IRGen: Fix crash trying to emit capture descriptor involving element archetype

We could encode local archetypes in reflection info by performing the
local archetype transform, to get an extended generic signature together
with substitutions for the added generic parameters.

However, for now, let's just not crash. This generalizes an earlier hack
for opened existential archetypes. Instead of replacing them with
nonsensical type parameters though, let's just not emit the whole
descriptor.

- Fixes https://github.com/swiftlang/swift/issues/83539.
- Fixes rdar://157554723.

lib/IRGen/GenHeap.cpp

@@ -1576,24 +1576,6 @@ public:
       boxedInterfaceType = boxedType.mapTypeOutOfContext();
     }
 
-    {
-      // FIXME: This seems wrong. We used to just mangle opened archetypes as
-      // their interface type. Let's make that explicit now.
-      auto astType = boxedInterfaceType.getASTType();
-      astType =
-          astType
-              .transformRec([](Type t) -> std::optional<Type> {
-                if (auto *openedExistential = t->getAs<ExistentialArchetypeType>()) {
-                  auto &ctx = openedExistential->getASTContext();
-                  return ctx.TheSelfType;
-                }
-                return std::nullopt;
-              })
-              ->getCanonicalType();
-      boxedInterfaceType = SILType::getPrimitiveType(
-          astType, boxedInterfaceType.getCategory());
-    }
-    
     auto boxDescriptor = IGF.IGM.getAddrOfBoxDescriptor(
         boxedInterfaceType,
         env ? env->getGenericSignature().getCanonicalSignature()

lib/IRGen/GenReflection.cpp

@@ -1693,6 +1693,9 @@ llvm::Constant *IRGenModule::getAddrOfFieldName(StringRef Name) {
 llvm::Constant *
 IRGenModule::getAddrOfBoxDescriptor(SILType BoxedType,
                                     CanGenericSignature genericSig) {
+  if (BoxedType.hasLocalArchetype())
+    return llvm::Constant::getNullValue(CaptureDescriptorPtrTy);
+
   if (IRGen.Opts.ReflectionMetadata != ReflectionMetadataMode::Runtime)
     return llvm::Constant::getNullValue(CaptureDescriptorPtrTy);
 

test/Reflection/box_descriptor_with_element_archetype.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-frontend -emit-ir %s -target %target-swift-5.9-abi-triple
+
+public protocol P {}
+
+public struct G<T>: P {
+  let s1: String
+  let s2: String
+}
+
+public func f<each T>(t: repeat G<each T>) {
+    var ts: [any P] = []
+    for x in repeat each t {
+        ts.append(x)
+    }
+}