[Property wrappers] Reject opaque result types when there is no initializer

lib/Sema/TypeCheckStorage.cpp

@@ -2683,16 +2683,21 @@ PropertyWrapperBackingPropertyInfoRequest::evaluate(Evaluator &evaluator,
     pbd->setInit(0, initializer);
     pbd->setInitializerChecked(0);
     wrappedValue = findWrappedValuePlaceholder(initializer);
-  } else if (!parentPBD->isInitialized(patternNumber) &&
-             wrapperInfo.defaultInit) {
-    // FIXME: Record this expression somewhere so that DI can perform the
-    // initialization itself.
-    auto typeExpr = TypeExpr::createImplicit(storageType, ctx);
-    Expr *initializer = CallExpr::createImplicit(ctx, typeExpr, {}, { });
-    typeCheckSynthesizedWrapperInitializer(pbd, backingVar, parentPBD,
-                                           initializer);
-    pbd->setInit(0, initializer);
-    pbd->setInitializerChecked(0);
+  } else {
+    if (!parentPBD->isInitialized(patternNumber) && wrapperInfo.defaultInit) {
+      // FIXME: Record this expression somewhere so that DI can perform the
+      // initialization itself.
+      auto typeExpr = TypeExpr::createImplicit(storageType, ctx);
+      Expr *initializer = CallExpr::createImplicit(ctx, typeExpr, {}, { });
+      typeCheckSynthesizedWrapperInitializer(pbd, backingVar, parentPBD,
+                                             initializer);
+      pbd->setInit(0, initializer);
+      pbd->setInitializerChecked(0);
+    }
+
+    if (var->getOpaqueResultTypeDecl()) {
+      var->diagnose(diag::opaque_type_var_no_underlying_type);
+    }
   }
 
   // If there is a projection property (projectedValue) in the wrapper,

test/decl/var/property_wrappers.swift

@@ -1972,4 +1972,4 @@ public struct NonVisibleImplicitInit {
   public var wrappedValue: Bool {
     return false
   }
-}
+}

test/decl/var/property_wrappers_opaque.swift

@@ -0,0 +1,27 @@
+// RUN: %target-typecheck-verify-swift -swift-version 5 -disable-availability-checking
+
+protocol P { }
+
+@propertyWrapper
+struct WrapperWithDefaultInit<T> {
+  private var stored: T?
+
+  var wrappedValue: T {
+    get { stored! }
+    set { stored = newValue }
+  }
+
+  init() {
+    self.stored = nil
+  }
+}
+
+// FB7699647 - crash with opaque result type and property wrappers.
+struct FB7699647 {
+  @WrapperWithDefaultInit var property: some P // expected-error{{property declares an opaque return type, but cannot infer the underlying type from its initializer expression}}
+  @WrapperWithDefaultInit() var otherProperty: some P // expected-error{{property declares an opaque return type, but cannot infer the underlying type from its initializer expression}}
+}
+
+struct FB7699647b {
+  @WrapperWithDefaultInit var property: some P // expected-error{{property declares an opaque return type, but cannot infer the underlying type from its initializer expression}}
+}