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

Fixes rdar://problem/63169705 / FB7699647.

(cherry picked from commit d43c515)

Author: DougGregor

lib/Sema/TypeCheckStorage.cpp

@@ -2694,6 +2694,10 @@ PropertyWrapperBackingPropertyInfoRequest::evaluate(Evaluator &evaluator,
                                            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 +1,24 @@
+// 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}}
+}
+