But the same does not compile with an optional enum:

enum MyEnum { case a, b }
let e: MyEnum? = .a

switch e {
case .a: print("a") // error: Enum case 'a' not found in type 'MyEnum?'
case .some(.a): print("some a") // OK
default: print("default")
}

Tested with Xcode 9.3.1, with both the default toolchain and the Swift 4.2 snapshot from May 20, 2018.

Related discussion in the Swift forum: Matching optionals in a switch statement, where Joe Groff said:

It ought to compile as well. That's a bug.

[belkadan]

@swift-ci create

[theblixguy]

@slavapestov @jckarter I have made a change in the type checker to look through optionals when checking an `EnumElementPattern`, however it seems like I need to handle it in SIL as well. For example, at the moment, SILVerification throws an error:

SIL verification failed: switch_enum dispatches on enum element that is not part of its type: elt->getDeclContext() == uDecl

for

enum Foo { case bar}
let foo: Foo? = .bar
switch foo { case .bar: print("bar") default: print("default") }

This makes sense, because we're switching over Foo? rather than Foo, so the emitted SIL looks like:

alloc_global @$s12testmetatype3fooAA3FooOSgvp   // id: %2
%3 = global_addr @$s12testmetatype3fooAA3FooOSgvp : $*Optional<Foo> // users: %8, %7
%4 = metatype $@thin Foo.Type
%5 = enum $Foo, #Foo.bar!enumelt                // user: %6
%6 = enum $Optional<Foo>, #Optional.some!enumelt.1, %5 : $Foo // user: %7
store %6 to [trivial] %3 : $*Optional<Foo>      // id: %7
%8 = load [trivial] %3 : $*Optional<Foo>        // user: %9
switch_enum %8 : $Optional<Foo>, case #Foo.bar!enumelt: bb1 // id: %9

What would be the best approach here? Do we switch over Foo instead, or do we wrap the case in `.some()`?

[theblixguy]

Or should we re-write the element in the type checker to wrap it in `.some()` so we don't have to make changes to SIL?

[theblixguy]

PR: #22486

[theblixguy]

Fixed on master, this will be available in the next Swift release!

[AnnaZaks]

@martinr448, Could you verify if the problem is fixed and if so move the JIRA to "Closed"?

Thanks!
Anna

[martinr448]

This compiles now, and `case .a:` matches the `Optional(MyEnum.a)`:

enum MyEnum { case a, b }
let e = Optional(MyEnum.a)

switch e {
case .a: print("a")
default: print("default")
}

Tested with Xcode 11.0 beta 5 (11M382q).