[embedded] Fix associated type conformances for specialized witness tables

[aschwaighofer]

No description provided.

[aschwaighofer]

@swift-ci test

[aschwaighofer]

This includes PR:
#85625 and #85426

[aschwaighofer]

@swift-ci test

[slavapestov]

This doesn't make sense, why is Conformance.getTypeWitness() returning the wrong thing? Is Conformance is not a SpecializedProtocolConformance? If not, it should be fixed to match up and the conditional removed.

[slavapestov]

You could use either Conformance.getTypeWitness() or entry.getAssociatedTypeWitness().Witness here, but they should be equal.

[aschwaighofer]

Conformance.getTypeWitness returns the wrong thing because Conformance is the root conformance not the specialized_conformance.

Example:

protocol PWithAssoc {
  associatedtype Assoc
  func a() -> Assoc
}

struct Conformer<T> {
  var t: T
  init(_ t: T) {
    self.t = t
  }
}

struct Container<T> : PWithAssoc {
  var x: Conformer<T>

  init(_ t: T) {
    self.x = Conformer(t)
  }

  func a() -> Conformer<T> {
    return x
  }
}

func test(_ p: PWithAssoc) {
  let _ = p.a()
}

test(Container(1))

In this example, we have a specialized SILWitness table:

sil_witness_table shared [specialized] Container<Int>: specialize <Int> (<T> Container<T>: PWithAssoc module t3) {
  associated_type Assoc: Conformer<Int>
  method #PWithAssoc.a: <Self where Self : PWithAssoc> (Self) -> () -> Self.Assoc : @$e2t39ContainerVyxGAA10PWithAssocA2aEP1a0D0QzyFTWSi_Tg5    // specialized protocol witness for PWithAssoc.a() in conformance Container<A>
}

In the witnesstablebuilder code Conformance is initialized to the root conformance SILWT->getConformance()->getRootConformance():

  class WitnessTableBuilderBase {                                               
  protected:                                                                    
    IRGenModule &IGM;                                                           
    SILWitnessTable *SILWT;                                                     
    const RootProtocolConformance &Conformance;                                 
    const ProtocolConformance &ConformanceInContext;                            
    CanType ConcreteType;                                                       
                                                                                
    std::optional<FulfillmentMap> Fulfillments;                                 
                                                                                
    WitnessTableBuilderBase(IRGenModule &IGM, SILWitnessTable *SILWT)           
        : IGM(IGM), SILWT(SILWT),                                               
          Conformance(*SILWT->getConformance()->getRootConformance()),          
          ConformanceInContext(*mapConformanceIntoContext(SILWT->getConformance()->getRootConformance())),
          ConcreteType(Conformance.getDeclContext()                             
                         ->mapTypeIntoEnvironment(Conformance.getType())        
                         ->getCanonicalType()) {}    

And that ends up being the normal conformance:

(normal_conformance type="Container<T>" protocol="PWithAssoc" explicit nonisolated
  (assoc_type req="Assoc" type="Conformer<T>")
  (value req="a()" witness="t3.(file).Container.a()@t3.swift:20:8")
  (assoc_conformance type="Self" proto="Copyable"
    (builtin_conformance type="Container<T>" protocol="Copyable" synthesized nonisolated))
  (assoc_conformance type="Self" proto="Escapable"
    (builtin_conformance type="Container<T>" protocol="Escapable" synthesized nonisolated))
  (assoc_conformance type="Self.Assoc" proto="Copyable"
    (builtin_conformance type="Conformer<T>" protocol="Copyable" synthesized nonisolated))
  (assoc_conformance type="Self.Assoc" proto="Escapable"
    (builtin_conformance type="Conformer<T>" protocol="Escapable" synthesized nonisolated)))

And then we end up with: (bound_generic_struct_type decl="t3.(file).Conformer@t3.swift:6:8" (generic_type_param_type depth=0 index=0 name="T" param_kind=type)) Instead of Conformer<Int> for

auto typeWitness = Conformance.getTypeWitness(assocType); 

If we used SILWT->getConformance().getTypeWitness(assocType) we would get the the right type (bound_generic_struct_type decl="t3.(file).Conformer@t3.swift:6:8" (struct_type decl="Swift.(file).Int"))

[slavapestov]

Ok, thanks for the investigation. I think the right fix is to not unwrap to get the root conformance there at all. If the witness table is not specialized, it should already be the root conformance. But that might be tricky to untangle so you don't have to do it if you don't want to.

Two more questions. Does this need to be conditional on langOpts.hasFeature(Feature::EmbeddedExistentials) then? Also there's another place where getAssociatedConformance() gets called on the unspecialized root conformance. Is that also wrong?

I'll clean this up some day. ConformanceInContext could probably be removed as well.

[aschwaighofer]

Failed Tests (1):
14:44:53 Swift(iphonesimulator-x86_64) :: TBD/linker-directives-ld-previous-ios.swift