[Property Wrappers] Ban non-member property wrappers with observers,

just like we do for regular non-member properties.

Author: hborla

lib/Sema/TypeCheckStorage.cpp

@@ -2737,6 +2737,8 @@ PropertyWrapperBackingPropertyInfoRequest::evaluate(Evaluator &evaluator,
                                              initializer);
       pbd->setInit(0, initializer);
       pbd->setInitializerChecked(0);
+    } else if (var->hasObservers() && !dc->isTypeContext()) {
+      var->diagnose(diag::observingprop_requires_initializer);
     }
 
     if (var->getOpaqueResultTypeDecl()) {

test/decl/var/property_wrappers.swift

@@ -167,6 +167,11 @@ func testLocalContext() {
   @WrapperWithStorageRef
   var hasProjection = 10
   let _: Wrapper = $hasProjection
+
+  @WrapperWithInitialValue
+  var uninitialized: Int { // expected-error {{non-member observing properties require an initializer}}
+    didSet {}
+  }
 }
 
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