[TypeChecker] InitAccessors: Synthesize default init for init accessor properties

Fixes a bug were default initializer for an init accessor property
hasn't been synthesized even when the property is marked as default
initializable.

Resolves: rdar://113421273

Author: xedin

lib/Sema/TypeCheckStorage.cpp

@@ -491,11 +491,16 @@ const PatternBindingEntry *PatternBindingEntryRequest::evaluate(
     }
   }
 
+  auto isInitAccessorProperty = [](Pattern *pattern) {
+    auto *var = pattern->getSingleVar();
+    return var && var->getAccessor(AccessorKind::Init);
+  };
+
   // If we have a type but no initializer, check whether the type is
   // default-initializable. If so, do it.
   if (!pbe.isInitialized() &&
       binding->isDefaultInitializable(entryNumber) &&
-      pattern->hasStorage()) {
+      (pattern->hasStorage() || isInitAccessorProperty(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/decl/var/init_accessors.swift

@@ -620,3 +620,33 @@ func test_invalid_storage_restrictions() {
     init() {}
   }
 }
+
+do {
+  struct Test1 {
+    var q: String
+    var a: Int? {
+      init {}
+      get { 42 }
+    }
+  }
+
+  _ = Test1(q: "some question") // Ok `a` is default initialized to `nil`
+  _ = Test1(q: "ultimate question", a: 42)
+
+  struct Test2 {
+    var q: String
+
+    var _a: Int?
+    var a: Int? {
+      @storageRestrictions(initializes: _a)
+      init {
+        _a = newValue
+      }
+      get { _a }
+      set { _a = newValue }
+    }
+  }
+
+  _ = Test2(q: "some question") // Ok `a` is default initialized to `nil`
+  _ = Test2(q: "ultimate question", a: 42)
+}