Break cyclic dependency due to implicit initialization of optional values

The fundamental problem here is that we don't know a priori whether an
accessor macro will convert a stored property into a computed one.
That can only be determined after macro expansion, which depends on
having a determined type for the property.

Implicit initialization of optional-typed values (e.g., "var
birthDate: Date?") adds the initializer when there is storage,
triggering the cycle. Introduce a very narrow fix that assumes that
properties that have an accessor macro on them do not have storage.
We probably want to enforce this, so that the "does this variable have
storage?" query can be made cheaper.

Author: DougGregor

lib/Sema/TypeCheckStorage.cpp

@@ -280,6 +280,27 @@ StoredPropertiesAndMissingMembersRequest::evaluate(Evaluator &evaluator,
   return decl->getASTContext().AllocateCopy(results);
 }
 
+/// Check whether the pattern may have storage.
+///
+/// This query is careful not to trigger accessor macro expansion, which
+/// creates a cycle. It conservatively assumes that all accessor macros
+/// produce computed properties, which is... incorrect.
+static bool mayHaveStorage(Pattern *pattern) {
+  // Check whether there are any accessor macros.
+  bool hasAccessorMacros = false;
+  pattern->forEachVariable([&](VarDecl *VD) {
+    VD->forEachAttachedMacro(MacroRole::Accessor,
+      [&](CustomAttr *customAttr, MacroDecl *macro) {
+        hasAccessorMacros = true;
+      });
+  });
+
+  if (hasAccessorMacros)
+    return false;
+
+  return pattern->hasStorage();
+}
+
 /// Validate the \c entryNumber'th entry in \c binding.
 const PatternBindingEntry *PatternBindingEntryRequest::evaluate(
     Evaluator &eval, PatternBindingDecl *binding, unsigned entryNumber,
@@ -380,7 +401,7 @@ const PatternBindingEntry *PatternBindingEntryRequest::evaluate(
   // default-initializable. If so, do it.
   if (!pbe.isInitialized() &&
       binding->isDefaultInitializable(entryNumber) &&
-      pattern->hasStorage()) {
+      mayHaveStorage(pattern)) {
     if (auto defaultInit = TypeChecker::buildDefaultInitializer(patternType)) {
       // If we got a default initializer, install it and re-type-check it
       // to make sure it is properly coerced to the pattern type.

test/Macros/accessor_macros.swift

@@ -53,7 +53,7 @@ struct MyStruct {
   // CHECK-DUMP: }
 
   @myPropertyWrapper
-  var birthDate: Date? = nil
+  var birthDate: Date?
   // CHECK-DUMP: @__swiftmacro_15accessor_macros8MyStructV9birthDateAA0F0VSgvp17myPropertyWrapperfMa_.swift 
   // CHECK-DUMP: get {
   // CHECK-DUMP:   _birthDate.wrappedValue