IRGen: Force emission of lazy witness table when conformance descriptor is referenced [5.0]

lib/IRGen/GenDecl.cpp

@@ -3539,6 +3539,8 @@ llvm::GlobalValue *IRGenModule::defineAssociatedConformanceDescriptor(
 llvm::Constant *IRGenModule::getAddrOfProtocolConformanceDescriptor(
                                 const RootProtocolConformance *conformance,
                                 ConstantInit definition) {
+  IRGen.addLazyWitnessTable(conformance);
+
   auto entity = LinkEntity::forProtocolConformanceDescriptor(conformance);
   return getAddrOfLLVMVariable(entity, definition,
                                DebugTypeInfo());

test/IRGen/lazy_conformances.swift

@@ -0,0 +1,41 @@
+// RUN: %target-swift-frontend -parse-as-library -O %s -emit-ir | %FileCheck %s
+
+// CHECK: @"$s17lazy_conformances12MyCollectionVyxGSKAASKRzrlMc" = hidden constant {
+// CHECK: @"$s17lazy_conformances12MyCollectionVyxGSTAAMc" = hidden constant {
+
+struct MyCollection<Base : Collection> : Collection {
+  typealias Index = Base.Index
+  typealias Element = Base.Element
+
+  var startIndex: Index {
+    fatalError()
+  }
+
+  var endIndex: Index {
+    fatalError()
+  }
+
+  func index(after i: Index) -> Index {
+    fatalError()
+  }
+
+  func formIndex(after i: inout Index) {
+    fatalError()
+  }
+
+  subscript(position: Index) -> Element {
+    fatalError()
+  }
+}
+
+extension MyCollection : BidirectionalCollection
+  where Base : BidirectionalCollection
+{
+  func index(before i: Index) -> Index {
+    fatalError()
+  }
+
+  func formIndex(before i: inout Index) {
+    fatalError()
+  }
+}