SIL: Fix signature of reabstraction thunk involving parameterized existential

Fixes https://github.com/swiftlang/swift/issues/74569
Fixes rdar://problem/130402282

lib/AST/LocalArchetypeRequirementCollector.cpp

@@ -136,7 +136,7 @@ GenericSignature swift::buildGenericSignatureWithCapturedEnvironments(
     case GenericEnvironment::Kind::OpenedExistential: {
       auto constraint = genericEnv->getOpenedExistentialType();
       if (auto existential = constraint->getAs<ExistentialType>())
-        constraint = existential->getConstraintType();
+        constraint = existential->getConstraintType()->mapTypeOutOfContext();
       collector.addOpenedExistential(constraint);
       continue;
     }

test/SILGen/parameterized_existentials_reabstraction.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-emit-silgen -disable-availability-checking %s | %FileCheck %s
+
+protocol P<A> {
+    associatedtype A
+    associatedtype B
+    var function: () -> B { get }
+}
+
+func f<A>(p: any P<A>) {
+    _ = p.function()
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s40parameterized_existentials_reabstraction1f1pyAA1P_px1ARts_XP_tlF : $@convention(thin) <A> (@in_guaranteed any P<A>) -> () {
+
+// CHECK-LABEL: sil shared [transparent] [serialized] [reabstraction_thunk] [ossa] @$s1B40parameterized_existentials_reabstraction1PPQyd__Iegr_ypIegr_1AQyd__RszAbCRd__r__lTR : $@convention(thin) <τ_0_0><τ_1_0 where τ_0_0 == τ_1_0.A, τ_1_0 : P> (@guaranteed @callee_guaranteed () -> @out τ_1_0.B) -> @out Any {

validation-test/compiler_crashers_2_fixed/issue-74569.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend -emit-ir %s -disable-availability-checking
+
+protocol Cache<Object> {
+    associatedtype Object
+    associatedtype Snapshot: Sequence<Object>
+
+    func entries(_ body: (Snapshot) -> Void) -> AsyncStream<Object>
+}
+
+func readFromCacheImpl<T>(cache: any Cache<T>) async {
+    let updateStream = cache.entries { _ in }
+
+    for await _ in updateStream {}
+}

validation-test/compiler_crashers_2_fixed/issue-74729.swift

@@ -0,0 +1,61 @@
+// RUN: %target-swift-frontend -emit-ir %s -disable-availability-checking
+
+protocol ContextDescriptor: Sendable { }
+
+protocol OutcomeDescriptor: Sendable { }
+
+protocol ResultDescriptor: Sendable { }
+
+protocol ErasedReducer<Context>: Sendable {
+    associatedtype Context: ContextDescriptor
+    associatedtype TriggerOutcome: OutcomeDescriptor
+    associatedtype Result: ResultDescriptor
+
+    var name: String { get }
+    var function: @Sendable (_ context: Context?, _ trigger: TriggerOutcome) -> Result  { get }
+}
+
+protocol Reducer<Context, TriggerOutcome>: ErasedReducer {
+    var name: String { get }
+    var function: @Sendable (_ context: Context?, _ trigger: TriggerOutcome) -> Result  { get }
+}
+
+struct ExampleContext: ContextDescriptor { }
+
+struct ExampleOutcome: OutcomeDescriptor { }
+
+struct ExampleResult: ResultDescriptor { }
+
+struct ExampleReducer<Context: ContextDescriptor, TriggerOutcome: OutcomeDescriptor, Result: ResultDescriptor>: Reducer {
+    let name: String
+    let function: @Sendable (_ context: Context?, _ trigger: TriggerOutcome) -> Result
+}
+
+class ExampleService<Context: ContextDescriptor> {
+    let reducers:  [any ErasedReducer<Context>]
+
+    public init(reducers: [any ErasedReducer<Context>]) {
+        self.reducers = reducers
+    }
+
+    func reduce<TriggerOutcome: OutcomeDescriptor>(outcome: TriggerOutcome, context: Context) -> (any ResultDescriptor)? {
+        // This line appears to be what's causing the crash
+        guard let reducer = (reducers.compactMap { reducer in reducer as? any Reducer<Context, TriggerOutcome> }).first else {
+            return nil
+        }
+        return reducer.function(context, outcome)
+    }
+}
+
+
+public func testReducing() {
+    let erasedReducers: [any ErasedReducer<ExampleContext>] = [
+        ExampleReducer<ExampleContext, ExampleOutcome, ExampleResult>(name: "Example", function: { (_, _) in ExampleResult() })
+    ]
+    let context = ExampleContext()
+    let trigger = ExampleOutcome()
+
+    let service = ExampleService<ExampleContext>(reducers: erasedReducers)
+
+    _ = service.reduce(outcome: trigger, context: context)
+}