Sema: Force SILGen emission of on-demand synthesized accessors

When an on-demand accessor is synthesized while checking a conformance,
make sure it ends up in the 'external declarations' list so that SILGen
can emit it.

Fixes <rdar://problem/46503121>, and part of <rdar://problem/46186045>.

Author: slavapestov

lib/SILGen/SILGenDecl.cpp

@@ -1396,7 +1396,19 @@ void SILGenModule::emitExternalWitnessTable(ProtocolConformance *c) {
   lastEmittedConformance = root;
 }
 
+static bool isDeclaredInPrimaryFile(SILModule &M, Decl *d) {
+  auto *dc = d->getDeclContext();
+  if (auto *sf = dyn_cast<SourceFile>(dc->getModuleScopeContext()))
+    if (M.isWholeModule() || M.getAssociatedContext() == sf)
+      return true;
+
+  return false;
+}
+
 void SILGenModule::emitExternalDefinition(Decl *d) {
+  if (isDeclaredInPrimaryFile(M, d))
+    return;
+
   switch (d->getKind()) {
   case DeclKind::Func:
   case DeclKind::Accessor: {

lib/Sema/CodeSynthesis.cpp

@@ -1083,6 +1083,11 @@ void TypeChecker::synthesizeWitnessAccessorsForStorage(
     if (isOnDemandAccessor(storage, kind)) {
       auto synthKind = getSynthKindForAccessorKind(kind);
       triggerSynthesis(*this, storage->getAccessor(kind), synthKind);
+
+      // Make sure SILGen emits the accessor; on-demand accessors have shared
+      // linkage, and if its defined in a different translation unit from the
+      // conformance we cannot simply generate an external declaration.
+      Context.addExternalDecl(storage->getAccessor(kind));
     }
   });
 

test/SILGen/Inputs/nsmanaged-witness-multi-other.swift

@@ -0,0 +1,5 @@
+import Foundation
+
+public class Fish {
+  @NSManaged public var name: String
+}

test/SILGen/nsmanaged-witness-multi.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -module-name main -emit-silgen -enable-sil-ownership -sdk %S/Inputs -primary-file %s %S/Inputs/nsmanaged-witness-multi-other.swift -I %S/Inputs -I %t -enable-source-import | %FileCheck %s
+
+// RUN: %target-swift-frontend -module-name main -emit-silgen -enable-sil-ownership -sdk %S/Inputs -primary-file %s -primary-file %S/Inputs/nsmanaged-witness-multi-other.swift -I %S/Inputs -I %t -enable-source-import | %FileCheck %s
+
+// RUN: %target-swift-frontend -module-name main -emit-silgen -enable-sil-ownership -sdk %S/Inputs %s %S/Inputs/nsmanaged-witness-multi-other.swift -I %S/Inputs -I %t -enable-source-import | %FileCheck %s
+
+// REQUIRES: objc_interop
+import Foundation
+
+public protocol FishProtocol {
+  var name: String { get set }
+}
+
+extension Fish : FishProtocol {}
+
+// Make sure the modify accessor for Fish.name is emitted here even though it
+// its storage was declared in a different translation unit
+
+// CHECK-LABEL: sil shared [transparent] [serialized] [thunk] @$s4main4FishCAA0B8ProtocolA2aDP4nameSSvMTW : $@yield_once @convention(witness_method: FishProtocol) (@inout Fish) -> @yields @inout String
+// CHECK: function_ref @$s4main4FishC4nameSSvM
+// CHECK: return
+
+// CHECK-LABEL: sil shared [serialized] @$s4main4FishC4nameSSvM : $@yield_once @convention(method) (@guaranteed Fish) -> @yields @inout String
+// CHECK: objc_method %0 : $Fish, #Fish.name!getter.1.foreign
+// CHECK: objc_method %0 : $Fish, #Fish.name!setter.1.foreign
+// CHECK: unwind