Sema: Fix crash with stored property that has on-demand 'modify' accessor

If a stored property has observers, the 'modify' coroutine body takes a
special form which directly yields the underlying storage and calls the
'didSet' and/or 'willSet' accessors.

However, if a property is 'dynamic', the 'modify' coroutine is synthesized
on-demand and it's SIL is serialized. It cannot reference the 'didSet'
or 'willSet' accessors, since they are private. Furthermore, an on-demand
accessor must also work if the property is overridden in a subclass.

So in this case, fall back to the more general 'modify' synthesis code,
which just calls the getter followed by the setter.

Fixes <rdar://problem/62339808>.

Author: slavapestov

lib/Sema/TypeCheckStorage.cpp

@@ -1760,9 +1760,16 @@ synthesizeCoroutineAccessorBody(AccessorDecl *accessor, ASTContext &ctx) {
 
   bool isModify = accessor->getAccessorKind() == AccessorKind::Modify;
 
+  // Special-case for a modify coroutine of a simple stored property with
+  // observers. We can yield a borrowed copy of the underlying storage
+  // in this case. However, if the accessor was synthesized on-demand,
+  // we do the more general thing, because on-demand accessors might be
+  // serialized, which prevents them from being able to directly reference
+  // didSet/willSet accessors, which are private.
   if (isModify &&
       (storageReadWriteImpl == ReadWriteImplKind::StoredWithSimpleDidSet ||
-       storageReadWriteImpl == ReadWriteImplKind::InheritedWithSimpleDidSet)) {
+       storageReadWriteImpl == ReadWriteImplKind::InheritedWithSimpleDidSet) &&
+      !accessor->hasForcedStaticDispatch()) {
     return synthesizeModifyCoroutineBodyWithSimpleDidSet(accessor, ctx);
   }
 

test/SILGen/observers_with_on_demand_modify_accessor.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-emit-silgen %s -disable-objc-attr-requires-foundation-module | %FileCheck %s
+// REQUIRES: objc_interop
+
+public protocol Proto {
+  var property: Int { get set }
+}
+
+public class ClassWithDynamicObservedProperty : Proto {
+  @objc public dynamic var property: Int = 0 {
+    didSet {}
+  }
+}
+
+// CHECK-LABEL: sil shared [serialized] [ossa] @$s40observers_with_on_demand_modify_accessor32ClassWithDynamicObservedPropertyC8propertySivM : $@yield_once @convention(method) (@guaranteed ClassWithDynamicObservedProperty) -> @yields @inout Int {
+// CHECK: objc_method %0 : $ClassWithDynamicObservedProperty, #ClassWithDynamicObservedProperty.property!getter.foreign
+// CHECK: objc_method %0 : $ClassWithDynamicObservedProperty, #ClassWithDynamicObservedProperty.property!setter.foreign
+// CHECK: return