[test] effectful properties + property wrappers

They're currently incompatible, but it should be
possible to enable this, with some care taken to
ensure that effectful wrappers are composed
correctly.

Author: kavon

test/decl/var/effectful_property_wrapper.swift

@@ -0,0 +1,41 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency
+
+// Currently, we don't support having property wrappers that are effectful.
+// Eventually we'd like to add this.
+
+@propertyWrapper
+struct Abstraction<T> {
+    private var value : T
+
+    init(_ initial : T) { self.value = initial }
+
+    var wrappedValue : T {
+        get throws { return value } // expected-error{{property wrappers currently cannot define an 'async' or 'throws' accessor}}
+    }
+
+    // its OK to have effectul props that are not `wrappedValue`
+
+    var prop1 : T {
+      get async { value }
+    }
+    var prop2 : T {
+      get throws { value }
+    }
+    var prop3 : T {
+      get async throws { value }
+    }
+}
+
+@propertyWrapper
+struct NeedlessIntWrapper {
+  var wrappedValue : Int {
+    get {}
+  }
+}
+
+struct S {
+  // expected-error@+1 {{property wrapper cannot be applied to a computed property}}
+  @NeedlessIntWrapper var throwingProp : Int {
+    get throws { 0 }
+  }
+}