--- title: Migrating away from experimental suppressed associated types (OldSuppressedAssociatedTypes) framework: swift-compiler role: article role_heading: Article path: swift-compiler/documentation/diagnostics/old-suppressed-associatedtypes --- # Migrating away from experimental suppressed associated types (OldSuppressedAssociatedTypes) SE-503 officially introduced the ability to define protocols with associated types that are ~Copyable and/or ~Escapable (i.e., “suppressed”), which relaxes the requirement on types conforming to that protocol. During the evolution process, a prototype version of this feature existed under the experimental feature named SuppressedAssociatedTypes. That experimental feature is now deprecated and is source-incompatible with the accepted version detailed in SE-503. ## Overview Overview This document provides high-level guidance for users of this experimental feature to help them update their code to be compatible with SE-503. Please see the evolution proposal for complete details of the accepted functionality. Source Changes note: The difference between the prototype experimental feature and SE-503 is with its treatment of primary associated types that are suppressed. Thus, if your suppressions were only on associated types that are not primary i.e., not mentioned within the angle brackets of the protocol in which they’re defined, then there are no source changes required. Primary associated types are those that are mentioned within angle brackets of a protocol: protocol Mailbox { // ^~~~~~~ declares that `Items` is a primary associated type associatedtype Items: ~Copyable associatedtype Generator: ~Copyable // ... } Relative to the prototype, SE-503 infers default requirements for primary associated types in places they were not already. The defaults are expanded within extensions of the protocol: extension Mailbox {} // OLD extension Mailbox where Items: ~Copyable {} // NEW The defaults are also expanded in generic contexts where a conformance requirement involving that protocol is written. So, to preserve existing behavior, you’ll need add explicit suppressions on the associated types induced by those conformance requirements within those contexts: func foo(..) where T: Mailbox {} // OLD func foo(..) where T: Mailbox, T.Items: ~Copyable {} // NEW struct PostOffice {} // OLD struct PostOffice where M.Items: ~Copyable {} // NEW ABI Changes note: This section is only of interest to who distribute pre-built shared libraries and declared a public protocol that has a suppressed primary associated type. The implementation of SE-503 changes the mangling scheme for generic signatures as a result of the inferred defaults. Suppose you have this protocol: protocol Drink { associatedtype Flavor: ~Copyable } and these two functions that are completely explicit about whether T.Flavor is Copyable or not: func foo(_ t: T) where T.Flavor: Copyable {} func bar(_ t: T) where T.Flavor: ~Copyable {} In the prototype version of the feature, bar’s requirement T.Flavor: ~Copyable was redundant, whereas foo’s requirement T.Flavor: Copyable was not redundant, so it would be included in the mangled symbol of foo: $s1X3fooyyxAA5DrinkRzs8Copyable6FlavorRpzlF ---> X.foo(A) -> () $s1X3baryyxAA5DrinkRzlF ---> X.bar(A) -> () With SE-503, this almost flips around. It’s now foo whose requirement is redundant, and bar’s mangled symbol now mentions the requirement T.Flavor: ~Copyable: $s1X3fooyyxAA5DrinkRzlF ---> X.foo(A) -> () $s1X3baryyxAA5DrinkRz6FlavorRj_zlF ---> X.bar(A) -> () It’s possible to preserve the old symbol for bar by using @abi to produce its symbol as if T.Flavor: Copyable: // $s1X3baryyxAA5DrinkRzlF ---> X.bar(A) -> () @abi(func bar(_ t: T) where T.Flavor: Copyable) func bar(_ t: T) where T.Flavor: ~Copyable {} But, only brittle internal compiler features can be used preserve foo’s old mangled symbol $s1X3fooyyxAA5DrinkRzs8Copyable6FlavorRpzlF. ## See Also - [@dynamicCallable implementation requirements (DynamicCallable)](swift-compiler/documentation/diagnostics/dynamic-callable-requirements.md) - [Add @preconcurrency import (AddPreconcurrencyImport)](swift-compiler/documentation/diagnostics/add-preconcurrency-import.md) - [Always enabled availability domains (AlwaysAvailableDomain)](swift-compiler/documentation/diagnostics/always-available-domain.md) - [Argument matching for trailing closures (TrailingClosureMatching)](swift-compiler/documentation/diagnostics/trailing-closure-matching.md) - [Calling a mutating async actor-isolated method (ActorIsolatedMutatingAsync)](swift-compiler/documentation/diagnostics/actor-isolated-mutating-async.md) - [Calling an actor-isolated method from a synchronous nonisolated context (ActorIsolatedCall)](swift-compiler/documentation/diagnostics/actor-isolated-call.md) - [Captures in a `@Sendable` closure (SendableClosureCaptures)](swift-compiler/documentation/diagnostics/sendable-closure-captures.md) - [Compilation caching (CompilationCaching)](swift-compiler/documentation/diagnostics/compilation-caching.md) - [Conforming to `StringInterpolationProtocol` (StringInterpolationConformance)](swift-compiler/documentation/diagnostics/string-interpolation-conformance.md) - [Conversion from `@isolated(any)` function type to synchronous function type (ConversionFromIsolatedAnyToSynchronous)](swift-compiler/documentation/diagnostics/conversion-from-isolated-any-to-synchronous.md) - [Cross-isolation data race (RegionIsolationCrossIsolationDataRace)](swift-compiler/documentation/diagnostics/region-isolation-cross-isolation-data-race.md) - [Deprecated declaration warnings (DeprecatedDeclaration)](swift-compiler/documentation/diagnostics/deprecated-declaration.md) - [Deprecated implementation-only imports (ImplementationOnlyDeprecated)](swift-compiler/documentation/diagnostics/implementation-only-deprecated.md) - [Dynamic exclusivity (DynamicExclusivity)](swift-compiler/documentation/diagnostics/dynamic-exclusivity.md) - [Embedded Swift language restrictions (EmbeddedRestrictions)](swift-compiler/documentation/diagnostics/embedded-restrictions.md)