Opaque return types are special type declarations that have it
own nested generic signature. Thus, given this:
```
protocol P<A> { associatedtype A: ~Copyable }
func f<T: ~Copyable>() -> some P<T> {}
```
The generic signature for f is <T where T Escapable>, and
for the opaque return type, its nested signature ends up as
```
<X where X: P, X.A == T>
```
With SE-503, we will now also expand a default for the suppressed
primary associated type, so the signature after expansion becomes
```
<X where X: P, X.A == T, X.A: Copyable>
```
It would be smarter to effectively have this rule
```
X.A == T, T: ~Copyable
----------------------
X.A: ~Copyable
```
where we infer the inverse on X.A to cancel-out the
expanded default X.A: Copyable. We already do this for
two in-scope type parameters, and it would be better if
we did it if one side was out-of-scope, but that would
be source-breaking to do in general.
In the case of opaque return types, the fact that
it has a nested generic signature seems more an
artifact of the implementation. There also is little
risk of source break, as the only kinds of same-type
requirements that can appear are from parameterized
protocol type.
The experimental suppressed associated types prior to
SE-503 wouldn't be broken by this change, as they do
not infer defaults that need suppression, and we only
filter-out requirements from defaults expansion, rather
than explicitly-written ones.
rdar://175500824
There's a need for more control over how default requirements
for conformance to Copyable/Escapable are expanded, and
subsequently how inverses are applied or inferred to cancel-out
those defaults.
The pattern of `/*applyInverses*/BOOL` is insufficient, so this
is a refactoring to grow that into a proper type that carries
an option that can be used in some future scenario about inferring
inverses for opaque return types.
The compiler synthesizes an `unownedExecutor` property for every `actor` type.
It was also synthesizing availability attributes for this property by
intersecting the availability of the actor type and the `UnownedExecutor` type.
This may have been necessary early in the development of Swift Concurrency but
now it amounts to little more than a convoluted procedure that must result in
repeating the availability of the actor on one of its properties (the property
can never be less available than the actor itself).
It should be possible to disable SerializationRequirement by assigning
Any to it. This effectively disables the checking, since any type
conforms to that.
Previously we would crash trying to check for this conformance; or
rather, emitting diagnostics about un-necessary casts then encountering
incomplete types in CSDiagnostics.
Avoiding the cast entirely sounds like a good solution here.
resolves rdar://159285863
This feature is essentially self-migrating, but fit it into the
migration flow by marking it as migratable, adding
`-strict-memory-safety:migrate`, and introducing a test.
Kavon Farvardin
Allan Shortlidge
Slava Pestov
Hamish Knight
Konrad Malawski
Doug Gregor
Pavel Yaskevich
Anthony Latsis